source: draft-ietf-httpbis/latest/p1-messaging.xml @ 2582

Last change on this file since 2582 was 2582, checked in by fielding@…, 7 years ago

(design) Strengthen advice on userinfo handling of untrusted URIs to a SHOULD; addresses #531

  • Property svn:eol-style set to native
  • Property svn:mime-type set to text/xml
File size: 236.3 KB
1<?xml version="1.0" encoding="utf-8"?>
2<?xml-stylesheet type='text/xsl' href='../myxml2rfc.xslt'?>
3<!DOCTYPE rfc [
4  <!ENTITY MAY "<bcp14 xmlns=''>MAY</bcp14>">
5  <!ENTITY MUST "<bcp14 xmlns=''>MUST</bcp14>">
6  <!ENTITY MUST-NOT "<bcp14 xmlns=''>MUST NOT</bcp14>">
7  <!ENTITY OPTIONAL "<bcp14 xmlns=''>OPTIONAL</bcp14>">
8  <!ENTITY RECOMMENDED "<bcp14 xmlns=''>RECOMMENDED</bcp14>">
9  <!ENTITY REQUIRED "<bcp14 xmlns=''>REQUIRED</bcp14>">
10  <!ENTITY SHALL "<bcp14 xmlns=''>SHALL</bcp14>">
11  <!ENTITY SHALL-NOT "<bcp14 xmlns=''>SHALL NOT</bcp14>">
12  <!ENTITY SHOULD "<bcp14 xmlns=''>SHOULD</bcp14>">
13  <!ENTITY SHOULD-NOT "<bcp14 xmlns=''>SHOULD NOT</bcp14>">
14  <!ENTITY ID-VERSION "latest">
15  <!ENTITY ID-MONTH "January">
16  <!ENTITY ID-YEAR "2014">
17  <!ENTITY mdash "&#8212;">
18  <!ENTITY Note "<x:h xmlns:x=''>Note:</x:h>">
19  <!ENTITY caching-overview       "<xref target='Part6' x:rel='#caching.overview' xmlns:x=''/>">
20  <!ENTITY cache-incomplete       "<xref target='Part6' x:rel='#response.cacheability' xmlns:x=''/>">
21  <!ENTITY payload                "<xref target='Part2' x:rel='#payload' xmlns:x=''/>">
22  <!ENTITY media-type            "<xref target='Part2' x:rel='#media.type' xmlns:x=''/>">
23  <!ENTITY content-codings        "<xref target='Part2' x:rel='#content.codings' xmlns:x=''/>">
24  <!ENTITY CONNECT                "<xref target='Part2' x:rel='#CONNECT' xmlns:x=''/>">
25  <!ENTITY content.negotiation    "<xref target='Part2' x:rel='#content.negotiation' xmlns:x=''/>">
26  <!ENTITY diff-mime              "<xref target='Part2' x:rel='#differences.between.http.and.mime' xmlns:x=''/>">
27  <!ENTITY representation         "<xref target='Part2' x:rel='#representations' xmlns:x=''/>">
28  <!ENTITY GET                    "<xref target='Part2' x:rel='#GET' xmlns:x=''/>">
29  <!ENTITY HEAD                   "<xref target='Part2' x:rel='#HEAD' xmlns:x=''/>">
30  <!ENTITY header-allow           "<xref target='Part2' x:rel='#header.allow' xmlns:x=''/>">
31  <!ENTITY header-cache-control   "<xref target='Part6' x:rel='#header.cache-control' xmlns:x=''/>">
32  <!ENTITY header-content-encoding    "<xref target='Part2' x:rel='#header.content-encoding' xmlns:x=''/>">
33  <!ENTITY header-content-location    "<xref target='Part2' x:rel='#header.content-location' xmlns:x=''/>">
34  <!ENTITY header-content-range   "<xref target='Part5' x:rel='#header.content-range' xmlns:x=''/>">
35  <!ENTITY header-content-type    "<xref target='Part2' x:rel='#header.content-type' xmlns:x=''/>">
36  <!ENTITY header-date            "<xref target='Part2' x:rel='' xmlns:x=''/>">
37  <!ENTITY header-etag            "<xref target='Part4' x:rel='#header.etag' xmlns:x=''/>">
38  <!ENTITY header-expect          "<xref target='Part2' x:rel='#header.expect' xmlns:x=''/>">
39  <!ENTITY header-expires         "<xref target='Part6' x:rel='#header.expires' xmlns:x=''/>">
40  <!ENTITY header-last-modified   "<xref target='Part4' x:rel='#header.last-modified' xmlns:x=''/>">
41  <!ENTITY header-mime-version    "<xref target='Part2' x:rel='#mime-version' xmlns:x=''/>">
42  <!ENTITY header-pragma          "<xref target='Part6' x:rel='#header.pragma' xmlns:x=''/>">
43  <!ENTITY header-proxy-authenticate  "<xref target='Part7' x:rel='#header.proxy-authenticate' xmlns:x=''/>">
44  <!ENTITY header-proxy-authorization "<xref target='Part7' x:rel='#header.proxy-authorization' xmlns:x=''/>">
45  <!ENTITY header-server          "<xref target='Part2' x:rel='#header.server' xmlns:x=''/>">
46  <!ENTITY header-warning         "<xref target='Part6' x:rel='#header.warning' xmlns:x=''/>">
47  <!ENTITY idempotent-methods     "<xref target='Part2' x:rel='#idempotent.methods' xmlns:x=''/>">
48  <!ENTITY safe-methods           "<xref target='Part2' x:rel='#safe.methods' xmlns:x=''/>">
49  <!ENTITY methods                "<xref target='Part2' x:rel='#methods' xmlns:x=''/>">
50  <!ENTITY OPTIONS                "<xref target='Part2' x:rel='#OPTIONS' xmlns:x=''/>">
51  <!ENTITY qvalue                 "<xref target='Part2' x:rel='#quality.values' xmlns:x=''/>">
52  <!ENTITY request-header-fields  "<xref target='Part2' x:rel='#request.header.fields' xmlns:x=''/>">
53  <!ENTITY response-control-data  "<xref target='Part2' x:rel='' xmlns:x=''/>">
54  <!ENTITY resource               "<xref target='Part2' x:rel='#resources' xmlns:x=''/>">
55  <!ENTITY semantics              "<xref target='Part2' xmlns:x=''/>">
56  <!ENTITY status-codes           "<xref target='Part2' x:rel='' xmlns:x=''/>">
57  <!ENTITY status-1xx             "<xref target='Part2' x:rel='#status.1xx' xmlns:x=''/>">
58  <!ENTITY status-203             "<xref target='Part2' x:rel='#status.203' xmlns:x=''/>">
59  <!ENTITY status-3xx             "<xref target='Part2' x:rel='#status.3xx' xmlns:x=''/>">
60  <!ENTITY status-304             "<xref target='Part4' x:rel='#status.304' xmlns:x=''/>">
61  <!ENTITY status-4xx             "<xref target='Part2' x:rel='#status.4xx' xmlns:x=''/>">
62  <!ENTITY status-414             "<xref target='Part2' x:rel='#status.414' xmlns:x=''/>">
63  <!ENTITY iana-header-registry   "<xref target='Part2' x:rel='#header.field.registry' xmlns:x=''/>">
65<?rfc toc="yes" ?>
66<?rfc symrefs="yes" ?>
67<?rfc sortrefs="yes" ?>
68<?rfc compact="yes"?>
69<?rfc subcompact="no" ?>
70<?rfc linkmailto="no" ?>
71<?rfc editing="no" ?>
72<?rfc comments="yes"?>
73<?rfc inline="yes"?>
74<?rfc rfcedstyle="yes"?>
75<?rfc-ext allow-markup-in-artwork="yes" ?>
76<?rfc-ext include-references-in-index="yes" ?>
77<rfc obsoletes="2145,2616" updates="2817,2818" category="std" x:maturity-level="proposed"
78     ipr="pre5378Trust200902" docName="draft-ietf-httpbis-p1-messaging-&ID-VERSION;"
79     xmlns:x=''>
80<x:link rel="next" basename="p2-semantics"/>
81<x:feedback template="{docname},%20%22{section}%22&amp;body=&lt;{ref}&gt;:"/>
84  <title abbrev="HTTP/1.1 Message Syntax and Routing">Hypertext Transfer Protocol (HTTP/1.1): Message Syntax and Routing</title>
86  <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
87    <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
88    <address>
89      <postal>
90        <street>345 Park Ave</street>
91        <city>San Jose</city>
92        <region>CA</region>
93        <code>95110</code>
94        <country>USA</country>
95      </postal>
96      <email></email>
97      <uri></uri>
98    </address>
99  </author>
101  <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
102    <organization abbrev="greenbytes">greenbytes GmbH</organization>
103    <address>
104      <postal>
105        <street>Hafenweg 16</street>
106        <city>Muenster</city><region>NW</region><code>48155</code>
107        <country>Germany</country>
108      </postal>
109      <email></email>
110      <uri></uri>
111    </address>
112  </author>
114  <date month="&ID-MONTH;" year="&ID-YEAR;"/>
115  <workgroup>HTTPbis Working Group</workgroup>
119   The Hypertext Transfer Protocol (HTTP) is a stateless application-level
120   protocol for distributed, collaborative, hypertext information systems.
121   This document provides an overview of HTTP architecture and its associated
122   terminology, defines the "http" and "https" Uniform Resource Identifier
123   (URI) schemes, defines the HTTP/1.1 message syntax and parsing
124   requirements, and describes related security concerns for implementations.
128<note title="Editorial Note (To be removed by RFC Editor)">
129  <t>
130    Discussion of this draft takes place on the HTTPBIS working group
131    mailing list (, which is archived at
132    <eref target=""/>.
133  </t>
134  <t>
135    The current issues list is at
136    <eref target=""/> and related
137    documents (including fancy diffs) can be found at
138    <eref target=""/>.
139  </t>
140  <t>
141    The changes in this draft are summarized in <xref target="changes.since.25"/>.
142  </t>
146<section title="Introduction" anchor="introduction">
148   The Hypertext Transfer Protocol (HTTP) is a stateless application-level
149   request/response protocol that uses extensible semantics and
150   self-descriptive message payloads for flexible interaction with
151   network-based hypertext information systems. This document is the first in
152   a series of documents that collectively form the HTTP/1.1 specification:
153   <list style="empty">
154    <t>RFC xxx1: Message Syntax and Routing</t>
155    <t><xref target="Part2" x:fmt="none">RFC xxx2</xref>: Semantics and Content</t>
156    <t><xref target="Part4" x:fmt="none">RFC xxx3</xref>: Conditional Requests</t>
157    <t><xref target="Part5" x:fmt="none">RFC xxx4</xref>: Range Requests</t>
158    <t><xref target="Part6" x:fmt="none">RFC xxx5</xref>: Caching</t>
159    <t><xref target="Part7" x:fmt="none">RFC xxx6</xref>: Authentication</t>
160   </list>
163   This HTTP/1.1 specification obsoletes
164   <xref target="RFC2616" x:fmt="none">RFC 2616</xref> and
165   <xref target="RFC2145" x:fmt="none">RFC 2145</xref> (on HTTP versioning).
166   This specification also updates the use of CONNECT to establish a tunnel,
167   previously defined in <xref target="RFC2817" x:fmt="none">RFC 2817</xref>,
168   and defines the "https" URI scheme that was described informally in
169   <xref target="RFC2818" x:fmt="none">RFC 2818</xref>.
172   HTTP is a generic interface protocol for information systems. It is
173   designed to hide the details of how a service is implemented by presenting
174   a uniform interface to clients that is independent of the types of
175   resources provided. Likewise, servers do not need to be aware of each
176   client's purpose: an HTTP request can be considered in isolation rather
177   than being associated with a specific type of client or a predetermined
178   sequence of application steps. The result is a protocol that can be used
179   effectively in many different contexts and for which implementations can
180   evolve independently over time.
183   HTTP is also designed for use as an intermediation protocol for translating
184   communication to and from non-HTTP information systems.
185   HTTP proxies and gateways can provide access to alternative information
186   services by translating their diverse protocols into a hypertext
187   format that can be viewed and manipulated by clients in the same way
188   as HTTP services.
191   One consequence of this flexibility is that the protocol cannot be
192   defined in terms of what occurs behind the interface. Instead, we
193   are limited to defining the syntax of communication, the intent
194   of received communication, and the expected behavior of recipients.
195   If the communication is considered in isolation, then successful
196   actions ought to be reflected in corresponding changes to the
197   observable interface provided by servers. However, since multiple
198   clients might act in parallel and perhaps at cross-purposes, we
199   cannot require that such changes be observable beyond the scope
200   of a single response.
203   This document describes the architectural elements that are used or
204   referred to in HTTP, defines the "http" and "https" URI schemes,
205   describes overall network operation and connection management,
206   and defines HTTP message framing and forwarding requirements.
207   Our goal is to define all of the mechanisms necessary for HTTP message
208   handling that are independent of message semantics, thereby defining the
209   complete set of requirements for message parsers and
210   message-forwarding intermediaries.
214<section title="Requirement Notation" anchor="intro.requirements">
216   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
217   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
218   document are to be interpreted as described in <xref target="RFC2119"/>.
221   Conformance criteria and considerations regarding error handling
222   are defined in <xref target="conformance"/>.
226<section title="Syntax Notation" anchor="notation">
227<iref primary="true" item="Grammar" subitem="ALPHA"/>
228<iref primary="true" item="Grammar" subitem="CR"/>
229<iref primary="true" item="Grammar" subitem="CRLF"/>
230<iref primary="true" item="Grammar" subitem="CTL"/>
231<iref primary="true" item="Grammar" subitem="DIGIT"/>
232<iref primary="true" item="Grammar" subitem="DQUOTE"/>
233<iref primary="true" item="Grammar" subitem="HEXDIG"/>
234<iref primary="true" item="Grammar" subitem="HTAB"/>
235<iref primary="true" item="Grammar" subitem="LF"/>
236<iref primary="true" item="Grammar" subitem="OCTET"/>
237<iref primary="true" item="Grammar" subitem="SP"/>
238<iref primary="true" item="Grammar" subitem="VCHAR"/>
240   This specification uses the Augmented Backus-Naur Form (ABNF) notation of
241   <xref target="RFC5234"/> with a list extension, defined in
242   <xref target="abnf.extension"/>, that allows for compact definition of
243   comma-separated lists using a '#' operator (similar to how the '*' operator
244   indicates repetition).
245   <xref target="collected.abnf"/> shows the collected grammar with all list
246   operators expanded to standard ABNF notation.
248<t anchor="core.rules">
249  <x:anchor-alias value="ALPHA"/>
250  <x:anchor-alias value="CTL"/>
251  <x:anchor-alias value="CR"/>
252  <x:anchor-alias value="CRLF"/>
253  <x:anchor-alias value="DIGIT"/>
254  <x:anchor-alias value="DQUOTE"/>
255  <x:anchor-alias value="HEXDIG"/>
256  <x:anchor-alias value="HTAB"/>
257  <x:anchor-alias value="LF"/>
258  <x:anchor-alias value="OCTET"/>
259  <x:anchor-alias value="SP"/>
260  <x:anchor-alias value="VCHAR"/>
261   The following core rules are included by
262   reference, as defined in <xref target="RFC5234" x:fmt="," x:sec="B.1"/>:
263   ALPHA (letters), CR (carriage return), CRLF (CR LF), CTL (controls),
264   DIGIT (decimal 0-9), DQUOTE (double quote),
265   HEXDIG (hexadecimal 0-9/A-F/a-f), HTAB (horizontal tab), LF (line feed),
266   OCTET (any 8-bit sequence of data), SP (space), and
267   VCHAR (any visible <xref target="USASCII"/> character).
270   As a convention, ABNF rule names prefixed with "obs-" denote
271   "obsolete" grammar rules that appear for historical reasons.
276<section title="Architecture" anchor="architecture">
278   HTTP was created for the World Wide Web (WWW) architecture
279   and has evolved over time to support the scalability needs of a worldwide
280   hypertext system. Much of that architecture is reflected in the terminology
281   and syntax productions used to define HTTP.
284<section title="Client/Server Messaging" anchor="operation">
285<iref primary="true" item="client"/>
286<iref primary="true" item="server"/>
287<iref primary="true" item="connection"/>
289   HTTP is a stateless request/response protocol that operates by exchanging
290   <x:dfn>messages</x:dfn> (<xref target="http.message"/>) across a reliable
291   transport or session-layer
292   "<x:dfn>connection</x:dfn>" (<xref target=""/>).
293   An HTTP "<x:dfn>client</x:dfn>" is a program that establishes a connection
294   to a server for the purpose of sending one or more HTTP requests.
295   An HTTP "<x:dfn>server</x:dfn>" is a program that accepts connections
296   in order to service HTTP requests by sending HTTP responses.
298<iref primary="true" item="user agent"/>
299<iref primary="true" item="origin server"/>
300<iref primary="true" item="browser"/>
301<iref primary="true" item="spider"/>
302<iref primary="true" item="sender"/>
303<iref primary="true" item="recipient"/>
305   The terms client and server refer only to the roles that
306   these programs perform for a particular connection.  The same program
307   might act as a client on some connections and a server on others.
308   The term "<x:dfn>user agent</x:dfn>" refers to any of the various
309   client programs that initiate a request, including (but not limited to)
310   browsers, spiders (web-based robots), command-line tools, custom
311   applications, and mobile apps.
312   The term "<x:dfn>origin server</x:dfn>" refers to the program that can
313   originate authoritative responses for a given target resource.
314   The terms "<x:dfn>sender</x:dfn>" and "<x:dfn>recipient</x:dfn>" refer to
315   any implementation that sends or receives a given message, respectively.
318   HTTP relies upon the Uniform Resource Identifier (URI)
319   standard <xref target="RFC3986"/> to indicate the target resource
320   (<xref target="target-resource"/>) and relationships between resources.
321   Messages are passed in a format similar to that used by Internet mail
322   <xref target="RFC5322"/> and the Multipurpose Internet Mail Extensions
323   (MIME) <xref target="RFC2045"/> (see &diff-mime; for the differences
324   between HTTP and MIME messages).
327   Most HTTP communication consists of a retrieval request (GET) for
328   a representation of some resource identified by a URI.  In the
329   simplest case, this might be accomplished via a single bidirectional
330   connection (===) between the user agent (UA) and the origin server (O).
332<figure><artwork type="drawing">
333         request   &gt;
334    <x:highlight>UA</x:highlight> ======================================= <x:highlight>O</x:highlight>
335                                &lt;   response
337<iref primary="true" item="message"/>
338<iref primary="true" item="request"/>
339<iref primary="true" item="response"/>
341   A client sends an HTTP request to a server in the form of a <x:dfn>request</x:dfn>
342   message, beginning with a request-line that includes a method, URI, and
343   protocol version (<xref target="request.line"/>),
344   followed by header fields containing
345   request modifiers, client information, and representation metadata
346   (<xref target="header.fields"/>),
347   an empty line to indicate the end of the header section, and finally
348   a message body containing the payload body (if any,
349   <xref target="message.body"/>).
352   A server responds to a client's request by sending one or more HTTP
353   <x:dfn>response</x:dfn>
354   messages, each beginning with a status line that
355   includes the protocol version, a success or error code, and textual
356   reason phrase (<xref target="status.line"/>),
357   possibly followed by header fields containing server
358   information, resource metadata, and representation metadata
359   (<xref target="header.fields"/>),
360   an empty line to indicate the end of the header section, and finally
361   a message body containing the payload body (if any,
362   <xref target="message.body"/>).
365   A connection might be used for multiple request/response exchanges,
366   as defined in <xref target="persistent.connections"/>.
369   The following example illustrates a typical message exchange for a
370   GET request (&GET;) on the URI "":
373Client request:
374</preamble><artwork type="message/http; msgtype=&#34;request&#34;" x:indent-with="  ">
375GET /hello.txt HTTP/1.1
376User-Agent: curl/7.16.3 libcurl/7.16.3 OpenSSL/0.9.7l zlib/1.2.3
378Accept-Language: en, mi
382Server response:
383</preamble><artwork type="message/http; msgtype=&#34;response&#34;" x:indent-with="  ">
384HTTP/1.1 200 OK
385Date: Mon, 27 Jul 2009 12:28:53 GMT
386Server: Apache
387Last-Modified: Wed, 22 Jul 2009 19:15:56 GMT
388ETag: "34aa387-d-1568eb00"
389Accept-Ranges: bytes
390Content-Length: <x:length-of target="exbody"/>
391Vary: Accept-Encoding
392Content-Type: text/plain
394<x:span anchor="exbody">Hello World! My payload includes a trailing CRLF.
399<section title="Implementation Diversity" anchor="implementation-diversity">
401   When considering the design of HTTP, it is easy to fall into a trap of
402   thinking that all user agents are general-purpose browsers and all origin
403   servers are large public websites. That is not the case in practice.
404   Common HTTP user agents include household appliances, stereos, scales,
405   firmware update scripts, command-line programs, mobile apps,
406   and communication devices in a multitude of shapes and sizes.  Likewise,
407   common HTTP origin servers include home automation units, configurable
408   networking components, office machines, autonomous robots, news feeds,
409   traffic cameras, ad selectors, and video delivery platforms.
412   The term "user agent" does not imply that there is a human user directly
413   interacting with the software agent at the time of a request. In many
414   cases, a user agent is installed or configured to run in the background
415   and save its results for later inspection (or save only a subset of those
416   results that might be interesting or erroneous). Spiders, for example, are
417   typically given a start URI and configured to follow certain behavior while
418   crawling the Web as a hypertext graph.
421   The implementation diversity of HTTP means that not all user agents can
422   make interactive suggestions to their user or provide adequate warning for
423   security or privacy concerns. In the few cases where this
424   specification requires reporting of errors to the user, it is acceptable
425   for such reporting to only be observable in an error console or log file.
426   Likewise, requirements that an automated action be confirmed by the user
427   before proceeding might be met via advance configuration choices,
428   run-time options, or simple avoidance of the unsafe action; confirmation
429   does not imply any specific user interface or interruption of normal
430   processing if the user has already made that choice.
434<section title="Intermediaries" anchor="intermediaries">
435<iref primary="true" item="intermediary"/>
437   HTTP enables the use of intermediaries to satisfy requests through
438   a chain of connections.  There are three common forms of HTTP
439   <x:dfn>intermediary</x:dfn>: proxy, gateway, and tunnel.  In some cases,
440   a single intermediary might act as an origin server, proxy, gateway,
441   or tunnel, switching behavior based on the nature of each request.
443<figure><artwork type="drawing">
444         &gt;             &gt;             &gt;             &gt;
445    <x:highlight>UA</x:highlight> =========== <x:highlight>A</x:highlight> =========== <x:highlight>B</x:highlight> =========== <x:highlight>C</x:highlight> =========== <x:highlight>O</x:highlight>
446               &lt;             &lt;             &lt;             &lt;
449   The figure above shows three intermediaries (A, B, and C) between the
450   user agent and origin server. A request or response message that
451   travels the whole chain will pass through four separate connections.
452   Some HTTP communication options
453   might apply only to the connection with the nearest, non-tunnel
454   neighbor, only to the end-points of the chain, or to all connections
455   along the chain. Although the diagram is linear, each participant might
456   be engaged in multiple, simultaneous communications. For example, B
457   might be receiving requests from many clients other than A, and/or
458   forwarding requests to servers other than C, at the same time that it
459   is handling A's request. Likewise, later requests might be sent through a
460   different path of connections, often based on dynamic configuration for
461   load balancing.   
464<iref primary="true" item="upstream"/><iref primary="true" item="downstream"/>
465<iref primary="true" item="inbound"/><iref primary="true" item="outbound"/>
466   The terms "<x:dfn>upstream</x:dfn>" and "<x:dfn>downstream</x:dfn>" are
467   used to describe directional requirements in relation to the message flow:
468   all messages flow from upstream to downstream.
469   The terms inbound and outbound are used to describe directional
470   requirements in relation to the request route:
471   "<x:dfn>inbound</x:dfn>" means toward the origin server and
472   "<x:dfn>outbound</x:dfn>" means toward the user agent.
474<t><iref primary="true" item="proxy"/>
475   A "<x:dfn>proxy</x:dfn>" is a message forwarding agent that is selected by the
476   client, usually via local configuration rules, to receive requests
477   for some type(s) of absolute URI and attempt to satisfy those
478   requests via translation through the HTTP interface.  Some translations
479   are minimal, such as for proxy requests for "http" URIs, whereas
480   other requests might require translation to and from entirely different
481   application-level protocols. Proxies are often used to group an
482   organization's HTTP requests through a common intermediary for the
483   sake of security, annotation services, or shared caching. Some proxies
484   are designed to apply transformations to selected messages or payloads
485   while they are being forwarded, as described in
486   <xref target="message.transformations"/>.
488<t><iref primary="true" item="gateway"/><iref primary="true" item="reverse proxy"/>
489<iref primary="true" item="accelerator"/>
490   A "<x:dfn>gateway</x:dfn>" (a.k.a., "<x:dfn>reverse proxy</x:dfn>") is an
491   intermediary that acts as an origin server for the outbound connection, but
492   translates received requests and forwards them inbound to another server or
493   servers. Gateways are often used to encapsulate legacy or untrusted
494   information services, to improve server performance through
495   "<x:dfn>accelerator</x:dfn>" caching, and to enable partitioning or load
496   balancing of HTTP services across multiple machines.
499   All HTTP requirements applicable to an origin server
500   also apply to the outbound communication of a gateway.
501   A gateway communicates with inbound servers using any protocol that
502   it desires, including private extensions to HTTP that are outside
503   the scope of this specification.  However, an HTTP-to-HTTP gateway
504   that wishes to interoperate with third-party HTTP servers ought to conform
505   to user agent requirements on the gateway's inbound connection.
507<t><iref primary="true" item="tunnel"/>
508   A "<x:dfn>tunnel</x:dfn>" acts as a blind relay between two connections
509   without changing the messages. Once active, a tunnel is not
510   considered a party to the HTTP communication, though the tunnel might
511   have been initiated by an HTTP request. A tunnel ceases to exist when
512   both ends of the relayed connection are closed. Tunnels are used to
513   extend a virtual connection through an intermediary, such as when
514   Transport Layer Security (TLS, <xref target="RFC5246"/>) is used to
515   establish confidential communication through a shared firewall proxy.
517<t><iref primary="true" item="interception proxy"/>
518<iref primary="true" item="transparent proxy"/>
519<iref primary="true" item="captive portal"/>
520   The above categories for intermediary only consider those acting as
521   participants in the HTTP communication.  There are also intermediaries
522   that can act on lower layers of the network protocol stack, filtering or
523   redirecting HTTP traffic without the knowledge or permission of message
524   senders. Network intermediaries often introduce security flaws or
525   interoperability problems by violating HTTP semantics.  For example, an
526   "<x:dfn>interception proxy</x:dfn>" <xref target="RFC3040"/> (also commonly
527   known as a "<x:dfn>transparent proxy</x:dfn>" <xref target="RFC1919"/> or
528   "<x:dfn>captive portal</x:dfn>")
529   differs from an HTTP proxy because it is not selected by the client.
530   Instead, an interception proxy filters or redirects outgoing TCP port 80
531   packets (and occasionally other common port traffic).
532   Interception proxies are commonly found on public network access points,
533   as a means of enforcing account subscription prior to allowing use of
534   non-local Internet services, and within corporate firewalls to enforce
535   network usage policies.
536   They are indistinguishable from a man-in-the-middle attack.
539   HTTP is defined as a stateless protocol, meaning that each request message
540   can be understood in isolation.  Many implementations depend on HTTP's
541   stateless design in order to reuse proxied connections or dynamically
542   load-balance requests across multiple servers.  Hence, a server &MUST-NOT;
543   assume that two requests on the same connection are from the same user
544   agent unless the connection is secured and specific to that agent.
545   Some non-standard HTTP extensions (e.g., <xref target="RFC4559"/>) have
546   been known to violate this requirement, resulting in security and
547   interoperability problems.
551<section title="Caches" anchor="caches">
552<iref primary="true" item="cache"/>
554   A "<x:dfn>cache</x:dfn>" is a local store of previous response messages and the
555   subsystem that controls its message storage, retrieval, and deletion.
556   A cache stores cacheable responses in order to reduce the response
557   time and network bandwidth consumption on future, equivalent
558   requests. Any client or server &MAY; employ a cache, though a cache
559   cannot be used by a server while it is acting as a tunnel.
562   The effect of a cache is that the request/response chain is shortened
563   if one of the participants along the chain has a cached response
564   applicable to that request. The following illustrates the resulting
565   chain if B has a cached copy of an earlier response from O (via C)
566   for a request that has not been cached by UA or A.
568<figure><artwork type="drawing">
569            &gt;             &gt;
570       <x:highlight>UA</x:highlight> =========== <x:highlight>A</x:highlight> =========== <x:highlight>B</x:highlight> - - - - - - <x:highlight>C</x:highlight> - - - - - - <x:highlight>O</x:highlight>
571                  &lt;             &lt;
573<t><iref primary="true" item="cacheable"/>
574   A response is "<x:dfn>cacheable</x:dfn>" if a cache is allowed to store a copy of
575   the response message for use in answering subsequent requests.
576   Even when a response is cacheable, there might be additional
577   constraints placed by the client or by the origin server on when
578   that cached response can be used for a particular request. HTTP
579   requirements for cache behavior and cacheable responses are
580   defined in &caching-overview;. 
583   There are a wide variety of architectures and configurations
584   of caches deployed across the World Wide Web and
585   inside large organizations. These include national hierarchies
586   of proxy caches to save transoceanic bandwidth, collaborative systems that
587   broadcast or multicast cache entries, archives of pre-fetched cache
588   entries for use in off-line or high-latency environments, and so on.
592<section title="Conformance and Error Handling" anchor="conformance">
594   This specification targets conformance criteria according to the role of
595   a participant in HTTP communication.  Hence, HTTP requirements are placed
596   on senders, recipients, clients, servers, user agents, intermediaries,
597   origin servers, proxies, gateways, or caches, depending on what behavior
598   is being constrained by the requirement. Additional (social) requirements
599   are placed on implementations, resource owners, and protocol element
600   registrations when they apply beyond the scope of a single communication.
603   The verb "generate" is used instead of "send" where a requirement
604   differentiates between creating a protocol element and merely forwarding a
605   received element downstream.
608   An implementation is considered conformant if it complies with all of the
609   requirements associated with the roles it partakes in HTTP.
612   Conformance includes both the syntax and semantics of protocol
613   elements. A sender &MUST-NOT; generate protocol elements that convey a
614   meaning that is known by that sender to be false. A sender &MUST-NOT;
615   generate protocol elements that do not match the grammar defined by the
616   corresponding ABNF rules. Within a given message, a sender &MUST-NOT;
617   generate protocol elements or syntax alternatives that are only allowed to
618   be generated by participants in other roles (i.e., a role that the sender
619   does not have for that message).
622   When a received protocol element is parsed, the recipient &MUST; be able to
623   parse any value of reasonable length that is applicable to the recipient's
624   role and matches the grammar defined by the corresponding ABNF rules.
625   Note, however, that some received protocol elements might not be parsed.
626   For example, an intermediary forwarding a message might parse a
627   header-field into generic field-name and field-value components, but then
628   forward the header field without further parsing inside the field-value.
631   HTTP does not have specific length limitations for many of its protocol
632   elements because the lengths that might be appropriate will vary widely,
633   depending on the deployment context and purpose of the implementation.
634   Hence, interoperability between senders and recipients depends on shared
635   expectations regarding what is a reasonable length for each protocol
636   element. Furthermore, what is commonly understood to be a reasonable length
637   for some protocol elements has changed over the course of the past two
638   decades of HTTP use, and is expected to continue changing in the future.
641   At a minimum, a recipient &MUST; be able to parse and process protocol
642   element lengths that are at least as long as the values that it generates
643   for those same protocol elements in other messages. For example, an origin
644   server that publishes very long URI references to its own resources needs
645   to be able to parse and process those same references when received as a
646   request target.
649   A recipient &MUST; interpret a received protocol element according to the
650   semantics defined for it by this specification, including extensions to
651   this specification, unless the recipient has determined (through experience
652   or configuration) that the sender incorrectly implements what is implied by
653   those semantics.
654   For example, an origin server might disregard the contents of a received
655   <x:ref>Accept-Encoding</x:ref> header field if inspection of the
656   <x:ref>User-Agent</x:ref> header field indicates a specific implementation
657   version that is known to fail on receipt of certain content codings.
660   Unless noted otherwise, a recipient &MAY; attempt to recover a usable
661   protocol element from an invalid construct.  HTTP does not define
662   specific error handling mechanisms except when they have a direct impact
663   on security, since different applications of the protocol require
664   different error handling strategies.  For example, a Web browser might
665   wish to transparently recover from a response where the
666   <x:ref>Location</x:ref> header field doesn't parse according to the ABNF,
667   whereas a systems control client might consider any form of error recovery
668   to be dangerous.
672<section title="Protocol Versioning" anchor="http.version">
673  <x:anchor-alias value="HTTP-version"/>
674  <x:anchor-alias value="HTTP-name"/>
676   HTTP uses a "&lt;major&gt;.&lt;minor&gt;" numbering scheme to indicate
677   versions of the protocol. This specification defines version "1.1".
678   The protocol version as a whole indicates the sender's conformance
679   with the set of requirements laid out in that version's corresponding
680   specification of HTTP.
683   The version of an HTTP message is indicated by an HTTP-version field
684   in the first line of the message. HTTP-version is case-sensitive.
686<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="HTTP-version"/><iref primary="true" item="Grammar" subitem="HTTP-name"/>
687  <x:ref>HTTP-version</x:ref>  = <x:ref>HTTP-name</x:ref> "/" <x:ref>DIGIT</x:ref> "." <x:ref>DIGIT</x:ref>
688  <x:ref>HTTP-name</x:ref>     = <x:abnf-char-sequence>"HTTP"</x:abnf-char-sequence> ; "HTTP", case-sensitive
691   The HTTP version number consists of two decimal digits separated by a "."
692   (period or decimal point).  The first digit ("major version") indicates the
693   HTTP messaging syntax, whereas the second digit ("minor version") indicates
694   the highest minor version within that major version to which the sender is
695   conformant and able to understand for future communication.  The minor
696   version advertises the sender's communication capabilities even when the
697   sender is only using a backwards-compatible subset of the protocol,
698   thereby letting the recipient know that more advanced features can
699   be used in response (by servers) or in future requests (by clients).
702   When an HTTP/1.1 message is sent to an HTTP/1.0 recipient
703   <xref target="RFC1945"/> or a recipient whose version is unknown,
704   the HTTP/1.1 message is constructed such that it can be interpreted
705   as a valid HTTP/1.0 message if all of the newer features are ignored.
706   This specification places recipient-version requirements on some
707   new features so that a conformant sender will only use compatible
708   features until it has determined, through configuration or the
709   receipt of a message, that the recipient supports HTTP/1.1.
712   The interpretation of a header field does not change between minor
713   versions of the same major HTTP version, though the default
714   behavior of a recipient in the absence of such a field can change.
715   Unless specified otherwise, header fields defined in HTTP/1.1 are
716   defined for all versions of HTTP/1.x.  In particular, the <x:ref>Host</x:ref>
717   and <x:ref>Connection</x:ref> header fields ought to be implemented by all
718   HTTP/1.x implementations whether or not they advertise conformance with
719   HTTP/1.1.
722   New header fields can be introduced without changing the protocol version
723   if their defined semantics allow them to be safely ignored by recipients
724   that do not recognize them. Header field extensibility is discussed in
725   <xref target="field.extensibility"/>.
728   Intermediaries that process HTTP messages (i.e., all intermediaries
729   other than those acting as tunnels) &MUST; send their own HTTP-version
730   in forwarded messages.  In other words, they are not allowed to blindly
731   forward the first line of an HTTP message without ensuring that the
732   protocol version in that message matches a version to which that
733   intermediary is conformant for both the receiving and
734   sending of messages.  Forwarding an HTTP message without rewriting
735   the HTTP-version might result in communication errors when downstream
736   recipients use the message sender's version to determine what features
737   are safe to use for later communication with that sender.
740   A client &SHOULD; send a request version equal to the highest
741   version to which the client is conformant and
742   whose major version is no higher than the highest version supported
743   by the server, if this is known.  A client &MUST-NOT; send a
744   version to which it is not conformant.
747   A client &MAY; send a lower request version if it is known that
748   the server incorrectly implements the HTTP specification, but only
749   after the client has attempted at least one normal request and determined
750   from the response status code or header fields (e.g., <x:ref>Server</x:ref>) that
751   the server improperly handles higher request versions.
754   A server &SHOULD; send a response version equal to the highest version to
755   which the server is conformant that has a major version less than or equal
756   to the one received in the request.
757   A server &MUST-NOT; send a version to which it is not conformant.
758   A server can send a <x:ref>505 (HTTP Version Not Supported)</x:ref>
759   response if it wishes, for any reason, to refuse service of the client's
760   major protocol version.
763   A server &MAY; send an HTTP/1.0 response to a request
764   if it is known or suspected that the client incorrectly implements the
765   HTTP specification and is incapable of correctly processing later
766   version responses, such as when a client fails to parse the version
767   number correctly or when an intermediary is known to blindly forward
768   the HTTP-version even when it doesn't conform to the given minor
769   version of the protocol. Such protocol downgrades &SHOULD-NOT; be
770   performed unless triggered by specific client attributes, such as when
771   one or more of the request header fields (e.g., <x:ref>User-Agent</x:ref>)
772   uniquely match the values sent by a client known to be in error.
775   The intention of HTTP's versioning design is that the major number
776   will only be incremented if an incompatible message syntax is
777   introduced, and that the minor number will only be incremented when
778   changes made to the protocol have the effect of adding to the message
779   semantics or implying additional capabilities of the sender.  However,
780   the minor version was not incremented for the changes introduced between
781   <xref target="RFC2068"/> and <xref target="RFC2616"/>, and this revision
782   has specifically avoided any such changes to the protocol.
785   When an HTTP message is received with a major version number that the
786   recipient implements, but a higher minor version number than what the
787   recipient implements, the recipient &SHOULD; process the message as if it
788   were in the highest minor version within that major version to which the
789   recipient is conformant. A recipient can assume that a message with a
790   higher minor version, when sent to a recipient that has not yet indicated
791   support for that higher version, is sufficiently backwards-compatible to be
792   safely processed by any implementation of the same major version.
796<section title="Uniform Resource Identifiers" anchor="uri">
797<iref primary="true" item="resource"/>
799   Uniform Resource Identifiers (URIs) <xref target="RFC3986"/> are used
800   throughout HTTP as the means for identifying resources (&resource;).
801   URI references are used to target requests, indicate redirects, and define
802   relationships.
804  <x:anchor-alias value="URI-reference"/>
805  <x:anchor-alias value="absolute-URI"/>
806  <x:anchor-alias value="relative-part"/>
807  <x:anchor-alias value="authority"/>
808  <x:anchor-alias value="uri-host"/>
809  <x:anchor-alias value="port"/>
810  <x:anchor-alias value="path-abempty"/>
811  <x:anchor-alias value="segment"/>
812  <x:anchor-alias value="query"/>
813  <x:anchor-alias value="fragment"/>
814  <x:anchor-alias value="absolute-path"/>
815  <x:anchor-alias value="partial-URI"/>
817   The definitions of "URI-reference",
818   "absolute-URI", "relative-part", "authority", "port", "host",
819   "path-abempty", "segment", "query", and "fragment" are adopted from the
820   URI generic syntax.
821   An "absolute-path" rule is defined, differing slightly from
822   RFC 3986's "path-absolute" in that it allows a leading "//".
823   A "partial-URI" rule is defined for protocol elements
824   that allow a relative URI but not a fragment.
826<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="URI-reference"><!--exported production--></iref><iref primary="true" item="Grammar" subitem="absolute-URI"/><iref primary="true" item="Grammar" subitem="authority"/><iref primary="true" item="Grammar" subitem="absolute-path"/><iref primary="true" item="Grammar" subitem="port"/><iref primary="true" item="Grammar" subitem="query"/><iref primary="true" item="Grammar" subitem="fragment"/><iref primary="true" item="Grammar" subitem="segment"/><iref primary="true" item="Grammar" subitem="uri-host"/><iref primary="true" item="Grammar" subitem="partial-URI"><!--exported production--></iref>
827  <x:ref>URI-reference</x:ref> = &lt;URI-reference, defined in <xref target="RFC3986" x:fmt="," x:sec="4.1"/>&gt;
828  <x:ref>absolute-URI</x:ref>  = &lt;absolute-URI, defined in <xref target="RFC3986" x:fmt="," x:sec="4.3"/>&gt;
829  <x:ref>relative-part</x:ref> = &lt;relative-part, defined in <xref target="RFC3986" x:fmt="," x:sec="4.2"/>&gt;
830  <x:ref>authority</x:ref>     = &lt;authority, defined in <xref target="RFC3986" x:fmt="," x:sec="3.2"/>&gt;
831  <x:ref>uri-host</x:ref>      = &lt;host, defined in <xref target="RFC3986" x:fmt="," x:sec="3.2.2"/>&gt;
832  <x:ref>port</x:ref>          = &lt;port, defined in <xref target="RFC3986" x:fmt="," x:sec="3.2.3"/>&gt;
833  <x:ref>path-abempty</x:ref>  = &lt;path-abempty, defined in <xref target="RFC3986" x:fmt="," x:sec="3.3"/>&gt;
834  <x:ref>segment</x:ref>       = &lt;segment, defined in <xref target="RFC3986" x:fmt="," x:sec="3.3"/>&gt;
835  <x:ref>query</x:ref>         = &lt;query, defined in <xref target="RFC3986" x:fmt="," x:sec="3.4"/>&gt;
836  <x:ref>fragment</x:ref>      = &lt;fragment, defined in <xref target="RFC3986" x:fmt="," x:sec="3.5"/>&gt;
838  <x:ref>absolute-path</x:ref> = 1*( "/" segment )
839  <x:ref>partial-URI</x:ref>   = relative-part [ "?" query ]
842   Each protocol element in HTTP that allows a URI reference will indicate
843   in its ABNF production whether the element allows any form of reference
844   (URI-reference), only a URI in absolute form (absolute-URI), only the
845   path and optional query components, or some combination of the above.
846   Unless otherwise indicated, URI references are parsed
847   relative to the effective request URI
848   (<xref target="effective.request.uri"/>).
851<section title="http URI scheme" anchor="http.uri">
852  <x:anchor-alias value="http-URI"/>
853  <iref item="http URI scheme" primary="true"/>
854  <iref item="URI scheme" subitem="http" primary="true"/>
856   The "http" URI scheme is hereby defined for the purpose of minting
857   identifiers according to their association with the hierarchical
858   namespace governed by a potential HTTP origin server listening for
859   TCP (<xref target="RFC0793"/>) connections on a given port.
861<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="http-URI"><!--terminal production--></iref>
862  <x:ref>http-URI</x:ref> = "http:" "//" <x:ref>authority</x:ref> <x:ref>path-abempty</x:ref> [ "?" <x:ref>query</x:ref> ]
863             [ "#" <x:ref>fragment</x:ref> ]
866   The HTTP origin server is identified by the generic syntax's
867   <x:ref>authority</x:ref> component, which includes a host identifier
868   and optional TCP port (<xref target="RFC3986" x:fmt="," x:sec="3.2.2"/>).
869   The remainder of the URI, consisting of both the hierarchical path
870   component and optional query component, serves as an identifier for
871   a potential resource within that origin server's name space.
874   A sender &MUST-NOT; generate an "http" URI with an empty host identifier.
875   A recipient that processes such a URI reference &MUST; reject it as invalid.
878   If the host identifier is provided as an IP address,
879   then the origin server is any listener on the indicated TCP port at
880   that IP address. If host is a registered name, then that name is
881   considered an indirect identifier and the recipient might use a name
882   resolution service, such as DNS, to find the address of a listener
883   for that host.
884   If the port subcomponent is empty or not given, then TCP port 80 is
885   assumed (the default reserved port for WWW services).
888   Regardless of the form of host identifier, access to that host is not
889   implied by the mere presence of its name or address. The host might or might
890   not exist and, even when it does exist, might or might not be running an
891   HTTP server or listening to the indicated port. The "http" URI scheme
892   makes use of the delegated nature of Internet names and addresses to
893   establish a naming authority (whatever entity has the ability to place
894   an HTTP server at that Internet name or address) and allows that
895   authority to determine which names are valid and how they might be used.
898   When an "http" URI is used within a context that calls for access to the
899   indicated resource, a client &MAY; attempt access by resolving
900   the host to an IP address, establishing a TCP connection to that address
901   on the indicated port, and sending an HTTP request message
902   (<xref target="http.message"/>) containing the URI's identifying data
903   (<xref target="message.routing"/>) to the server.
904   If the server responds to that request with a non-interim HTTP response
905   message, as described in &status-codes;, then that response
906   is considered an authoritative answer to the client's request.
909   Although HTTP is independent of the transport protocol, the "http"
910   scheme is specific to TCP-based services because the name delegation
911   process depends on TCP for establishing authority.
912   An HTTP service based on some other underlying connection protocol
913   would presumably be identified using a different URI scheme, just as
914   the "https" scheme (below) is used for resources that require an
915   end-to-end secured connection. Other protocols might also be used to
916   provide access to "http" identified resources &mdash; it is only the
917   authoritative interface that is specific to TCP.
920   The URI generic syntax for authority also includes a deprecated
921   userinfo subcomponent (<xref target="RFC3986" x:fmt="," x:sec="3.2.1"/>)
922   for including user authentication information in the URI.  Some
923   implementations make use of the userinfo component for internal
924   configuration of authentication information, such as within command
925   invocation options, configuration files, or bookmark lists, even
926   though such usage might expose a user identifier or password.
927   A sender &MUST-NOT; generate the userinfo subcomponent (and its "@"
928   delimiter) when an "http" URI reference is generated within a message as a
929   request target or header field value.
930   Before making use of an "http" URI reference received from an untrusted
931   source, a recipient &SHOULD; parse for userinfo and treat its presence as
932   an error; it is likely being used to obscure the authority for the sake of
933   phishing attacks.
937<section title="https URI scheme" anchor="https.uri">
938   <x:anchor-alias value="https-URI"/>
939   <iref item="https URI scheme"/>
940   <iref item="URI scheme" subitem="https"/>
942   The "https" URI scheme is hereby defined for the purpose of minting
943   identifiers according to their association with the hierarchical
944   namespace governed by a potential HTTP origin server listening to a
945   given TCP port for TLS-secured connections
946   (<xref target="RFC0793"/>, <xref target="RFC5246"/>).
949   All of the requirements listed above for the "http" scheme are also
950   requirements for the "https" scheme, except that a default TCP port
951   of 443 is assumed if the port subcomponent is empty or not given,
952   and the user agent &MUST; ensure that its connection to the origin
953   server is secured through the use of strong encryption, end-to-end,
954   prior to sending the first HTTP request.
956<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="https-URI"><!--terminal production--></iref>
957  <x:ref>https-URI</x:ref> = "https:" "//" <x:ref>authority</x:ref> <x:ref>path-abempty</x:ref> [ "?" <x:ref>query</x:ref> ]
958              [ "#" <x:ref>fragment</x:ref> ]
961   Note that the "https" URI scheme depends on both TLS and TCP for
962   establishing authority.
963   Resources made available via the "https" scheme have no shared
964   identity with the "http" scheme even if their resource identifiers
965   indicate the same authority (the same host listening to the same
966   TCP port).  They are distinct name spaces and are considered to be
967   distinct origin servers.  However, an extension to HTTP that is
968   defined to apply to entire host domains, such as the Cookie protocol
969   <xref target="RFC6265"/>, can allow information
970   set by one service to impact communication with other services
971   within a matching group of host domains.
974   The process for authoritative access to an "https" identified
975   resource is defined in <xref target="RFC2818"/>.
979<section title="http and https URI Normalization and Comparison" anchor="uri.comparison">
981   Since the "http" and "https" schemes conform to the URI generic syntax,
982   such URIs are normalized and compared according to the algorithm defined
983   in <xref target="RFC3986" x:fmt="of" x:sec="6"/>, using the defaults
984   described above for each scheme.
987   If the port is equal to the default port for a scheme, the normal form is
988   to omit the port subcomponent. When not being used in absolute form as the
989   request target of an OPTIONS request, an empty path component is equivalent
990   to an absolute path of "/", so the normal form is to provide a path of "/"
991   instead. The scheme and host are case-insensitive and normally provided in
992   lowercase; all other components are compared in a case-sensitive manner.
993   Characters other than those in the "reserved" set are equivalent to their
994   percent-encoded octets: the normal form is to not encode them
995   (see Sections <xref target="RFC3986" x:fmt="number" x:sec="2.1"/> and
996   <xref target="RFC3986" x:fmt="number" x:sec="2.2"/> of
997   <xref target="RFC3986"/>).
1000   For example, the following three URIs are equivalent:
1002<figure><artwork type="example">
1011<section title="Message Format" anchor="http.message">
1012<x:anchor-alias value="generic-message"/>
1013<x:anchor-alias value="message.types"/>
1014<x:anchor-alias value="HTTP-message"/>
1015<x:anchor-alias value="start-line"/>
1016<iref item="header section"/>
1017<iref item="headers"/>
1018<iref item="header field"/>
1020   All HTTP/1.1 messages consist of a start-line followed by a sequence of
1021   octets in a format similar to the Internet Message Format
1022   <xref target="RFC5322"/>: zero or more header fields (collectively
1023   referred to as the "headers" or the "header section"), an empty line
1024   indicating the end of the header section, and an optional message body.
1026<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="HTTP-message"><!--terminal production--></iref>
1027  <x:ref>HTTP-message</x:ref>   = <x:ref>start-line</x:ref>
1028                   *( <x:ref>header-field</x:ref> <x:ref>CRLF</x:ref> )
1029                   <x:ref>CRLF</x:ref>
1030                   [ <x:ref>message-body</x:ref> ]
1033   The normal procedure for parsing an HTTP message is to read the
1034   start-line into a structure, read each header field into a hash
1035   table by field name until the empty line, and then use the parsed
1036   data to determine if a message body is expected.  If a message body
1037   has been indicated, then it is read as a stream until an amount
1038   of octets equal to the message body length is read or the connection
1039   is closed.
1042   A recipient &MUST; parse an HTTP message as a sequence of octets in an
1043   encoding that is a superset of US-ASCII <xref target="USASCII"/>.
1044   Parsing an HTTP message as a stream of Unicode characters, without regard
1045   for the specific encoding, creates security vulnerabilities due to the
1046   varying ways that string processing libraries handle invalid multibyte
1047   character sequences that contain the octet LF (%x0A).  String-based
1048   parsers can only be safely used within protocol elements after the element
1049   has been extracted from the message, such as within a header field-value
1050   after message parsing has delineated the individual fields.
1053   An HTTP message can be parsed as a stream for incremental processing or
1054   forwarding downstream.  However, recipients cannot rely on incremental
1055   delivery of partial messages, since some implementations will buffer or
1056   delay message forwarding for the sake of network efficiency, security
1057   checks, or payload transformations.
1060   A sender &MUST-NOT; send whitespace between the start-line and
1061   the first header field.
1062   A recipient that receives whitespace between the start-line and
1063   the first header field &MUST; either reject the message as invalid or
1064   consume each whitespace-preceded line without further processing of it
1065   (i.e., ignore the entire line, along with any subsequent lines preceded
1066   by whitespace, until a properly formed header field is received or the
1067   header section is terminated).
1070   The presence of such whitespace in a request
1071   might be an attempt to trick a server into ignoring that field or
1072   processing the line after it as a new request, either of which might
1073   result in a security vulnerability if other implementations within
1074   the request chain interpret the same message differently.
1075   Likewise, the presence of such whitespace in a response might be
1076   ignored by some clients or cause others to cease parsing.
1079<section title="Start Line" anchor="start.line">
1080  <x:anchor-alias value="Start-Line"/>
1082   An HTTP message can either be a request from client to server or a
1083   response from server to client.  Syntactically, the two types of message
1084   differ only in the start-line, which is either a request-line (for requests)
1085   or a status-line (for responses), and in the algorithm for determining
1086   the length of the message body (<xref target="message.body"/>).
1089   In theory, a client could receive requests and a server could receive
1090   responses, distinguishing them by their different start-line formats,
1091   but in practice servers are implemented to only expect a request
1092   (a response is interpreted as an unknown or invalid request method)
1093   and clients are implemented to only expect a response.
1095<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="start-line"/>
1096  <x:ref>start-line</x:ref>     = <x:ref>request-line</x:ref> / <x:ref>status-line</x:ref>
1099<section title="Request Line" anchor="request.line">
1100  <x:anchor-alias value="Request"/>
1101  <x:anchor-alias value="request-line"/>
1103   A request-line begins with a method token, followed by a single
1104   space (SP), the request-target, another single space (SP), the
1105   protocol version, and ending with CRLF.
1107<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="request-line"/>
1108  <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>
1110<iref primary="true" item="method"/>
1111<t anchor="method">
1112   The method token indicates the request method to be performed on the
1113   target resource. The request method is case-sensitive.
1115<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="method"/>
1116  <x:ref>method</x:ref>         = <x:ref>token</x:ref>
1119   The request methods defined by this specification can be found in
1120   &methods;, along with information regarding the HTTP method registry
1121   and considerations for defining new methods.
1123<iref item="request-target"/>
1125   The request-target identifies the target resource upon which to apply
1126   the request, as defined in <xref target="request-target"/>.
1129   Recipients typically parse the request-line into its component parts by
1130   splitting on whitespace (see <xref target="message.robustness"/>), since
1131   no whitespace is allowed in the three components.
1132   Unfortunately, some user agents fail to properly encode or exclude
1133   whitespace found in hypertext references, resulting in those disallowed
1134   characters being sent in a request-target.
1137   Recipients of an invalid request-line &SHOULD; respond with either a
1138   <x:ref>400 (Bad Request)</x:ref> error or a <x:ref>301 (Moved Permanently)</x:ref>
1139   redirect with the request-target properly encoded.  A recipient &SHOULD-NOT;
1140   attempt to autocorrect and then process the request without a redirect,
1141   since the invalid request-line might be deliberately crafted to bypass
1142   security filters along the request chain.
1145   HTTP does not place a pre-defined limit on the length of a request-line.
1146   A server that receives a method longer than any that it implements
1147   &SHOULD; respond with a <x:ref>501 (Not Implemented)</x:ref> status code.
1148   A server ought to be prepared to receive URIs of unbounded length, as
1149   described in <xref target="conformance"/>, and &MUST; respond with a
1150   <x:ref>414 (URI Too Long)</x:ref> status code if the received
1151   request-target is longer than the server wishes to parse (see &status-414;).
1154   Various ad-hoc limitations on request-line length are found in practice.
1155   It is &RECOMMENDED; that all HTTP senders and recipients support, at a
1156   minimum, request-line lengths of 8000 octets.
1160<section title="Status Line" anchor="status.line">
1161  <x:anchor-alias value="response"/>
1162  <x:anchor-alias value="status-line"/>
1163  <x:anchor-alias value="status-code"/>
1164  <x:anchor-alias value="reason-phrase"/>
1166   The first line of a response message is the status-line, consisting
1167   of the protocol version, a space (SP), the status code, another space,
1168   a possibly-empty textual phrase describing the status code, and
1169   ending with CRLF.
1171<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="status-line"/>
1172  <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>
1175   The status-code element is a 3-digit integer code describing the
1176   result of the server's attempt to understand and satisfy the client's
1177   corresponding request. The rest of the response message is to be
1178   interpreted in light of the semantics defined for that status code.
1179   See &status-codes; for information about the semantics of status codes,
1180   including the classes of status code (indicated by the first digit),
1181   the status codes defined by this specification, considerations for the
1182   definition of new status codes, and the IANA registry.
1184<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="status-code"/>
1185  <x:ref>status-code</x:ref>    = 3<x:ref>DIGIT</x:ref>
1188   The reason-phrase element exists for the sole purpose of providing a
1189   textual description associated with the numeric status code, mostly
1190   out of deference to earlier Internet application protocols that were more
1191   frequently used with interactive text clients. A client &SHOULD; ignore
1192   the reason-phrase content.
1194<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="reason-phrase"/>
1195  <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> )
1200<section title="Header Fields" anchor="header.fields">
1201  <x:anchor-alias value="header-field"/>
1202  <x:anchor-alias value="field-content"/>
1203  <x:anchor-alias value="field-name"/>
1204  <x:anchor-alias value="field-value"/>
1205  <x:anchor-alias value="obs-fold"/>
1207   Each header field consists of a case-insensitive field name
1208   followed by a colon (":"), optional leading whitespace, the field value,
1209   and optional trailing whitespace.
1211<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="header-field"/><iref primary="true" item="Grammar" subitem="field-name"/><iref primary="true" item="Grammar" subitem="field-value"/><iref primary="true" item="Grammar" subitem="field-content"/><iref primary="true" item="Grammar" subitem="obs-fold"/>
1212  <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>
1213  <x:ref>field-name</x:ref>     = <x:ref>token</x:ref>
1214  <x:ref>field-value</x:ref>    = *( <x:ref>field-content</x:ref> / <x:ref>obs-fold</x:ref> )
1215  <x:ref>field-content</x:ref>  = *( <x:ref>HTAB</x:ref> / <x:ref>SP</x:ref> / <x:ref>VCHAR</x:ref> / <x:ref>obs-text</x:ref> )
1216  <x:ref>obs-fold</x:ref>       = <x:ref>CRLF</x:ref> ( <x:ref>SP</x:ref> / <x:ref>HTAB</x:ref> )
1217                 ; obsolete line folding
1218                 ; see <xref target="field.parsing"/>
1221   The field-name token labels the corresponding field-value as having the
1222   semantics defined by that header field.  For example, the <x:ref>Date</x:ref>
1223   header field is defined in &header-date; as containing the origination
1224   timestamp for the message in which it appears.
1227<section title="Field Extensibility" anchor="field.extensibility">
1229   Header fields are fully extensible: there is no limit on the
1230   introduction of new field names, each presumably defining new semantics,
1231   nor on the number of header fields used in a given message.  Existing
1232   fields are defined in each part of this specification and in many other
1233   specifications outside the core standard.
1236   New header fields can be defined such that, when they are understood by a
1237   recipient, they might override or enhance the interpretation of previously
1238   defined header fields, define preconditions on request evaluation, or
1239   refine the meaning of responses.
1242   A proxy &MUST; forward unrecognized header fields unless the
1243   field-name is listed in the <x:ref>Connection</x:ref> header field
1244   (<xref target="header.connection"/>) or the proxy is specifically
1245   configured to block, or otherwise transform, such fields.
1246   Other recipients &SHOULD; ignore unrecognized header fields.
1247   These requirements allow HTTP's functionality to be enhanced without
1248   requiring prior update of deployed intermediaries.
1251   All defined header fields ought to be registered with IANA in the
1252   Message Header Field Registry, as described in &iana-header-registry;.
1256<section title="Field Order" anchor="field.order">
1258   The order in which header fields with differing field names are
1259   received is not significant. However, it is "good practice" to send
1260   header fields that contain control data first, such as <x:ref>Host</x:ref>
1261   on requests and <x:ref>Date</x:ref> on responses, so that implementations
1262   can decide when not to handle a message as early as possible.  A server
1263   &MUST; wait until the entire header section is received before interpreting
1264   a request message, since later header fields might include conditionals,
1265   authentication credentials, or deliberately misleading duplicate
1266   header fields that would impact request processing.
1269   A sender &MUST-NOT; generate multiple header fields with the same field
1270   name in a message unless either the entire field value for that
1271   header field is defined as a comma-separated list [i.e., #(values)]
1272   or the header field is a well-known exception (as noted below).
1275   A recipient &MAY; combine multiple header fields with the same field name
1276   into one "field-name: field-value" pair, without changing the semantics of
1277   the message, by appending each subsequent field value to the combined
1278   field value in order, separated by a comma. The order in which
1279   header fields with the same field name are received is therefore
1280   significant to the interpretation of the combined field value;
1281   a proxy &MUST-NOT; change the order of these field values when
1282   forwarding a message.
1285  <t>
1286   &Note; In practice, the "Set-Cookie" header field (<xref target="RFC6265"/>)
1287   often appears multiple times in a response message and does not use the
1288   list syntax, violating the above requirements on multiple header fields
1289   with the same name. Since it cannot be combined into a single field-value,
1290   recipients ought to handle "Set-Cookie" as a special case while processing
1291   header fields. (See Appendix A.2.3 of <xref target="Kri2001"/> for details.)
1292  </t>
1296<section title="Whitespace" anchor="whitespace">
1297<t anchor="rule.LWS">
1298   This specification uses three rules to denote the use of linear
1299   whitespace: OWS (optional whitespace), RWS (required whitespace), and
1300   BWS ("bad" whitespace).
1302<t anchor="rule.OWS">
1303   The OWS rule is used where zero or more linear whitespace octets might
1304   appear. For protocol elements where optional whitespace is preferred to
1305   improve readability, a sender &SHOULD; generate the optional whitespace
1306   as a single SP; otherwise, a sender &SHOULD-NOT; generate optional
1307   whitespace except as needed to white-out invalid or unwanted protocol
1308   elements during in-place message filtering.
1310<t anchor="rule.RWS">
1311   The RWS rule is used when at least one linear whitespace octet is required
1312   to separate field tokens. A sender &SHOULD; generate RWS as a single SP.
1314<t anchor="rule.BWS">
1315   The BWS rule is used where the grammar allows optional whitespace only for
1316   historical reasons. A sender &MUST-NOT; generate BWS in messages.
1317   A recipient &MUST; parse for such bad whitespace and remove it before
1318   interpreting the protocol element.
1320<t anchor="rule.whitespace">
1321  <x:anchor-alias value="BWS"/>
1322  <x:anchor-alias value="OWS"/>
1323  <x:anchor-alias value="RWS"/>
1325<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"/>
1326  <x:ref>OWS</x:ref>            = *( <x:ref>SP</x:ref> / <x:ref>HTAB</x:ref> )
1327                 ; optional whitespace
1328  <x:ref>RWS</x:ref>            = 1*( <x:ref>SP</x:ref> / <x:ref>HTAB</x:ref> )
1329                 ; required whitespace
1330  <x:ref>BWS</x:ref>            = <x:ref>OWS</x:ref>
1331                 ; "bad" whitespace
1335<section title="Field Parsing" anchor="field.parsing">
1337   Messages are parsed using a generic algorithm, independent of the
1338   individual header field names. The contents within a given field value are
1339   not parsed until a later stage of message interpretation (usually after the
1340   message's entire header section has been processed).
1341   Consequently, this specification does not use ABNF rules to define each
1342   "Field-Name: Field Value" pair, as was done in previous editions.
1343   Instead, this specification uses ABNF rules which are named according to
1344   each registered field name, wherein the rule defines the valid grammar for
1345   that field's corresponding field values (i.e., after the field-value
1346   has been extracted from the header section by a generic field parser).
1349   No whitespace is allowed between the header field-name and colon.
1350   In the past, differences in the handling of such whitespace have led to
1351   security vulnerabilities in request routing and response handling.
1352   A server &MUST; reject any received request message that contains
1353   whitespace between a header field-name and colon with a response code of
1354   <x:ref>400 (Bad Request)</x:ref>. A proxy &MUST; remove any such whitespace
1355   from a response message before forwarding the message downstream.
1358   A field value is preceded by optional whitespace (OWS); a single SP is
1359   preferred. The field value does not include any leading or trailing white
1360   space: OWS occurring before the first non-whitespace octet of the field
1361   value or after the last non-whitespace octet of the field value ought to be
1362   excluded by parsers when extracting the field value from a header field.
1365   A recipient of field-content containing multiple sequential octets of
1366   optional (OWS) or required (RWS) whitespace &SHOULD; either replace the
1367   sequence with a single SP or transform any non-SP octets in the sequence to
1368   SP octets before interpreting the field value or forwarding the message
1369   downstream.
1372   Historically, HTTP header field values could be extended over multiple
1373   lines by preceding each extra line with at least one space or horizontal
1374   tab (obs-fold). This specification deprecates such line folding except
1375   within the message/http media type
1376   (<xref target=""/>).
1377   A sender &MUST-NOT; generate a message that includes line folding
1378   (i.e., that has any field-value that contains a match to the
1379   <x:ref>obs-fold</x:ref> rule) unless the message is intended for packaging
1380   within the message/http media type.
1383   A server that receives an <x:ref>obs-fold</x:ref> in a request message that
1384   is not within a message/http container &MUST; either reject the message by
1385   sending a <x:ref>400 (Bad Request)</x:ref>, preferably with a
1386   representation explaining that obsolete line folding is unacceptable, or
1387   replace each received <x:ref>obs-fold</x:ref> with one or more
1388   <x:ref>SP</x:ref> octets prior to interpreting the field value or
1389   forwarding the message downstream.
1392   A proxy or gateway that receives an <x:ref>obs-fold</x:ref> in a response
1393   message that is not within a message/http container &MUST; either discard
1394   the message and replace it with a <x:ref>502 (Bad Gateway)</x:ref>
1395   response, preferably with a representation explaining that unacceptable
1396   line folding was received, or replace each received <x:ref>obs-fold</x:ref>
1397   with one or more <x:ref>SP</x:ref> octets prior to interpreting the field
1398   value or forwarding the message downstream.
1401   A user agent that receives an <x:ref>obs-fold</x:ref> in a response message
1402   that is not within a message/http container &MUST; replace each received
1403   <x:ref>obs-fold</x:ref> with one or more <x:ref>SP</x:ref> octets prior to
1404   interpreting the field value.
1407   Historically, HTTP has allowed field content with text in the ISO-8859-1
1408   <xref target="ISO-8859-1"/> charset, supporting other charsets only
1409   through use of <xref target="RFC2047"/> encoding.
1410   In practice, most HTTP header field values use only a subset of the
1411   US-ASCII charset <xref target="USASCII"/>. Newly defined
1412   header fields &SHOULD; limit their field values to US-ASCII octets.
1413   A recipient &SHOULD; treat other octets in field content (obs-text) as
1414   opaque data.
1418<section title="Field Limits" anchor="field.limits">
1420   HTTP does not place a pre-defined limit on the length of each header field
1421   or on the length of the header section as a whole, as described in
1422   <xref target="conformance"/>. Various ad-hoc limitations on individual
1423   header field length are found in practice, often depending on the specific
1424   field semantics.
1427   A server ought to be prepared to receive request header fields of unbounded
1428   length and &MUST; respond with an appropriate
1429   <x:ref>4xx (Client Error)</x:ref> status code if the received header
1430   field(s) are larger than the server wishes to process.
1433   A client ought to be prepared to receive response header fields of
1434   unbounded length.
1435   A client &MAY; discard or truncate received header fields that are larger
1436   than the client wishes to process if the field semantics are such that the
1437   dropped value(s) can be safely ignored without changing the
1438   message framing or response semantics.
1442<section title="Field value components" anchor="field.components">
1443<t anchor="rule.token.separators">
1444  <x:anchor-alias value="tchar"/>
1445  <x:anchor-alias value="token"/>
1446  <iref item="Delimiters"/>
1447   Most HTTP header field values are defined using common syntax components
1448   (token, quoted-string, and comment) separated by whitespace or specific
1449   delimiting characters. Delimiters are chosen from the set of US-ASCII
1450   visual characters not allowed in a <x:ref>token</x:ref>
1451   (DQUOTE and "(),/:;&lt;=>?@[\]{}").
1453<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="token"/><iref primary="true" item="Grammar" subitem="tchar"/>
1454  <x:ref>token</x:ref>          = 1*<x:ref>tchar</x:ref>
1456  NOTE: the definition of tchar and the prose above about special characters need to match!
1457 -->
1458  <x:ref>tchar</x:ref>          = "!" / "#" / "$" / "%" / "&amp;" / "'" / "*"
1459                 / "+" / "-" / "." / "^" / "_" / "`" / "|" / "~"
1460                 / <x:ref>DIGIT</x:ref> / <x:ref>ALPHA</x:ref>
1461                 ; any <x:ref>VCHAR</x:ref>, except delimiters
1463<t anchor="rule.quoted-string">
1464  <x:anchor-alias value="quoted-string"/>
1465  <x:anchor-alias value="qdtext"/>
1466  <x:anchor-alias value="obs-text"/>
1467   A string of text is parsed as a single value if it is quoted using
1468   double-quote marks.
1470<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"/>
1471  <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>
1472  <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>
1473  <x:ref>obs-text</x:ref>       = %x80-FF
1475<t anchor="rule.comment">
1476  <x:anchor-alias value="comment"/>
1477  <x:anchor-alias value="ctext"/>
1478   Comments can be included in some HTTP header fields by surrounding
1479   the comment text with parentheses. Comments are only allowed in
1480   fields containing "comment" as part of their field value definition.
1482<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="comment"/><iref primary="true" item="Grammar" subitem="ctext"/>
1483  <x:ref>comment</x:ref>        = "(" *( <x:ref>ctext</x:ref> / <x:ref>quoted-pair</x:ref> / <x:ref>comment</x:ref> ) ")"
1484  <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>
1486<t anchor="rule.quoted-pair">
1487  <x:anchor-alias value="quoted-pair"/>
1488   The backslash octet ("\") can be used as a single-octet
1489   quoting mechanism within quoted-string and comment constructs.
1490   Recipients that process the value of a quoted-string &MUST; handle a
1491   quoted-pair as if it were replaced by the octet following the backslash.
1493<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="quoted-pair"/>
1494  <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> )
1497   A sender &SHOULD-NOT; generate a quoted-pair in a quoted-string except
1498   where necessary to quote DQUOTE and backslash octets occurring within that
1499   string.
1500   A sender &SHOULD-NOT; generate a quoted-pair in a comment except
1501   where necessary to quote parentheses ["(" and ")"] and backslash octets
1502   occurring within that comment.
1508<section title="Message Body" anchor="message.body">
1509  <x:anchor-alias value="message-body"/>
1511   The message body (if any) of an HTTP message is used to carry the
1512   payload body of that request or response.  The message body is
1513   identical to the payload body unless a transfer coding has been
1514   applied, as described in <xref target="header.transfer-encoding"/>.
1516<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="message-body"/>
1517  <x:ref>message-body</x:ref> = *OCTET
1520   The rules for when a message body is allowed in a message differ for
1521   requests and responses.
1524   The presence of a message body in a request is signaled by a
1525   <x:ref>Content-Length</x:ref> or <x:ref>Transfer-Encoding</x:ref> header
1526   field. Request message framing is independent of method semantics,
1527   even if the method does not define any use for a message body.
1530   The presence of a message body in a response depends on both
1531   the request method to which it is responding and the response
1532   status code (<xref target="status.line"/>).
1533   Responses to the HEAD request method (&HEAD;) never include a message body
1534   because the associated response header fields (e.g.,
1535   <x:ref>Transfer-Encoding</x:ref>, <x:ref>Content-Length</x:ref>, etc.),
1536   if present, indicate only what their values would have been if the request
1537   method had been GET (&GET;).
1538   <x:ref>2xx (Successful)</x:ref> responses to a CONNECT request method
1539   (&CONNECT;) switch to tunnel mode instead of having a message body.
1540   All <x:ref>1xx (Informational)</x:ref>, <x:ref>204 (No Content)</x:ref>, and
1541   <x:ref>304 (Not Modified)</x:ref> responses do not include a message body.
1542   All other responses do include a message body, although the body
1543   might be of zero length.
1546<section title="Transfer-Encoding" anchor="header.transfer-encoding">
1547  <iref primary="true" item="Transfer-Encoding header field" x:for-anchor=""/>
1548  <iref item="chunked (Coding Format)"/>
1549  <x:anchor-alias value="Transfer-Encoding"/>
1551   The Transfer-Encoding header field lists the transfer coding names
1552   corresponding to the sequence of transfer codings that have been
1553   (or will be) applied to the payload body in order to form the message body.
1554   Transfer codings are defined in <xref target="transfer.codings"/>.
1556<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Transfer-Encoding"/>
1557  <x:ref>Transfer-Encoding</x:ref> = 1#<x:ref>transfer-coding</x:ref>
1560   Transfer-Encoding is analogous to the Content-Transfer-Encoding field of
1561   MIME, which was designed to enable safe transport of binary data over a
1562   7-bit transport service (<xref target="RFC2045" x:fmt="," x:sec="6"/>).
1563   However, safe transport has a different focus for an 8bit-clean transfer
1564   protocol. In HTTP's case, Transfer-Encoding is primarily intended to
1565   accurately delimit a dynamically generated payload and to distinguish
1566   payload encodings that are only applied for transport efficiency or
1567   security from those that are characteristics of the selected resource.
1570   A recipient &MUST; be able to parse the chunked transfer coding
1571   (<xref target="chunked.encoding"/>) because it plays a crucial role in
1572   framing messages when the payload body size is not known in advance.
1573   A sender &MUST-NOT; apply chunked more than once to a message body
1574   (i.e., chunking an already chunked message is not allowed).
1575   If any transfer coding other than chunked is applied to a request payload
1576   body, the sender &MUST; apply chunked as the final transfer coding to
1577   ensure that the message is properly framed.
1578   If any transfer coding other than chunked is applied to a response payload
1579   body, the sender &MUST; either apply chunked as the final transfer coding
1580   or terminate the message by closing the connection.
1583   For example,
1584</preamble><artwork type="example">
1585  Transfer-Encoding: gzip, chunked
1587   indicates that the payload body has been compressed using the gzip
1588   coding and then chunked using the chunked coding while forming the
1589   message body.
1592   Unlike <x:ref>Content-Encoding</x:ref> (&content-codings;),
1593   Transfer-Encoding is a property of the message, not of the representation, and
1594   any recipient along the request/response chain &MAY; decode the received
1595   transfer coding(s) or apply additional transfer coding(s) to the message
1596   body, assuming that corresponding changes are made to the Transfer-Encoding
1597   field-value. Additional information about the encoding parameters &MAY; be
1598   provided by other header fields not defined by this specification.
1601   Transfer-Encoding &MAY; be sent in a response to a HEAD request or in a
1602   <x:ref>304 (Not Modified)</x:ref> response (&status-304;) to a GET request,
1603   neither of which includes a message body,
1604   to indicate that the origin server would have applied a transfer coding
1605   to the message body if the request had been an unconditional GET.
1606   This indication is not required, however, because any recipient on
1607   the response chain (including the origin server) can remove transfer
1608   codings when they are not needed.
1611   A server &MUST-NOT; send a Transfer-Encoding header field in any response
1612   with a status code of
1613   <x:ref>1xx (Informational)</x:ref> or <x:ref>204 (No Content)</x:ref>.
1614   A server &MUST-NOT; send a Transfer-Encoding header field in any
1615   <x:ref>2xx (Successful)</x:ref> response to a CONNECT request (&CONNECT;).
1618   Transfer-Encoding was added in HTTP/1.1.  It is generally assumed that
1619   implementations advertising only HTTP/1.0 support will not understand
1620   how to process a transfer-encoded payload.
1621   A client &MUST-NOT; send a request containing Transfer-Encoding unless it
1622   knows the server will handle HTTP/1.1 (or later) requests; such knowledge
1623   might be in the form of specific user configuration or by remembering the
1624   version of a prior received response.
1625   A server &MUST-NOT; send a response containing Transfer-Encoding unless
1626   the corresponding request indicates HTTP/1.1 (or later).
1629   A server that receives a request message with a transfer coding it does
1630   not understand &SHOULD; respond with <x:ref>501 (Not Implemented)</x:ref>.
1634<section title="Content-Length" anchor="header.content-length">
1635  <iref primary="true" item="Content-Length header field" x:for-anchor=""/>
1636  <x:anchor-alias value="Content-Length"/>
1638   When a message does not have a <x:ref>Transfer-Encoding</x:ref> header
1639   field, a Content-Length header field can provide the anticipated size,
1640   as a decimal number of octets, for a potential payload body.
1641   For messages that do include a payload body, the Content-Length field-value
1642   provides the framing information necessary for determining where the body
1643   (and message) ends.  For messages that do not include a payload body, the
1644   Content-Length indicates the size of the selected representation
1645   (&representation;).
1647<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Content-Length"/>
1648  <x:ref>Content-Length</x:ref> = 1*<x:ref>DIGIT</x:ref>
1651   An example is
1653<figure><artwork type="example">
1654  Content-Length: 3495
1657   A sender &MUST-NOT; send a Content-Length header field in any message that
1658   contains a <x:ref>Transfer-Encoding</x:ref> header field.
1661   A user agent &SHOULD; send a Content-Length in a request message when no
1662   <x:ref>Transfer-Encoding</x:ref> is sent and the request method defines
1663   a meaning for an enclosed payload body. For example, a Content-Length
1664   header field is normally sent in a POST request even when the value is
1665   0 (indicating an empty payload body).  A user agent &SHOULD-NOT; send a
1666   Content-Length header field when the request message does not contain a
1667   payload body and the method semantics do not anticipate such a body.
1670   A server &MAY; send a Content-Length header field in a response to a HEAD
1671   request (&HEAD;); a server &MUST-NOT; send Content-Length in such a
1672   response unless its field-value equals the decimal number of octets that
1673   would have been sent in the payload body of a response if the same
1674   request had used the GET method.
1677   A server &MAY; send a Content-Length header field in a
1678   <x:ref>304 (Not Modified)</x:ref> response to a conditional GET request
1679   (&status-304;); a server &MUST-NOT; send Content-Length in such a
1680   response unless its field-value equals the decimal number of octets that
1681   would have been sent in the payload body of a <x:ref>200 (OK)</x:ref>
1682   response to the same request.
1685   A server &MUST-NOT; send a Content-Length header field in any response
1686   with a status code of
1687   <x:ref>1xx (Informational)</x:ref> or <x:ref>204 (No Content)</x:ref>.
1688   A server &MUST-NOT; send a Content-Length header field in any
1689   <x:ref>2xx (Successful)</x:ref> response to a CONNECT request (&CONNECT;).
1692   Aside from the cases defined above, in the absence of Transfer-Encoding,
1693   an origin server &SHOULD; send a Content-Length header field when the
1694   payload body size is known prior to sending the complete header section.
1695   This will allow downstream recipients to measure transfer progress,
1696   know when a received message is complete, and potentially reuse the
1697   connection for additional requests.
1700   Any Content-Length field value greater than or equal to zero is valid.
1701   Since there is no predefined limit to the length of a payload, a
1702   recipient &MUST; anticipate potentially large decimal numerals and
1703   prevent parsing errors due to integer conversion overflows
1704   (<xref target="attack.protocol.element.size.overflows"/>).
1707   If a message is received that has multiple Content-Length header fields
1708   with field-values consisting of the same decimal value, or a single
1709   Content-Length header field with a field value containing a list of
1710   identical decimal values (e.g., "Content-Length: 42, 42"), indicating that
1711   duplicate Content-Length header fields have been generated or combined by an
1712   upstream message processor, then the recipient &MUST; either reject the
1713   message as invalid or replace the duplicated field-values with a single
1714   valid Content-Length field containing that decimal value prior to
1715   determining the message body length or forwarding the message.
1718  <t>
1719   &Note; HTTP's use of Content-Length for message framing differs
1720   significantly from the same field's use in MIME, where it is an optional
1721   field used only within the "message/external-body" media-type.
1722  </t>
1726<section title="Message Body Length" anchor="message.body.length">
1727  <iref item="chunked (Coding Format)"/>
1729   The length of a message body is determined by one of the following
1730   (in order of precedence):
1733  <list style="numbers">
1734    <x:lt><t>
1735     Any response to a HEAD request and any response with a
1736     <x:ref>1xx (Informational)</x:ref>, <x:ref>204 (No Content)</x:ref>, or
1737     <x:ref>304 (Not Modified)</x:ref> status code is always
1738     terminated by the first empty line after the header fields, regardless of
1739     the header fields present in the message, and thus cannot contain a
1740     message body.
1741    </t></x:lt>
1742    <x:lt><t>
1743     Any <x:ref>2xx (Successful)</x:ref> response to a CONNECT request implies that the
1744     connection will become a tunnel immediately after the empty line that
1745     concludes the header fields.  A client &MUST; ignore any
1746     <x:ref>Content-Length</x:ref> or <x:ref>Transfer-Encoding</x:ref> header
1747     fields received in such a message.
1748    </t></x:lt>
1749    <x:lt><t>
1750     If a <x:ref>Transfer-Encoding</x:ref> header field is present
1751     and the chunked transfer coding (<xref target="chunked.encoding"/>)
1752     is the final encoding, the message body length is determined by reading
1753     and decoding the chunked data until the transfer coding indicates the
1754     data is complete.
1755    </t>
1756    <t>
1757     If a <x:ref>Transfer-Encoding</x:ref> header field is present in a
1758     response and the chunked transfer coding is not the final encoding, the
1759     message body length is determined by reading the connection until it is
1760     closed by the server.
1761     If a <x:ref>Transfer-Encoding</x:ref> header field is present in a request and the
1762     chunked transfer coding is not the final encoding, the message body
1763     length cannot be determined reliably; the server &MUST; respond with
1764     the <x:ref>400 (Bad Request)</x:ref> status code and then close the connection.
1765    </t>
1766    <t>
1767     If a message is received with both a <x:ref>Transfer-Encoding</x:ref>
1768     and a <x:ref>Content-Length</x:ref> header field, the Transfer-Encoding
1769     overrides the Content-Length. Such a message might indicate an attempt
1770     to perform request or response smuggling (bypass of security-related
1771     checks on message routing or content) and thus ought to be handled as
1772     an error.  A sender &MUST; remove the received Content-Length field
1773     prior to forwarding such a message downstream.
1774    </t></x:lt>
1775    <x:lt><t>
1776     If a message is received without <x:ref>Transfer-Encoding</x:ref> and with
1777     either multiple <x:ref>Content-Length</x:ref> header fields having
1778     differing field-values or a single Content-Length header field having an
1779     invalid value, then the message framing is invalid and
1780     the recipient &MUST; treat it as an unrecoverable error to prevent
1781     request or response smuggling.
1782     If this is a request message, the server &MUST; respond with
1783     a <x:ref>400 (Bad Request)</x:ref> status code and then close the connection.
1784     If this is a response message received by a proxy,
1785     the proxy &MUST; close the connection to the server, discard the received
1786     response, and send a <x:ref>502 (Bad Gateway)</x:ref> response to the
1787     client.
1788     If this is a response message received by a user agent,
1789     the user agent &MUST; close the connection to the server and discard the
1790     received response.
1791    </t></x:lt>
1792    <x:lt><t>
1793     If a valid <x:ref>Content-Length</x:ref> header field is present without
1794     <x:ref>Transfer-Encoding</x:ref>, its decimal value defines the
1795     expected message body length in octets.
1796     If the sender closes the connection or the recipient times out before the
1797     indicated number of octets are received, the recipient &MUST; consider
1798     the message to be incomplete and close the connection.
1799    </t></x:lt>
1800    <x:lt><t>
1801     If this is a request message and none of the above are true, then the
1802     message body length is zero (no message body is present).
1803    </t></x:lt>
1804    <x:lt><t>
1805     Otherwise, this is a response message without a declared message body
1806     length, so the message body length is determined by the number of octets
1807     received prior to the server closing the connection.
1808    </t></x:lt>
1809  </list>
1812   Since there is no way to distinguish a successfully completed,
1813   close-delimited message from a partially-received message interrupted
1814   by network failure, a server &SHOULD; generate encoding or
1815   length-delimited messages whenever possible.  The close-delimiting
1816   feature exists primarily for backwards compatibility with HTTP/1.0.
1819   A server &MAY; reject a request that contains a message body but
1820   not a <x:ref>Content-Length</x:ref> by responding with
1821   <x:ref>411 (Length Required)</x:ref>.
1824   Unless a transfer coding other than chunked has been applied,
1825   a client that sends a request containing a message body &SHOULD;
1826   use a valid <x:ref>Content-Length</x:ref> header field if the message body
1827   length is known in advance, rather than the chunked transfer coding, since some
1828   existing services respond to chunked with a <x:ref>411 (Length Required)</x:ref>
1829   status code even though they understand the chunked transfer coding.  This
1830   is typically because such services are implemented via a gateway that
1831   requires a content-length in advance of being called and the server
1832   is unable or unwilling to buffer the entire request before processing.
1835   A user agent that sends a request containing a message body &MUST; send a
1836   valid <x:ref>Content-Length</x:ref> header field if it does not know the
1837   server will handle HTTP/1.1 (or later) requests; such knowledge can be in
1838   the form of specific user configuration or by remembering the version of a
1839   prior received response.
1842   If the final response to the last request on a connection has been
1843   completely received and there remains additional data to read, a user agent
1844   &MAY; discard the remaining data or attempt to determine if that data
1845   belongs as part of the prior response body, which might be the case if the
1846   prior message's Content-Length value is incorrect. A client &MUST-NOT;
1847   process, cache, or forward such extra data as a separate response, since
1848   such behavior would be vulnerable to cache poisoning.
1853<section anchor="incomplete.messages" title="Handling Incomplete Messages">
1855   A server that receives an incomplete request message, usually due to a
1856   canceled request or a triggered time-out exception, &MAY; send an error
1857   response prior to closing the connection.
1860   A client that receives an incomplete response message, which can occur
1861   when a connection is closed prematurely or when decoding a supposedly
1862   chunked transfer coding fails, &MUST; record the message as incomplete.
1863   Cache requirements for incomplete responses are defined in
1864   &cache-incomplete;.
1867   If a response terminates in the middle of the header section (before the
1868   empty line is received) and the status code might rely on header fields to
1869   convey the full meaning of the response, then the client cannot assume
1870   that meaning has been conveyed; the client might need to repeat the
1871   request in order to determine what action to take next.
1874   A message body that uses the chunked transfer coding is
1875   incomplete if the zero-sized chunk that terminates the encoding has not
1876   been received.  A message that uses a valid <x:ref>Content-Length</x:ref> is
1877   incomplete if the size of the message body received (in octets) is less than
1878   the value given by Content-Length.  A response that has neither chunked
1879   transfer coding nor Content-Length is terminated by closure of the
1880   connection, and thus is considered complete regardless of the number of
1881   message body octets received, provided that the header section was received
1882   intact.
1886<section title="Message Parsing Robustness" anchor="message.robustness">
1888   Older HTTP/1.0 user agent implementations might send an extra CRLF
1889   after a POST request as a workaround for some early server
1890   applications that failed to read message body content that was
1891   not terminated by a line-ending. An HTTP/1.1 user agent &MUST-NOT;
1892   preface or follow a request with an extra CRLF.  If terminating
1893   the request message body with a line-ending is desired, then the
1894   user agent &MUST; count the terminating CRLF octets as part of the
1895   message body length.
1898   In the interest of robustness, a server that is expecting to receive and
1899   parse a request-line &SHOULD; ignore at least one empty line (CRLF)
1900   received prior to the request-line.
1903   Although the line terminator for the start-line and header
1904   fields is the sequence CRLF, a recipient &MAY; recognize a
1905   single LF as a line terminator and ignore any preceding CR.
1908   Although the request-line and status-line grammar rules require that each
1909   of the component elements be separated by a single SP octet, recipients
1910   &MAY; instead parse on whitespace-delimited word boundaries and, aside
1911   from the CRLF terminator, treat any form of whitespace as the SP separator
1912   while ignoring preceding or trailing whitespace;
1913   such whitespace includes one or more of the following octets:
1914   SP, HTAB, VT (%x0B), FF (%x0C), or bare CR.
1917   When a server listening only for HTTP request messages, or processing
1918   what appears from the start-line to be an HTTP request message,
1919   receives a sequence of octets that does not match the HTTP-message
1920   grammar aside from the robustness exceptions listed above, the
1921   server &SHOULD; respond with a <x:ref>400 (Bad Request)</x:ref> response. 
1926<section title="Transfer Codings" anchor="transfer.codings">
1927  <x:anchor-alias value="transfer-coding"/>
1928  <x:anchor-alias value="transfer-extension"/>
1930   Transfer coding names are used to indicate an encoding
1931   transformation that has been, can be, or might need to be applied to a
1932   payload body in order to ensure "safe transport" through the network.
1933   This differs from a content coding in that the transfer coding is a
1934   property of the message rather than a property of the representation
1935   that is being transferred.
1937<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="transfer-coding"/><iref primary="true" item="Grammar" subitem="transfer-extension"/>
1938  <x:ref>transfer-coding</x:ref>    = "chunked" ; <xref target="chunked.encoding"/>
1939                     / "compress" ; <xref target="compress.coding"/>
1940                     / "deflate" ; <xref target="deflate.coding"/>
1941                     / "gzip" ; <xref target="gzip.coding"/>
1942                     / <x:ref>transfer-extension</x:ref>
1943  <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> )
1945<t anchor="rule.parameter">
1946  <x:anchor-alias value="transfer-parameter"/>
1947   Parameters are in the form of a name or name=value pair.
1949<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="transfer-parameter"/>
1950  <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> )
1953   All transfer-coding names are case-insensitive and ought to be registered
1954   within the HTTP Transfer Coding registry, as defined in
1955   <xref target="transfer.coding.registry"/>.
1956   They are used in the <x:ref>TE</x:ref> (<xref target="header.te"/>) and
1957   <x:ref>Transfer-Encoding</x:ref> (<xref target="header.transfer-encoding"/>)
1958   header fields.
1961<section title="Chunked Transfer Coding" anchor="chunked.encoding">
1962  <iref primary="true" item="chunked (Coding Format)"/>
1963  <x:anchor-alias value="chunk"/>
1964  <x:anchor-alias value="chunked-body"/>
1965  <x:anchor-alias value="chunk-data"/>
1966  <x:anchor-alias value="chunk-size"/>
1967  <x:anchor-alias value="last-chunk"/>
1969   The chunked transfer coding wraps the payload body in order to transfer it
1970   as a series of chunks, each with its own size indicator, followed by an
1971   &OPTIONAL; trailer containing header fields. Chunked enables content
1972   streams of unknown size to be transferred as a sequence of length-delimited
1973   buffers, which enables the sender to retain connection persistence and the
1974   recipient to know when it has received the entire message.
1976<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"/>
1977  <x:ref>chunked-body</x:ref>   = *<x:ref>chunk</x:ref>
1978                   <x:ref>last-chunk</x:ref>
1979                   <x:ref>trailer-part</x:ref>
1980                   <x:ref>CRLF</x:ref>
1982  <x:ref>chunk</x:ref>          = <x:ref>chunk-size</x:ref> [ <x:ref>chunk-ext</x:ref> ] <x:ref>CRLF</x:ref>
1983                   <x:ref>chunk-data</x:ref> <x:ref>CRLF</x:ref>
1984  <x:ref>chunk-size</x:ref>     = 1*<x:ref>HEXDIG</x:ref>
1985  <x:ref>last-chunk</x:ref>     = 1*("0") [ <x:ref>chunk-ext</x:ref> ] <x:ref>CRLF</x:ref>
1987  <x:ref>chunk-data</x:ref>     = 1*<x:ref>OCTET</x:ref> ; a sequence of chunk-size octets
1990   The chunk-size field is a string of hex digits indicating the size of
1991   the chunk-data in octets. The chunked transfer coding is complete when a
1992   chunk with a chunk-size of zero is received, possibly followed by a
1993   trailer, and finally terminated by an empty line.
1996   A recipient &MUST; be able to parse and decode the chunked transfer coding.
1999<section title="Chunk Extensions" anchor="chunked.extension">
2000  <x:anchor-alias value="chunk-ext"/>
2001  <x:anchor-alias value="chunk-ext-name"/>
2002  <x:anchor-alias value="chunk-ext-val"/>
2004   The chunked encoding allows each chunk to include zero or more chunk
2005   extensions, immediately following the <x:ref>chunk-size</x:ref>, for the
2006   sake of supplying per-chunk metadata (such as a signature or hash),
2007   mid-message control information, or randomization of message body size.
2009<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"/>
2010  <x:ref>chunk-ext</x:ref>      = *( ";" <x:ref>chunk-ext-name</x:ref> [ "=" <x:ref>chunk-ext-val</x:ref> ] )
2012  <x:ref>chunk-ext-name</x:ref> = <x:ref>token</x:ref>
2013  <x:ref>chunk-ext-val</x:ref>  = <x:ref>token</x:ref> / <x:ref>quoted-string</x:ref>
2016   The chunked encoding is specific to each connection and is likely to be
2017   removed or recoded by each recipient (including intermediaries) before any
2018   higher-level application would have a chance to inspect the extensions.
2019   Hence, use of chunk extensions is generally limited to specialized HTTP
2020   services such as "long polling" (where client and server can have shared
2021   expectations regarding the use of chunk extensions) or for padding within
2022   an end-to-end secured connection.
2025   A recipient &MUST; ignore unrecognized chunk extensions.
2026   A server ought to limit the total length of chunk extensions received in a
2027   request to an amount reasonable for the services provided, in the same way
2028   that it applies length limitations and timeouts for other parts of a
2029   message, and generate an appropriate <x:ref>4xx (Client Error)</x:ref>
2030   response if that amount is exceeded.
2034<section title="Chunked Trailer Part" anchor="chunked.trailer.part">
2035  <x:anchor-alias value="trailer-part"/>
2037   A trailer allows the sender to include additional fields at the end of a
2038   chunked message in order to supply metadata that might be dynamically
2039   generated while the message body is sent, such as a message integrity
2040   check, digital signature, or post-processing status. The trailer fields are
2041   identical to header fields, except they are sent in a chunked trailer
2042   instead of the message's header section.
2044<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="trailer-part"/><iref primary="false" item="Grammar" subitem="header-field"/>
2045  <x:ref>trailer-part</x:ref>   = *( <x:ref>header-field</x:ref> <x:ref>CRLF</x:ref> )
2048   A sender &MUST-NOT; generate a trailer that contains a field necessary for
2049   message framing (e.g., <x:ref>Transfer-Encoding</x:ref> and
2050   <x:ref>Content-Length</x:ref>), routing (e.g., <x:ref>Host</x:ref>),
2051   request modifiers (e.g., controls and conditionals in
2052   &request-header-fields;), authentication (e.g., see <xref target="Part7"/>
2053   and <xref target="RFC6265"/>), response control data (e.g., see
2054   &response-control-data;), or determining how to process the payload
2055   (e.g., <x:ref>Content-Encoding</x:ref>, <x:ref>Content-Type</x:ref>,
2056   <x:ref>Content-Range</x:ref>, and <x:ref>Trailer</x:ref>).
2059   When a chunked message containing a non-empty trailer is received, the
2060   recipient &MAY; process the fields (aside from those forbidden above)
2061   as if they were appended to the message's header section.
2062   A recipient &MUST; ignore (or consider as an error) any fields that are
2063   forbidden to be sent in a trailer, since processing them as if they were
2064   present in the header section might bypass external security filters.
2067   Unless the request includes a <x:ref>TE</x:ref> header field indicating
2068   "trailers" is acceptable, as described in <xref target="header.te"/>, a
2069   server &SHOULD-NOT; generate trailer fields that it believes are necessary
2070   for the user agent to receive. Without a TE containing "trailers", the
2071   server ought to assume that the trailer fields might be silently discarded
2072   along the path to the user agent. This requirement allows intermediaries to
2073   forward a de-chunked message to an HTTP/1.0 recipient without buffering the
2074   entire response.
2078<section title="Decoding Chunked" anchor="decoding.chunked">
2080   A process for decoding the chunked transfer coding
2081   can be represented in pseudo-code as:
2083<figure><artwork type="code">
2084  length := 0
2085  read chunk-size, chunk-ext (if any), and CRLF
2086  while (chunk-size &gt; 0) {
2087     read chunk-data and CRLF
2088     append chunk-data to decoded-body
2089     length := length + chunk-size
2090     read chunk-size, chunk-ext (if any), and CRLF
2091  }
2092  read trailer field
2093  while (trailer field is not empty) {
2094     if trailer field is allowed to be sent in a trailer,
2095         append trailer field to existing header fields
2096     read trailer-field
2097  }
2098  Content-Length := length
2099  Remove "chunked" from Transfer-Encoding
2100  Remove Trailer from existing header fields
2105<section title="Compression Codings" anchor="compression.codings">
2107   The codings defined below can be used to compress the payload of a
2108   message.
2111<section title="Compress Coding" anchor="compress.coding">
2112<iref item="compress (Coding Format)"/>
2114   The "compress" coding is an adaptive Lempel-Ziv-Welch (LZW) coding
2115   <xref target="Welch"/> that is commonly produced by the UNIX file
2116   compression program "compress".
2117   A recipient &SHOULD; consider "x-compress" to be equivalent to "compress".
2121<section title="Deflate Coding" anchor="deflate.coding">
2122<iref item="deflate (Coding Format)"/>
2124   The "deflate" coding is a "zlib" data format <xref target="RFC1950"/>
2125   containing a "deflate" compressed data stream <xref target="RFC1951"/>
2126   that uses a combination of the Lempel-Ziv (LZ77) compression algorithm and
2127   Huffman coding.
2130  <t>
2131    &Note; Some incorrect implementations send the "deflate"
2132    compressed data without the zlib wrapper.
2133   </t>
2137<section title="Gzip Coding" anchor="gzip.coding">
2138<iref item="gzip (Coding Format)"/>
2140   The "gzip" coding is an LZ77 coding with a 32 bit CRC that is commonly
2141   produced by the gzip file compression program <xref target="RFC1952"/>.
2142   A recipient &SHOULD; consider "x-gzip" to be equivalent to "gzip".
2148<section title="TE" anchor="header.te">
2149  <iref primary="true" item="TE header field" x:for-anchor=""/>
2150  <x:anchor-alias value="TE"/>
2151  <x:anchor-alias value="t-codings"/>
2152  <x:anchor-alias value="t-ranking"/>
2153  <x:anchor-alias value="rank"/>
2155   The "TE" header field in a request indicates what transfer codings,
2156   besides chunked, the client is willing to accept in response, and
2157   whether or not the client is willing to accept trailer fields in a
2158   chunked transfer coding.
2161   The TE field-value consists of a comma-separated list of transfer coding
2162   names, each allowing for optional parameters (as described in
2163   <xref target="transfer.codings"/>), and/or the keyword "trailers".
2164   A client &MUST-NOT; send the chunked transfer coding name in TE;
2165   chunked is always acceptable for HTTP/1.1 recipients.
2167<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"/>
2168  <x:ref>TE</x:ref>        = #<x:ref>t-codings</x:ref>
2169  <x:ref>t-codings</x:ref> = "trailers" / ( <x:ref>transfer-coding</x:ref> [ <x:ref>t-ranking</x:ref> ] )
2170  <x:ref>t-ranking</x:ref> = <x:ref>OWS</x:ref> ";" <x:ref>OWS</x:ref> "q=" <x:ref>rank</x:ref>
2171  <x:ref>rank</x:ref>      = ( "0" [ "." 0*3<x:ref>DIGIT</x:ref> ] )
2172             / ( "1" [ "." 0*3("0") ] )
2175   Three examples of TE use are below.
2177<figure><artwork type="example">
2178  TE: deflate
2179  TE:
2180  TE: trailers, deflate;q=0.5
2183   The presence of the keyword "trailers" indicates that the client is willing
2184   to accept trailer fields in a chunked transfer coding, as defined in
2185   <xref target="chunked.trailer.part"/>, on behalf of itself and any downstream
2186   clients. For requests from an intermediary, this implies that either:
2187   (a) all downstream clients are willing to accept trailer fields in the
2188   forwarded response; or,
2189   (b) the intermediary will attempt to buffer the response on behalf of
2190   downstream recipients.
2191   Note that HTTP/1.1 does not define any means to limit the size of a
2192   chunked response such that an intermediary can be assured of buffering the
2193   entire response.
2196   When multiple transfer codings are acceptable, the client &MAY; rank the
2197   codings by preference using a case-insensitive "q" parameter (similar to
2198   the qvalues used in content negotiation fields, &qvalue;). The rank value
2199   is a real number in the range 0 through 1, where 0.001 is the least
2200   preferred and 1 is the most preferred; a value of 0 means "not acceptable".
2203   If the TE field-value is empty or if no TE field is present, the only
2204   acceptable transfer coding is chunked. A message with no transfer coding
2205   is always acceptable.
2208   Since the TE header field only applies to the immediate connection,
2209   a sender of TE &MUST; also send a "TE" connection option within the
2210   <x:ref>Connection</x:ref> header field (<xref target="header.connection"/>)
2211   in order to prevent the TE field from being forwarded by intermediaries
2212   that do not support its semantics.
2216<section title="Trailer" anchor="header.trailer">
2217  <iref primary="true" item="Trailer header field" x:for-anchor=""/>
2218  <x:anchor-alias value="Trailer"/>
2220   When a message includes a message body encoded with the chunked
2221   transfer coding and the sender desires to send metadata in the form of
2222   trailer fields at the end of the message, the sender &SHOULD; generate a
2223   <x:ref>Trailer</x:ref> header field before the message body to indicate
2224   which fields will be present in the trailers. This allows the recipient
2225   to prepare for receipt of that metadata before it starts processing the body,
2226   which is useful if the message is being streamed and the recipient wishes
2227   to confirm an integrity check on the fly.
2229<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Trailer"/><iref primary="false" item="Grammar" subitem="field-name"/>
2230  <x:ref>Trailer</x:ref> = 1#<x:ref>field-name</x:ref>
2235<section title="Message Routing" anchor="message.routing">
2237   HTTP request message routing is determined by each client based on the
2238   target resource, the client's proxy configuration, and
2239   establishment or reuse of an inbound connection.  The corresponding
2240   response routing follows the same connection chain back to the client.
2243<section title="Identifying a Target Resource" anchor="target-resource">
2244  <iref primary="true" item="target resource"/>
2245  <iref primary="true" item="target URI"/>
2246  <x:anchor-alias value="target resource"/>
2247  <x:anchor-alias value="target URI"/>
2249   HTTP is used in a wide variety of applications, ranging from
2250   general-purpose computers to home appliances.  In some cases,
2251   communication options are hard-coded in a client's configuration.
2252   However, most HTTP clients rely on the same resource identification
2253   mechanism and configuration techniques as general-purpose Web browsers.
2256   HTTP communication is initiated by a user agent for some purpose.
2257   The purpose is a combination of request semantics, which are defined in
2258   <xref target="Part2"/>, and a target resource upon which to apply those
2259   semantics.  A URI reference (<xref target="uri"/>) is typically used as
2260   an identifier for the "<x:dfn>target resource</x:dfn>", which a user agent
2261   would resolve to its absolute form in order to obtain the
2262   "<x:dfn>target URI</x:dfn>".  The target URI
2263   excludes the reference's fragment component, if any,
2264   since fragment identifiers are reserved for client-side processing
2265   (<xref target="RFC3986" x:fmt="," x:sec="3.5"/>).
2269<section title="Connecting Inbound" anchor="connecting.inbound">
2271   Once the target URI is determined, a client needs to decide whether
2272   a network request is necessary to accomplish the desired semantics and,
2273   if so, where that request is to be directed.
2276   If the client has a cache <xref target="Part6"/> and the request can be
2277   satisfied by it, then the request is
2278   usually directed there first.
2281   If the request is not satisfied by a cache, then a typical client will
2282   check its configuration to determine whether a proxy is to be used to
2283   satisfy the request.  Proxy configuration is implementation-dependent,
2284   but is often based on URI prefix matching, selective authority matching,
2285   or both, and the proxy itself is usually identified by an "http" or
2286   "https" URI.  If a proxy is applicable, the client connects inbound by
2287   establishing (or reusing) a connection to that proxy.
2290   If no proxy is applicable, a typical client will invoke a handler routine,
2291   usually specific to the target URI's scheme, to connect directly
2292   to an authority for the target resource.  How that is accomplished is
2293   dependent on the target URI scheme and defined by its associated
2294   specification, similar to how this specification defines origin server
2295   access for resolution of the "http" (<xref target="http.uri"/>) and
2296   "https" (<xref target="https.uri"/>) schemes.
2299   HTTP requirements regarding connection management are defined in
2300   <xref target=""/>.
2304<section title="Request Target" anchor="request-target">
2306   Once an inbound connection is obtained,
2307   the client sends an HTTP request message (<xref target="http.message"/>)
2308   with a request-target derived from the target URI.
2309   There are four distinct formats for the request-target, depending on both
2310   the method being requested and whether the request is to a proxy.
2312<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="request-target"/><iref primary="true" item="Grammar" subitem="origin-form"/><iref primary="true" item="Grammar" subitem="absolute-form"/><iref primary="true" item="Grammar" subitem="authority-form"/><iref primary="true" item="Grammar" subitem="asterisk-form"/>
2313  <x:ref>request-target</x:ref> = <x:ref>origin-form</x:ref>
2314                 / <x:ref>absolute-form</x:ref>
2315                 / <x:ref>authority-form</x:ref>
2316                 / <x:ref>asterisk-form</x:ref>
2318  <x:ref>origin-form</x:ref>    = <x:ref>absolute-path</x:ref> [ "?" <x:ref>query</x:ref> ]
2319  <x:ref>absolute-form</x:ref>  = <x:ref>absolute-URI</x:ref>
2320  <x:ref>authority-form</x:ref> = <x:ref>authority</x:ref>
2321  <x:ref>asterisk-form</x:ref>  = "*"
2323<t anchor="origin-form"><iref item="origin-form (of request-target)"/>
2324  <x:h>origin-form</x:h>
2327   The most common form of request-target is the <x:dfn>origin-form</x:dfn>.
2328   When making a request directly to an origin server, other than a CONNECT
2329   or server-wide OPTIONS request (as detailed below),
2330   a client &MUST; send only the absolute path and query components of
2331   the target URI as the request-target.
2332   If the target URI's path component is empty, then the client &MUST; send
2333   "/" as the path within the origin-form of request-target.
2334   A <x:ref>Host</x:ref> header field is also sent, as defined in
2335   <xref target=""/>.
2338   For example, a client wishing to retrieve a representation of the resource
2339   identified as
2341<figure><artwork x:indent-with="  " type="example">
2345   directly from the origin server would open (or reuse) a TCP connection
2346   to port 80 of the host "" and send the lines:
2348<figure><artwork type="message/http; msgtype=&#34;request&#34;" x:indent-with="  ">
2349GET /where?q=now HTTP/1.1
2353   followed by the remainder of the request message.
2355<t anchor="absolute-form"><iref item="absolute-form (of request-target)"/>
2356  <x:h>absolute-form</x:h>
2359   When making a request to a proxy, other than a CONNECT or server-wide
2360   OPTIONS request (as detailed below), a client &MUST; send the target URI
2361   in <x:dfn>absolute-form</x:dfn> as the request-target.
2362   The proxy is requested to either service that request from a valid cache,
2363   if possible, or make the same request on the client's behalf to either
2364   the next inbound proxy server or directly to the origin server indicated
2365   by the request-target.  Requirements on such "forwarding" of messages are
2366   defined in <xref target="message.forwarding"/>.
2369   An example absolute-form of request-line would be:
2371<figure><artwork type="message/http; msgtype=&#34;request&#34;" x:indent-with="  ">
2372GET HTTP/1.1
2375   To allow for transition to the absolute-form for all requests in some
2376   future version of HTTP, a server &MUST; accept the absolute-form
2377   in requests, even though HTTP/1.1 clients will only send them in requests
2378   to proxies.
2380<t anchor="authority-form"><iref item="authority-form (of request-target)"/>
2381  <x:h>authority-form</x:h>
2384   The <x:dfn>authority-form</x:dfn> of request-target is only used for
2385   CONNECT requests (&CONNECT;). When making a CONNECT request to establish a
2386   tunnel through one or more proxies, a client &MUST; send only the target
2387   URI's authority component (excluding any userinfo and its "@" delimiter) as
2388   the request-target. For example,
2390<figure><artwork type="message/http; msgtype=&#34;request&#34;" x:indent-with="  ">
2393<t anchor="asterisk-form"><iref item="asterisk-form (of request-target)"/>
2394  <x:h>asterisk-form</x:h>
2397   The <x:dfn>asterisk-form</x:dfn> of request-target is only used for a server-wide
2398   OPTIONS request (&OPTIONS;).  When a client wishes to request OPTIONS
2399   for the server as a whole, as opposed to a specific named resource of
2400   that server, the client &MUST; send only "*" (%x2A) as the request-target.
2401   For example,
2403<figure><artwork type="message/http; msgtype=&#34;request&#34;" x:indent-with="  ">
2404OPTIONS * HTTP/1.1
2407   If a proxy receives an OPTIONS request with an absolute-form of
2408   request-target in which the URI has an empty path and no query component,
2409   then the last proxy on the request chain &MUST; send a request-target
2410   of "*" when it forwards the request to the indicated origin server.
2413   For example, the request
2414</preamble><artwork type="message/http; msgtype=&#34;request&#34;" x:indent-with="  ">
2418  would be forwarded by the final proxy as
2419</preamble><artwork type="message/http; msgtype=&#34;request&#34;" x:indent-with="  ">
2420OPTIONS * HTTP/1.1
2424   after connecting to port 8001 of host "".
2429<section title="Host" anchor="">
2430  <iref primary="true" item="Host header field" x:for-anchor=""/>
2431  <x:anchor-alias value="Host"/>
2433   The "Host" header field in a request provides the host and port
2434   information from the target URI, enabling the origin
2435   server to distinguish among resources while servicing requests
2436   for multiple host names on a single IP address.
2438<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Host"/>
2439  <x:ref>Host</x:ref> = <x:ref>uri-host</x:ref> [ ":" <x:ref>port</x:ref> ] ; <xref target="http.uri"/>
2442   A client &MUST; send a Host header field in all HTTP/1.1 request messages.
2443   If the target URI includes an authority component, then a client &MUST;
2444   send a field-value for Host that is identical to that authority
2445   component, excluding any userinfo subcomponent and its "@" delimiter
2446   (<xref target="http.uri"/>).
2447   If the authority component is missing or undefined for the target URI,
2448   then a client &MUST; send a Host header field with an empty field-value.
2451   Since the Host field-value is critical information for handling a request,
2452   a user agent &SHOULD; generate Host as the first header field following the
2453   request-line.
2456   For example, a GET request to the origin server for
2457   &lt;; would begin with:
2459<figure><artwork type="message/http; msgtype=&#34;request&#34;" x:indent-with="  ">
2460GET /pub/WWW/ HTTP/1.1
2464   A client &MUST; send a Host header field in an HTTP/1.1 request even
2465   if the request-target is in the absolute-form, since this
2466   allows the Host information to be forwarded through ancient HTTP/1.0
2467   proxies that might not have implemented Host.
2470   When a proxy receives a request with an absolute-form of
2471   request-target, the proxy &MUST; ignore the received
2472   Host header field (if any) and instead replace it with the host
2473   information of the request-target.  A proxy that forwards such a request
2474   &MUST; generate a new Host field-value based on the received
2475   request-target rather than forward the received Host field-value.
2478   Since the Host header field acts as an application-level routing
2479   mechanism, it is a frequent target for malware seeking to poison
2480   a shared cache or redirect a request to an unintended server.
2481   An interception proxy is particularly vulnerable if it relies on
2482   the Host field-value for redirecting requests to internal
2483   servers, or for use as a cache key in a shared cache, without
2484   first verifying that the intercepted connection is targeting a
2485   valid IP address for that host.
2488   A server &MUST; respond with a <x:ref>400 (Bad Request)</x:ref> status code
2489   to any HTTP/1.1 request message that lacks a Host header field and
2490   to any request message that contains more than one Host header field
2491   or a Host header field with an invalid field-value.
2495<section title="Effective Request URI" anchor="effective.request.uri">
2496  <iref primary="true" item="effective request URI"/>
2497  <x:anchor-alias value="effective request URI"/>
2499   A server that receives an HTTP request message &MUST; reconstruct
2500   the user agent's original target URI, based on the pieces of information
2501   learned from the request-target, <x:ref>Host</x:ref> header field, and
2502   connection context, in order to identify the intended target resource and
2503   properly service the request. The URI derived from this reconstruction
2504   process is referred to as the "<x:dfn>effective request URI</x:dfn>".
2507   For a user agent, the effective request URI is the target URI.
2510   If the request-target is in absolute-form, then the effective request URI
2511   is the same as the request-target.  Otherwise, the effective request URI
2512   is constructed as follows.
2515   If the request is received over a TLS-secured TCP connection,
2516   then the effective request URI's scheme is "https"; otherwise, the
2517   scheme is "http".
2520   If the request-target is in authority-form, then the effective
2521   request URI's authority component is the same as the request-target.
2522   Otherwise, if a <x:ref>Host</x:ref> header field is supplied with a
2523   non-empty field-value, then the authority component is the same as the
2524   Host field-value. Otherwise, the authority component is the concatenation of
2525   the default host name configured for the server, a colon (":"), and the
2526   connection's incoming TCP port number in decimal form.
2529   If the request-target is in authority-form or asterisk-form, then the
2530   effective request URI's combined path and query component is empty.
2531   Otherwise, the combined path and query component is the same as the
2532   request-target.
2535   The components of the effective request URI, once determined as above,
2536   can be combined into absolute-URI form by concatenating the scheme,
2537   "://", authority, and combined path and query component.
2541   Example 1: the following message received over an insecure TCP connection
2543<artwork type="example" x:indent-with="  ">
2544GET /pub/WWW/TheProject.html HTTP/1.1
2550  has an effective request URI of
2552<artwork type="example" x:indent-with="  ">
2558   Example 2: the following message received over a TLS-secured TCP connection
2560<artwork type="example" x:indent-with="  ">
2561OPTIONS * HTTP/1.1
2567  has an effective request URI of
2569<artwork type="example" x:indent-with="  ">
2574   An origin server that does not allow resources to differ by requested
2575   host &MAY; ignore the <x:ref>Host</x:ref> field-value and instead replace it
2576   with a configured server name when constructing the effective request URI.
2579   Recipients of an HTTP/1.0 request that lacks a <x:ref>Host</x:ref> header
2580   field &MAY; attempt to use heuristics (e.g., examination of the URI path for
2581   something unique to a particular host) in order to guess the
2582   effective request URI's authority component.
2586<section title="Associating a Response to a Request" anchor="">
2588   HTTP does not include a request identifier for associating a given
2589   request message with its corresponding one or more response messages.
2590   Hence, it relies on the order of response arrival to correspond exactly
2591   to the order in which requests are made on the same connection.
2592   More than one response message per request only occurs when one or more
2593   informational responses (<x:ref>1xx</x:ref>, see &status-1xx;) precede a
2594   final response to the same request.
2597   A client that has more than one outstanding request on a connection &MUST;
2598   maintain a list of outstanding requests in the order sent and &MUST;
2599   associate each received response message on that connection to the highest
2600   ordered request that has not yet received a final (non-<x:ref>1xx</x:ref>)
2601   response.
2605<section title="Message Forwarding" anchor="message.forwarding">
2607   As described in <xref target="intermediaries"/>, intermediaries can serve
2608   a variety of roles in the processing of HTTP requests and responses.
2609   Some intermediaries are used to improve performance or availability.
2610   Others are used for access control or to filter content.
2611   Since an HTTP stream has characteristics similar to a pipe-and-filter
2612   architecture, there are no inherent limits to the extent an intermediary
2613   can enhance (or interfere) with either direction of the stream.
2616   An intermediary not acting as a tunnel &MUST; implement the
2617   <x:ref>Connection</x:ref> header field, as specified in
2618   <xref target="header.connection"/>, and exclude fields from being forwarded
2619   that are only intended for the incoming connection.
2622   An intermediary &MUST-NOT; forward a message to itself unless it is
2623   protected from an infinite request loop. In general, an intermediary ought
2624   to recognize its own server names, including any aliases, local variations,
2625   or literal IP addresses, and respond to such requests directly.
2628<section title="Via" anchor="header.via">
2629  <iref primary="true" item="Via header field" x:for-anchor=""/>
2630  <x:anchor-alias value="pseudonym"/>
2631  <x:anchor-alias value="received-by"/>
2632  <x:anchor-alias value="received-protocol"/>
2633  <x:anchor-alias value="Via"/>
2635   The "Via" header field indicates the presence of intermediate protocols and
2636   recipients between the user agent and the server (on requests) or between
2637   the origin server and the client (on responses), similar to the
2638   "Received" header field in email
2639   (<xref target="RFC5322" x:fmt="of" x:sec="3.6.7"/>).
2640   Via can be used for tracking message forwards,
2641   avoiding request loops, and identifying the protocol capabilities of
2642   senders along the request/response chain.
2644<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"/>
2645  <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> ] )
2647  <x:ref>received-protocol</x:ref> = [ <x:ref>protocol-name</x:ref> "/" ] <x:ref>protocol-version</x:ref>
2648                      ; see <xref target="header.upgrade"/>
2649  <x:ref>received-by</x:ref>       = ( <x:ref>uri-host</x:ref> [ ":" <x:ref>port</x:ref> ] ) / <x:ref>pseudonym</x:ref>
2650  <x:ref>pseudonym</x:ref>         = <x:ref>token</x:ref>
2653   Multiple Via field values represent each proxy or gateway that has
2654   forwarded the message. Each intermediary appends its own information
2655   about how the message was received, such that the end result is ordered
2656   according to the sequence of forwarding recipients.
2659   A proxy &MUST; send an appropriate Via header field, as described below, in
2660   each message that it forwards.
2661   An HTTP-to-HTTP gateway &MUST; send an appropriate Via header field in
2662   each inbound request message and &MAY; send a Via header field in
2663   forwarded response messages.
2666   For each intermediary, the received-protocol indicates the protocol and
2667   protocol version used by the upstream sender of the message. Hence, the
2668   Via field value records the advertised protocol capabilities of the
2669   request/response chain such that they remain visible to downstream
2670   recipients; this can be useful for determining what backwards-incompatible
2671   features might be safe to use in response, or within a later request, as
2672   described in <xref target="http.version"/>. For brevity, the protocol-name
2673   is omitted when the received protocol is HTTP.
2676   The received-by portion of the field value is normally the host and optional
2677   port number of a recipient server or client that subsequently forwarded the
2678   message.
2679   However, if the real host is considered to be sensitive information, a
2680   sender &MAY; replace it with a pseudonym. If a port is not provided,
2681   a recipient &MAY; interpret that as meaning it was received on the default
2682   TCP port, if any, for the received-protocol.
2685   A sender &MAY; generate comments in the Via header field to identify the
2686   software of each recipient, analogous to the <x:ref>User-Agent</x:ref> and
2687   <x:ref>Server</x:ref> header fields. However, all comments in the Via field
2688   are optional and a recipient &MAY; remove them prior to forwarding the
2689   message.
2692   For example, a request message could be sent from an HTTP/1.0 user
2693   agent to an internal proxy code-named "fred", which uses HTTP/1.1 to
2694   forward the request to a public proxy at, which completes
2695   the request by forwarding it to the origin server at
2696   The request received by would then have the following
2697   Via header field:
2699<figure><artwork type="example">
2700  Via: 1.0 fred, 1.1
2703   An intermediary used as a portal through a network firewall
2704   &SHOULD-NOT; forward the names and ports of hosts within the firewall
2705   region unless it is explicitly enabled to do so. If not enabled, such an
2706   intermediary &SHOULD; replace each received-by host of any host behind the
2707   firewall by an appropriate pseudonym for that host.
2710   An intermediary &MAY; combine an ordered subsequence of Via header
2711   field entries into a single such entry if the entries have identical
2712   received-protocol values. For example,
2714<figure><artwork type="example">
2715  Via: 1.0 ricky, 1.1 ethel, 1.1 fred, 1.0 lucy
2718  could be collapsed to
2720<figure><artwork type="example">
2721  Via: 1.0 ricky, 1.1 mertz, 1.0 lucy
2724   A sender &SHOULD-NOT; combine multiple entries unless they are all
2725   under the same organizational control and the hosts have already been
2726   replaced by pseudonyms. A sender &MUST-NOT; combine entries that
2727   have different received-protocol values.
2731<section title="Transformations" anchor="message.transformations">
2732   <iref primary="true" item="transforming proxy"/>
2733   <iref primary="true" item="non-transforming proxy"/>
2735   Some intermediaries include features for transforming messages and their
2736   payloads. A proxy might, for example, convert between image formats in
2737   order to save cache space or to reduce the amount of traffic on a slow
2738   link. However, operational problems might occur when these transformations
2739   are applied to payloads intended for critical applications, such as medical
2740   imaging or scientific data analysis, particularly when integrity checks or
2741   digital signatures are used to ensure that the payload received is
2742   identical to the original.
2745   An HTTP-to-HTTP proxy is called a "<x:dfn>transforming proxy</x:dfn>"
2746   if it is designed or configured to modify messages in a semantically
2747   meaningful way (i.e., modifications, beyond those required by normal
2748   HTTP processing, that change the message in a way that would be
2749   significant to the original sender or potentially significant to
2750   downstream recipients).  For example, a transforming proxy might be
2751   acting as a shared annotation server (modifying responses to include
2752   references to a local annotation database), a malware filter, a
2753   format transcoder, or a privacy filter. Such transformations are presumed
2754   to be desired by whichever client (or client organization) selected the
2755   proxy.
2758   If a proxy receives a request-target with a host name that is not a
2759   fully qualified domain name, it &MAY; add its own domain to the host name
2760   it received when forwarding the request.  A proxy &MUST-NOT; change the
2761   host name if it is a fully qualified domain name.
2764   A proxy &MUST-NOT; modify the "absolute-path" and "query" parts of the
2765   received request-target when forwarding it to the next inbound server,
2766   except as noted above to replace an empty path with "/" or "*".
2769   A proxy &MUST-NOT; modify header fields that provide information about the
2770   end points of the communication chain, the resource state, or the selected
2771   representation. A proxy &MAY; change the message body through application
2772   or removal of a transfer coding (<xref target="transfer.codings"/>).
2775   A proxy &MUST-NOT; modify the payload (&payload;) of a message that
2776   contains a no-transform cache-control directive (&header-cache-control;).
2779   A proxy &MAY; transform the payload of a message
2780   that does not contain a no-transform cache-control directive.
2781   A proxy that transforms a payload &MUST; add a
2782   Warning header field with the warn-code of 214 ("Transformation Applied")
2783   if one is not already in the message (see &header-warning;).
2784   A proxy that transforms the payload of a <x:ref>200 (OK)</x:ref> response
2785   can further inform downstream recipients that a transformation has been
2786   applied by changing the response status code to
2787   <x:ref>203 (Non-Authoritative Information)</x:ref> (&status-203;).
2793<section title="Connection Management" anchor="">
2795   HTTP messaging is independent of the underlying transport or
2796   session-layer connection protocol(s).  HTTP only presumes a reliable
2797   transport with in-order delivery of requests and the corresponding
2798   in-order delivery of responses.  The mapping of HTTP request and
2799   response structures onto the data units of an underlying transport
2800   protocol is outside the scope of this specification.
2803   As described in <xref target="connecting.inbound"/>, the specific
2804   connection protocols to be used for an HTTP interaction are determined by
2805   client configuration and the <x:ref>target URI</x:ref>.
2806   For example, the "http" URI scheme
2807   (<xref target="http.uri"/>) indicates a default connection of TCP
2808   over IP, with a default TCP port of 80, but the client might be
2809   configured to use a proxy via some other connection, port, or protocol.
2812   HTTP implementations are expected to engage in connection management,
2813   which includes maintaining the state of current connections,
2814   establishing a new connection or reusing an existing connection,
2815   processing messages received on a connection, detecting connection
2816   failures, and closing each connection.
2817   Most clients maintain multiple connections in parallel, including
2818   more than one connection per server endpoint.
2819   Most servers are designed to maintain thousands of concurrent connections,
2820   while controlling request queues to enable fair use and detect
2821   denial of service attacks.
2824<section title="Connection" anchor="header.connection">
2825  <iref primary="true" item="Connection header field" x:for-anchor=""/>
2826  <iref primary="true" item="close" x:for-anchor=""/>
2827  <x:anchor-alias value="Connection"/>
2828  <x:anchor-alias value="connection-option"/>
2829  <x:anchor-alias value="close"/>
2831   The "Connection" header field allows the sender to indicate desired
2832   control options for the current connection.  In order to avoid confusing
2833   downstream recipients, a proxy or gateway &MUST; remove or replace any
2834   received connection options before forwarding the message.
2837   When a header field aside from Connection is used to supply control
2838   information for or about the current connection, the sender &MUST; list
2839   the corresponding field-name within the "Connection" header field.
2840   A proxy or gateway &MUST; parse a received Connection
2841   header field before a message is forwarded and, for each
2842   connection-option in this field, remove any header field(s) from
2843   the message with the same name as the connection-option, and then
2844   remove the Connection header field itself (or replace it with the
2845   intermediary's own connection options for the forwarded message).
2848   Hence, the Connection header field provides a declarative way of
2849   distinguishing header fields that are only intended for the
2850   immediate recipient ("hop-by-hop") from those fields that are
2851   intended for all recipients on the chain ("end-to-end"), enabling the
2852   message to be self-descriptive and allowing future connection-specific
2853   extensions to be deployed without fear that they will be blindly
2854   forwarded by older intermediaries.
2857   The Connection header field's value has the following grammar:
2859<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Connection"/><iref primary="true" item="Grammar" subitem="connection-option"/>
2860  <x:ref>Connection</x:ref>        = 1#<x:ref>connection-option</x:ref>
2861  <x:ref>connection-option</x:ref> = <x:ref>token</x:ref>
2864   Connection options are case-insensitive.
2867   A sender &MUST-NOT; send a connection option corresponding to a header
2868   field that is intended for all recipients of the payload.
2869   For example, <x:ref>Cache-Control</x:ref> is never appropriate as a
2870   connection option (&header-cache-control;).
2873   The connection options do not always correspond to a header field
2874   present in the message, since a connection-specific header field
2875   might not be needed if there are no parameters associated with a
2876   connection option. In contrast, a connection-specific header field that
2877   is received without a corresponding connection option usually indicates
2878   that the field has been improperly forwarded by an intermediary and
2879   ought to be ignored by the recipient.
2882   When defining new connection options, specification authors ought to survey
2883   existing header field names and ensure that the new connection option does
2884   not share the same name as an already deployed header field.
2885   Defining a new connection option essentially reserves that potential
2886   field-name for carrying additional information related to the
2887   connection option, since it would be unwise for senders to use
2888   that field-name for anything else.
2891   The "<x:dfn>close</x:dfn>" connection option is defined for a
2892   sender to signal that this connection will be closed after completion of
2893   the response. For example,
2895<figure><artwork type="example">
2896  Connection: close
2899   in either the request or the response header fields indicates that the
2900   sender is going to close the connection after the current request/response
2901   is complete (<xref target="persistent.tear-down"/>).
2904   A client that does not support <x:ref>persistent connections</x:ref> &MUST;
2905   send the "close" connection option in every request message.
2908   A server that does not support <x:ref>persistent connections</x:ref> &MUST;
2909   send the "close" connection option in every response message that
2910   does not have a <x:ref>1xx (Informational)</x:ref> status code.
2914<section title="Establishment" anchor="persistent.establishment">
2916   It is beyond the scope of this specification to describe how connections
2917   are established via various transport or session-layer protocols.
2918   Each connection applies to only one transport link.
2922<section title="Persistence" anchor="persistent.connections">
2923   <x:anchor-alias value="persistent connections"/>
2925   HTTP/1.1 defaults to the use of "<x:dfn>persistent connections</x:dfn>",
2926   allowing multiple requests and responses to be carried over a single
2927   connection. The "<x:ref>close</x:ref>" connection-option is used to signal
2928   that a connection will not persist after the current request/response.
2929   HTTP implementations &SHOULD; support persistent connections.
2932   A recipient determines whether a connection is persistent or not based on
2933   the most recently received message's protocol version and
2934   <x:ref>Connection</x:ref> header field (if any):
2935   <list style="symbols">
2936     <t>If the <x:ref>close</x:ref> connection option is present, the
2937        connection will not persist after the current response; else,</t>
2938     <t>If the received protocol is HTTP/1.1 (or later), the connection will
2939        persist after the current response; else,</t>
2940     <t>If the received protocol is HTTP/1.0, the "keep-alive"
2941        connection option is present, the recipient is not a proxy, and
2942        the recipient wishes to honor the HTTP/1.0 "keep-alive" mechanism,
2943        the connection will persist after the current response; otherwise,</t>
2944     <t>The connection will close after the current response.</t>
2945   </list>
2948   A server &MAY; assume that an HTTP/1.1 client intends to maintain a
2949   persistent connection until a <x:ref>close</x:ref> connection option
2950   is received in a request.
2953   A client &MAY; reuse a persistent connection until it sends or receives
2954   a <x:ref>close</x:ref> connection option or receives an HTTP/1.0 response
2955   without a "keep-alive" connection option.
2958   In order to remain persistent, all messages on a connection need to
2959   have a self-defined message length (i.e., one not defined by closure
2960   of the connection), as described in <xref target="message.body"/>.
2961   A server &MUST; read the entire request message body or close
2962   the connection after sending its response, since otherwise the
2963   remaining data on a persistent connection would be misinterpreted
2964   as the next request.  Likewise,
2965   a client &MUST; read the entire response message body if it intends
2966   to reuse the same connection for a subsequent request.
2969   A proxy server &MUST-NOT; maintain a persistent connection with an
2970   HTTP/1.0 client (see <xref x:sec="19.7.1" x:fmt="of" target="RFC2068"/> for
2971   information and discussion of the problems with the Keep-Alive header field
2972   implemented by many HTTP/1.0 clients).
2975   Clients and servers &SHOULD-NOT; assume that a persistent connection is
2976   maintained for HTTP versions less than 1.1 unless it is explicitly
2977   signaled.
2978   See <xref target="compatibility.with.http.1.0.persistent.connections"/>
2979   for more information on backward compatibility with HTTP/1.0 clients.
2982<section title="Retrying Requests" anchor="persistent.retrying.requests">
2984   Connections can be closed at any time, with or without intention.
2985   Implementations ought to anticipate the need to recover
2986   from asynchronous close events.
2989   When an inbound connection is closed prematurely, a client &MAY; open a new
2990   connection and automatically retransmit an aborted sequence of requests if
2991   all of those requests have idempotent methods (&idempotent-methods;).
2992   A proxy &MUST-NOT; automatically retry non-idempotent requests.
2995   A user agent &MUST-NOT; automatically retry a request with a non-idempotent
2996   method unless it has some means to know that the request semantics are
2997   actually idempotent, regardless of the method, or some means to detect that
2998   the original request was never applied. For example, a user agent that
2999   knows (through design or configuration) that a POST request to a given
3000   resource is safe can repeat that request automatically.
3001   Likewise, a user agent designed specifically to operate on a version
3002   control repository might be able to recover from partial failure conditions
3003   by checking the target resource revision(s) after a failed connection,
3004   reverting or fixing any changes that were partially applied, and then
3005   automatically retrying the requests that failed.
3008   A client &SHOULD-NOT; automatically retry a failed automatic retry.
3012<section title="Pipelining" anchor="pipelining">
3013   <x:anchor-alias value="pipeline"/>
3015   A client that supports persistent connections &MAY; "<x:dfn>pipeline</x:dfn>"
3016   its requests (i.e., send multiple requests without waiting for each
3017   response). A server &MAY; process a sequence of pipelined requests in
3018   parallel if they all have safe methods (&safe-methods;), but &MUST; send
3019   the corresponding responses in the same order that the requests were
3020   received.
3023   A client that pipelines requests &SHOULD; retry unanswered requests if the
3024   connection closes before it receives all of the corresponding responses.
3025   When retrying pipelined requests after a failed connection (a connection
3026   not explicitly closed by the server in its last complete response), a
3027   client &MUST-NOT; pipeline immediately after connection establishment,
3028   since the first remaining request in the prior pipeline might have caused
3029   an error response that can be lost again if multiple requests are sent on a
3030   prematurely closed connection (see the TCP reset problem described in
3031   <xref target="persistent.tear-down"/>).
3034   Idempotent methods (&idempotent-methods;) are significant to pipelining
3035   because they can be automatically retried after a connection failure.
3036   A user agent &SHOULD-NOT; pipeline requests after a non-idempotent method,
3037   until the final response status code for that method has been received,
3038   unless the user agent has a means to detect and recover from partial
3039   failure conditions involving the pipelined sequence.
3042   An intermediary that receives pipelined requests &MAY; pipeline those
3043   requests when forwarding them inbound, since it can rely on the outbound
3044   user agent(s) to determine what requests can be safely pipelined. If the
3045   inbound connection fails before receiving a response, the pipelining
3046   intermediary &MAY; attempt to retry a sequence of requests that have yet
3047   to receive a response if the requests all have idempotent methods;
3048   otherwise, the pipelining intermediary &SHOULD; forward any received
3049   responses and then close the corresponding outbound connection(s) so that
3050   the outbound user agent(s) can recover accordingly.
3055<section title="Concurrency" anchor="persistent.concurrency">
3057   A client &SHOULD; limit the number of simultaneous open
3058   connections that it maintains to a given server.
3061   Previous revisions of HTTP gave a specific number of connections as a
3062   ceiling, but this was found to be impractical for many applications. As a
3063   result, this specification does not mandate a particular maximum number of
3064   connections, but instead encourages clients to be conservative when opening
3065   multiple connections.
3068   Multiple connections are typically used to avoid the "head-of-line
3069   blocking" problem, wherein a request that takes significant server-side
3070   processing and/or has a large payload blocks subsequent requests on the
3071   same connection. However, each connection consumes server resources.
3072   Furthermore, using multiple connections can cause undesirable side effects
3073   in congested networks.
3076   Note that servers might reject traffic that they deem abusive, including an
3077   excessive number of connections from a client.
3081<section title="Failures and Time-outs" anchor="persistent.failures">
3083   Servers will usually have some time-out value beyond which they will
3084   no longer maintain an inactive connection. Proxy servers might make
3085   this a higher value since it is likely that the client will be making
3086   more connections through the same proxy server. The use of persistent
3087   connections places no requirements on the length (or existence) of
3088   this time-out for either the client or the server.
3091   A client or server that wishes to time-out &SHOULD; issue a graceful close
3092   on the connection. Implementations &SHOULD; constantly monitor open
3093   connections for a received closure signal and respond to it as appropriate,
3094   since prompt closure of both sides of a connection enables allocated system
3095   resources to be reclaimed.
3098   A client, server, or proxy &MAY; close the transport connection at any
3099   time. For example, a client might have started to send a new request
3100   at the same time that the server has decided to close the "idle"
3101   connection. From the server's point of view, the connection is being
3102   closed while it was idle, but from the client's point of view, a
3103   request is in progress.
3106   A server &SHOULD; sustain persistent connections, when possible, and allow
3107   the underlying
3108   transport's flow control mechanisms to resolve temporary overloads, rather
3109   than terminate connections with the expectation that clients will retry.
3110   The latter technique can exacerbate network congestion.
3113   A client sending a message body &SHOULD; monitor
3114   the network connection for an error response while it is transmitting
3115   the request. If the client sees a response that indicates the server does
3116   not wish to receive the message body and is closing the connection, the
3117   client &SHOULD; immediately cease transmitting the body and close its side
3118   of the connection.
3122<section title="Tear-down" anchor="persistent.tear-down">
3123  <iref primary="false" item="Connection header field" x:for-anchor=""/>
3124  <iref primary="false" item="close" x:for-anchor=""/>
3126   The <x:ref>Connection</x:ref> header field
3127   (<xref target="header.connection"/>) provides a "<x:ref>close</x:ref>"
3128   connection option that a sender &SHOULD; send when it wishes to close
3129   the connection after the current request/response pair.
3132   A client that sends a <x:ref>close</x:ref> connection option &MUST-NOT;
3133   send further requests on that connection (after the one containing
3134   <x:ref>close</x:ref>) and &MUST; close the connection after reading the
3135   final response message corresponding to this request.
3138   A server that receives a <x:ref>close</x:ref> connection option &MUST;
3139   initiate a close of the connection (see below) after it sends the
3140   final response to the request that contained <x:ref>close</x:ref>.
3141   The server &SHOULD; send a <x:ref>close</x:ref> connection option
3142   in its final response on that connection. The server &MUST-NOT; process
3143   any further requests received on that connection.
3146   A server that sends a <x:ref>close</x:ref> connection option &MUST;
3147   initiate a close of the connection (see below) after it sends the
3148   response containing <x:ref>close</x:ref>. The server &MUST-NOT; process
3149   any further requests received on that connection.
3152   A client that receives a <x:ref>close</x:ref> connection option &MUST;
3153   cease sending requests on that connection and close the connection
3154   after reading the response message containing the close; if additional
3155   pipelined requests had been sent on the connection, the client &SHOULD-NOT;
3156   assume that they will be processed by the server.
3159   If a server performs an immediate close of a TCP connection, there is a
3160   significant risk that the client will not be able to read the last HTTP
3161   response.  If the server receives additional data from the client on a
3162   fully-closed connection, such as another request that was sent by the
3163   client before receiving the server's response, the server's TCP stack will
3164   send a reset packet to the client; unfortunately, the reset packet might
3165   erase the client's unacknowledged input buffers before they can be read
3166   and interpreted by the client's HTTP parser.
3169   To avoid the TCP reset problem, servers typically close a connection in
3170   stages. First, the server performs a half-close by closing only the write
3171   side of the read/write connection. The server then continues to read from
3172   the connection until it receives a corresponding close by the client, or
3173   until the server is reasonably certain that its own TCP stack has received
3174   the client's acknowledgement of the packet(s) containing the server's last
3175   response. Finally, the server fully closes the connection.
3178   It is unknown whether the reset problem is exclusive to TCP or might also
3179   be found in other transport connection protocols.
3183<section title="Upgrade" anchor="header.upgrade">
3184  <iref primary="true" item="Upgrade header field" x:for-anchor=""/>
3185  <x:anchor-alias value="Upgrade"/>
3186  <x:anchor-alias value="protocol"/>
3187  <x:anchor-alias value="protocol-name"/>
3188  <x:anchor-alias value="protocol-version"/>
3190   The "Upgrade" header field is intended to provide a simple mechanism
3191   for transitioning from HTTP/1.1 to some other protocol on the same
3192   connection.  A client &MAY; send a list of protocols in the Upgrade
3193   header field of a request to invite the server to switch to one or
3194   more of those protocols, in order of descending preference, before sending
3195   the final response. A server &MAY; ignore a received Upgrade header field
3196   if it wishes to continue using the current protocol on that connection.
3198<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Upgrade"/>
3199  <x:ref>Upgrade</x:ref>          = 1#<x:ref>protocol</x:ref>
3201  <x:ref>protocol</x:ref>         = <x:ref>protocol-name</x:ref> ["/" <x:ref>protocol-version</x:ref>]
3202  <x:ref>protocol-name</x:ref>    = <x:ref>token</x:ref>
3203  <x:ref>protocol-version</x:ref> = <x:ref>token</x:ref>
3206   A server that sends a <x:ref>101 (Switching Protocols)</x:ref> response
3207   &MUST; send an Upgrade header field to indicate the new protocol(s) to
3208   which the connection is being switched; if multiple protocol layers are
3209   being switched, the sender &MUST; list the protocols in layer-ascending
3210   order. A server &MUST-NOT; switch to a protocol that was not indicated by
3211   the client in the corresponding request's Upgrade header field.
3212   A server &MAY; choose to ignore the order of preference indicated by the
3213   client and select the new protocol(s) based on other factors, such as the
3214   nature of the request or the current load on the server.
3217   A server that sends a <x:ref>426 (Upgrade Required)</x:ref> response
3218   &MUST; send an Upgrade header field to indicate the acceptable protocols,
3219   in order of descending preference.
3222   A server &MAY; send an Upgrade header field in any other response to
3223   advertise that it implements support for upgrading to the listed protocols,
3224   in order of descending preference, when appropriate for a future request.
3227   The following is a hypothetical example sent by a client:
3228</preamble><artwork type="message/http; msgtype=&#34;request&#34;" x:indent-with="  ">
3229GET /hello.txt HTTP/1.1
3231Connection: upgrade
3232Upgrade: HTTP/2.0, SHTTP/1.3, IRC/6.9, RTA/x11
3236   Upgrade cannot be used to insist on a protocol change; its acceptance and
3237   use by the server is optional. The capabilities and nature of the
3238   application-level communication after the protocol change is entirely
3239   dependent upon the new protocol(s) chosen. However, immediately after
3240   sending the 101 response, the server is expected to continue responding to
3241   the original request as if it had received its equivalent within the new
3242   protocol (i.e., the server still has an outstanding request to satisfy
3243   after the protocol has been changed, and is expected to do so without
3244   requiring the request to be repeated).
3247   For example, if the Upgrade header field is received in a GET request
3248   and the server decides to switch protocols, it first responds
3249   with a <x:ref>101 (Switching Protocols)</x:ref> message in HTTP/1.1 and
3250   then immediately follows that with the new protocol's equivalent of a
3251   response to a GET on the target resource.  This allows a connection to be
3252   upgraded to protocols with the same semantics as HTTP without the
3253   latency cost of an additional round-trip.  A server &MUST-NOT; switch
3254   protocols unless the received message semantics can be honored by the new
3255   protocol; an OPTIONS request can be honored by any protocol.
3258   The following is an example response to the above hypothetical request:
3259</preamble><artwork type="message/http; msgtype=&#34;response&#34;" x:indent-with="  ">
3260HTTP/1.1 101 Switching Protocols
3261Connection: upgrade
3262Upgrade: HTTP/2.0
3264[... data stream switches to HTTP/2.0 with an appropriate response
3265(as defined by new protocol) to the "GET /hello.txt" request ...]
3268   When Upgrade is sent, the sender &MUST; also send a
3269   <x:ref>Connection</x:ref> header field (<xref target="header.connection"/>)
3270   that contains an "upgrade" connection option, in order to prevent Upgrade
3271   from being accidentally forwarded by intermediaries that might not implement
3272   the listed protocols.  A server &MUST; ignore an Upgrade header field that
3273   is received in an HTTP/1.0 request.
3276   A client cannot begin using an upgraded protocol on the connection until
3277   it has completely sent the request message (i.e., the client can't change
3278   the protocol it is sending in the middle of a message).
3279   If a server receives both Upgrade and an <x:ref>Expect</x:ref> header field
3280   with the "100-continue" expectation (&header-expect;), the
3281   server &MUST; send a <x:ref>100 (Continue)</x:ref> response before sending
3282   a <x:ref>101 (Switching Protocols)</x:ref> response.
3285   The Upgrade header field only applies to switching protocols on top of the
3286   existing connection; it cannot be used to switch the underlying connection
3287   (transport) protocol, nor to switch the existing communication to a
3288   different connection. For those purposes, it is more appropriate to use a
3289   <x:ref>3xx (Redirection)</x:ref> response (&status-3xx;).
3292   This specification only defines the protocol name "HTTP" for use by
3293   the family of Hypertext Transfer Protocols, as defined by the HTTP
3294   version rules of <xref target="http.version"/> and future updates to this
3295   specification. Additional tokens ought to be registered with IANA using the
3296   registration procedure defined in <xref target="upgrade.token.registry"/>.
3301<section title="ABNF list extension: #rule" anchor="abnf.extension">
3303  A #rule extension to the ABNF rules of <xref target="RFC5234"/> is used to
3304  improve readability in the definitions of some header field values.
3307  A construct "#" is defined, similar to "*", for defining comma-delimited
3308  lists of elements. The full form is "&lt;n&gt;#&lt;m&gt;element" indicating
3309  at least &lt;n&gt; and at most &lt;m&gt; elements, each separated by a single
3310  comma (",") and optional whitespace (OWS).   
3313  Thus, a sender &MUST; expand the list construct as follows:
3314</preamble><artwork type="example">
3315  1#element =&gt; element *( OWS "," OWS element )
3318  and:
3319</preamble><artwork type="example">
3320  #element =&gt; [ 1#element ]
3323  and for n &gt;= 1 and m &gt; 1:
3324</preamble><artwork type="example">
3325  &lt;n&gt;#&lt;m&gt;element =&gt; element &lt;n-1&gt;*&lt;m-1&gt;( OWS "," OWS element )
3328  For compatibility with legacy list rules, a recipient &MUST; parse and ignore
3329  a reasonable number of empty list elements: enough to handle common mistakes
3330  by senders that merge values, but not so much that they could be used as a
3331  denial of service mechanism. In other words, a recipient &MUST; expand the
3332  list construct as follows:
3334<figure><artwork type="example">
3335  #element =&gt; [ ( "," / element ) *( OWS "," [ OWS element ] ) ]
3337  1#element =&gt; *( "," OWS ) element *( OWS "," [ OWS element ] )
3340  Empty elements do not contribute to the count of elements present.
3341  For example, given these ABNF productions:
3343<figure><artwork type="example">
3344  example-list      = 1#example-list-elmt
3345  example-list-elmt = token ; see <xref target="field.components"/>
3348  Then the following are valid values for example-list (not including the
3349  double quotes, which are present for delimitation only):
3351<figure><artwork type="example">
3352  "foo,bar"
3353  "foo ,bar,"
3354  "foo , ,bar,charlie   "
3357  In contrast, the following values would be invalid, since at least one
3358  non-empty element is required by the example-list production:
3360<figure><artwork type="example">
3361  ""
3362  ","
3363  ",   ,"
3366  <xref target="collected.abnf"/> shows the collected ABNF after the list
3367  constructs have been expanded, as described above, for recipients.
3371<section title="IANA Considerations" anchor="IANA.considerations">
3373<section title="Header Field Registration" anchor="header.field.registration">
3375   HTTP header fields are registered within the Message Header Field Registry
3376   maintained at
3377   <eref target=""/>.
3380   This document defines the following HTTP header fields, so their
3381   associated registry entries shall be updated according to the permanent
3382   registrations below (see <xref target="BCP90"/>):
3384<?BEGININC p1-messaging.iana-headers ?>
3385<!--AUTOGENERATED FROM extract-header-defs.xslt, do not edit manually-->
3386<texttable align="left" suppress-title="true" anchor="iana.header.registration.table">
3387   <ttcol>Header Field Name</ttcol>
3388   <ttcol>Protocol</ttcol>
3389   <ttcol>Status</ttcol>
3390   <ttcol>Reference</ttcol>
3392   <c>Connection</c>
3393   <c>http</c>
3394   <c>standard</c>
3395   <c>
3396      <xref target="header.connection"/>
3397   </c>
3398   <c>Content-Length</c>
3399   <c>http</c>
3400   <c>standard</c>
3401   <c>
3402      <xref target="header.content-length"/>
3403   </c>
3404   <c>Host</c>
3405   <c>http</c>
3406   <c>standard</c>
3407   <c>
3408      <xref target=""/>
3409   </c>
3410   <c>TE</c>
3411   <c>http</c>
3412   <c>standard</c>
3413   <c>
3414      <xref target="header.te"/>
3415   </c>
3416   <c>Trailer</c>
3417   <c>http</c>
3418   <c>standard</c>
3419   <c>
3420      <xref target="header.trailer"/>
3421   </c>
3422   <c>Transfer-Encoding</c>
3423   <c>http</c>
3424   <c>standard</c>
3425   <c>
3426      <xref target="header.transfer-encoding"/>
3427   </c>
3428   <c>Upgrade</c>
3429   <c>http</c>
3430   <c>standard</c>
3431   <c>
3432      <xref target="header.upgrade"/>
3433   </c>
3434   <c>Via</c>
3435   <c>http</c>
3436   <c>standard</c>
3437   <c>
3438      <xref target="header.via"/>
3439   </c>
3442<?ENDINC p1-messaging.iana-headers ?>
3444   Furthermore, the header field-name "Close" shall be registered as
3445   "reserved", since using that name as an HTTP header field might
3446   conflict with the "close" connection option of the "<x:ref>Connection</x:ref>"
3447   header field (<xref target="header.connection"/>).
3449<texttable align="left" suppress-title="true">
3450   <ttcol>Header Field Name</ttcol>
3451   <ttcol>Protocol</ttcol>
3452   <ttcol>Status</ttcol>
3453   <ttcol>Reference</ttcol>
3455   <c>Close</c>
3456   <c>http</c>
3457   <c>reserved</c>
3458   <c>
3459      <xref target="header.field.registration"/>
3460   </c>
3463   The change controller is: "IETF ( - Internet Engineering Task Force".
3467<section title="URI Scheme Registration" anchor="uri.scheme.registration">
3469   IANA maintains the registry of URI Schemes <xref target="BCP115"/> at
3470   <eref target=""/>.
3473   This document defines the following URI schemes, so their
3474   associated registry entries shall be updated according to the permanent
3475   registrations below:
3477<texttable align="left" suppress-title="true">
3478   <ttcol>URI Scheme</ttcol>
3479   <ttcol>Description</ttcol>
3480   <ttcol>Reference</ttcol>
3482   <c>http</c>
3483   <c>Hypertext Transfer Protocol</c>
3484   <c><xref target="http.uri"/></c>
3486   <c>https</c>
3487   <c>Hypertext Transfer Protocol Secure</c>
3488   <c><xref target="https.uri"/></c>
3492<section title="Internet Media Type Registration" anchor="">
3494   IANA maintains the registry of Internet media types <xref target="BCP13"/> at
3495   <eref target=""/>.
3498   This document serves as the specification for the Internet media types
3499   "message/http" and "application/http". The following is to be registered with
3500   IANA.
3502<section title="Internet Media Type message/http" anchor="">
3503<iref item="Media Type" subitem="message/http" primary="true"/>
3504<iref item="message/http Media Type" primary="true"/>
3506   The message/http type can be used to enclose a single HTTP request or
3507   response message, provided that it obeys the MIME restrictions for all
3508   "message" types regarding line length and encodings.
3511  <list style="hanging" x:indent="12em">
3512    <t hangText="Type name:">
3513      message
3514    </t>
3515    <t hangText="Subtype name:">
3516      http
3517    </t>
3518    <t hangText="Required parameters:">
3519      N/A
3520    </t>
3521    <t hangText="Optional parameters:">
3522      version, msgtype
3523      <list style="hanging">
3524        <t hangText="version:">
3525          The HTTP-version number of the enclosed message
3526          (e.g., "1.1"). If not present, the version can be
3527          determined from the first line of the body.
3528        </t>
3529        <t hangText="msgtype:">
3530          The message type &mdash; "request" or "response". If not
3531          present, the type can be determined from the first
3532          line of the body.
3533        </t>
3534      </list>
3535    </t>
3536    <t hangText="Encoding considerations:">
3537      only "7bit", "8bit", or "binary" are permitted
3538    </t>
3539    <t hangText="Security considerations:">
3540      see <xref target="security.considerations"/>
3541    </t>
3542    <t hangText="Interoperability considerations:">
3543      N/A
3544    </t>
3545    <t hangText="Published specification:">
3546      This specification (see <xref target=""/>).
3547    </t>
3548    <t hangText="Applications that use this media type:">
3549      N/A
3550    </t>
3551    <t hangText="Fragment identifier considerations:">
3552      N/A
3553    </t>
3554    <t hangText="Additional information:">
3555      <list style="hanging">
3556        <t hangText="Magic number(s):">N/A</t>
3557        <t hangText="Deprecated alias names for this type:">N/A</t>
3558        <t hangText="File extension(s):">N/A</t>
3559        <t hangText="Macintosh file type code(s):">N/A</t>
3560      </list>
3561    </t>
3562    <t hangText="Person and email address to contact for further information:">
3563      See Authors Section.
3564    </t>
3565    <t hangText="Intended usage:">
3566      COMMON
3567    </t>
3568    <t hangText="Restrictions on usage:">
3569      N/A
3570    </t>
3571    <t hangText="Author:">
3572      See Authors Section.
3573    </t>
3574    <t hangText="Change controller:">
3575      IESG
3576    </t>
3577  </list>
3580<section title="Internet Media Type application/http" anchor="">
3581<iref item="Media Type" subitem="application/http" primary="true"/>
3582<iref item="application/http Media Type" primary="true"/>
3584   The application/http type can be used to enclose a pipeline of one or more
3585   HTTP request or response messages (not intermixed).
3588  <list style="hanging" x:indent="12em">
3589    <t hangText="Type name:">
3590      application
3591    </t>
3592    <t hangText="Subtype name:">
3593      http
3594    </t>
3595    <t hangText="Required parameters:">
3596      N/A
3597    </t>
3598    <t hangText="Optional parameters:">
3599      version, msgtype
3600      <list style="hanging">
3601        <t hangText="version:">
3602          The HTTP-version number of the enclosed messages
3603          (e.g., "1.1"). If not present, the version can be
3604          determined from the first line of the body.
3605        </t>
3606        <t hangText="msgtype:">
3607          The message type &mdash; "request" or "response". If not
3608          present, the type can be determined from the first
3609          line of the body.
3610        </t>
3611      </list>
3612    </t>
3613    <t hangText="Encoding considerations:">
3614      HTTP messages enclosed by this type
3615      are in "binary" format; use of an appropriate
3616      Content-Transfer-Encoding is required when
3617      transmitted via E-mail.
3618    </t>
3619    <t hangText="Security considerations:">
3620      see <xref target="security.considerations"/>
3621    </t>
3622    <t hangText="Interoperability considerations:">
3623      N/A
3624    </t>
3625    <t hangText="Published specification:">
3626      This specification (see <xref target=""/>).
3627    </t>
3628    <t hangText="Applications that use this media type:">
3629      N/A
3630    </t>
3631    <t hangText="Fragment identifier considerations:">
3632      N/A
3633    </t>
3634    <t hangText="Additional information:">
3635      <list style="hanging">
3636        <t hangText="Deprecated alias names for this type:">N/A</t>
3637        <t hangText="Magic number(s):">N/A</t>
3638        <t hangText="File extension(s):">N/A</t>
3639        <t hangText="Macintosh file type code(s):">N/A</t>
3640      </list>
3641    </t>
3642    <t hangText="Person and email address to contact for further information:">
3643      See Authors Section.
3644    </t>
3645    <t hangText="Intended usage:">
3646      COMMON
3647    </t>
3648    <t hangText="Restrictions on usage:">
3649      N/A
3650    </t>
3651    <t hangText="Author:">
3652      See Authors Section.
3653    </t>
3654    <t hangText="Change controller:">
3655      IESG
3656    </t>
3657  </list>
3662<section title="Transfer Coding Registry" anchor="transfer.coding.registry">
3664   The HTTP Transfer Coding Registry defines the name space for transfer
3665   coding names. It is maintained at <eref target=""/>.
3668<section title="Procedure" anchor="transfer.coding.registry.procedure">
3670   Registrations &MUST; include the following fields:
3671   <list style="symbols">
3672     <t>Name</t>
3673     <t>Description</t>
3674     <t>Pointer to specification text</t>
3675   </list>
3678   Names of transfer codings &MUST-NOT; overlap with names of content codings
3679   (&content-codings;) unless the encoding transformation is identical, as
3680   is the case for the compression codings defined in
3681   <xref target="compression.codings"/>.
3684   Values to be added to this name space require IETF Review (see
3685   <xref target="RFC5226" x:fmt="of" x:sec="4.1"/>), and &MUST;
3686   conform to the purpose of transfer coding defined in this specification.
3689   Use of program names for the identification of encoding formats
3690   is not desirable and is discouraged for future encodings.
3694<section title="Registration" anchor="transfer.coding.registration">
3696   The HTTP Transfer Coding Registry shall be updated with the registrations
3697   below:
3699<texttable align="left" suppress-title="true" anchor="iana.transfer.coding.registration.table">
3700   <ttcol>Name</ttcol>
3701   <ttcol>Description</ttcol>
3702   <ttcol>Reference</ttcol>
3703   <c>chunked</c>
3704   <c>Transfer in a series of chunks</c>
3705   <c>
3706      <xref target="chunked.encoding"/>
3707   </c>
3708   <c>compress</c>
3709   <c>UNIX "compress" data format <xref target="Welch"/></c>
3710   <c>
3711      <xref target="compress.coding"/>
3712   </c>
3713   <c>deflate</c>
3714   <c>"deflate" compressed data (<xref target="RFC1951"/>) inside
3715   the "zlib" data format (<xref target="RFC1950"/>)
3716   </c>
3717   <c>
3718      <xref target="deflate.coding"/>
3719   </c>
3720   <c>gzip</c>
3721   <c>GZIP file format <xref target="RFC1952"/></c>
3722   <c>
3723      <xref target="gzip.coding"/>
3724   </c>
3725   <c>x-compress</c>
3726   <c>Deprecated (alias for compress)</c>
3727   <c>
3728      <xref target="compress.coding"/>
3729   </c>
3730   <c>x-gzip</c>
3731   <c>Deprecated (alias for gzip)</c>
3732   <c>
3733      <xref target="gzip.coding"/>
3734   </c>
3739<section title="Content Coding Registration" anchor="content.coding.registration">
3741   IANA maintains the registry of HTTP Content Codings at
3742   <eref target=""/>.
3745   The HTTP Content Codings Registry shall be updated with the registrations
3746   below:
3748<texttable align="left" suppress-title="true" anchor="iana.content.coding.registration.table">
3749   <ttcol>Name</ttcol>
3750   <ttcol>Description</ttcol>
3751   <ttcol>Reference</ttcol>
3752   <c>compress</c>
3753   <c>UNIX "compress" data format <xref target="Welch"/></c>
3754   <c>
3755      <xref target="compress.coding"/>
3756   </c>
3757   <c>deflate</c>
3758   <c>"deflate" compressed data (<xref target="RFC1951"/>) inside
3759   the "zlib" data format (<xref target="RFC1950"/>)</c>
3760   <c>
3761      <xref target="deflate.coding"/>
3762   </c>
3763   <c>gzip</c>
3764   <c>GZIP file format <xref target="RFC1952"/></c>
3765   <c>
3766      <xref target="gzip.coding"/>
3767   </c>
3768   <c>x-compress</c>
3769   <c>Deprecated (alias for compress)</c>
3770   <c>
3771      <xref target="compress.coding"/>
3772   </c>
3773   <c>x-gzip</c>
3774   <c>Deprecated (alias for gzip)</c>
3775   <c>
3776      <xref target="gzip.coding"/>
3777   </c>
3781<section title="Upgrade Token Registry" anchor="upgrade.token.registry">
3783   The HTTP Upgrade Token Registry defines the name space for protocol-name
3784   tokens used to identify protocols in the <x:ref>Upgrade</x:ref> header
3785   field. The registry is maintained at <eref target=""/>.
3788<section title="Procedure" anchor="upgrade.token.registry.procedure">  
3790   Each registered protocol name is associated with contact information
3791   and an optional set of specifications that details how the connection
3792   will be processed after it has been upgraded.
3795   Registrations happen on a "First Come First Served" basis (see
3796   <xref target="RFC5226" x:sec="4.1" x:fmt="of"/>) and are subject to the
3797   following rules:
3798  <list style="numbers">
3799    <t>A protocol-name token, once registered, stays registered forever.</t>
3800    <t>The registration &MUST; name a responsible party for the
3801       registration.</t>
3802    <t>The registration &MUST; name a point of contact.</t>
3803    <t>The registration &MAY; name a set of specifications associated with
3804       that token. Such specifications need not be publicly available.</t>
3805    <t>The registration &SHOULD; name a set of expected "protocol-version"
3806       tokens associated with that token at the time of registration.</t>
3807    <t>The responsible party &MAY; change the registration at any time.
3808       The IANA will keep a record of all such changes, and make them
3809       available upon request.</t>
3810    <t>The IESG &MAY; reassign responsibility for a protocol token.
3811       This will normally only be used in the case when a
3812       responsible party cannot be contacted.</t>
3813  </list>
3816   This registration procedure for HTTP Upgrade Tokens replaces that
3817   previously defined in <xref target="RFC2817" x:fmt="of" x:sec="7.2"/>.
3821<section title="Upgrade Token Registration" anchor="upgrade.token.registration">
3823   The "HTTP" entry in the HTTP Upgrade Token Registry shall be updated with
3824   the registration below:
3826<texttable align="left" suppress-title="true">
3827   <ttcol>Value</ttcol>
3828   <ttcol>Description</ttcol>
3829   <ttcol>Expected Version Tokens</ttcol>
3830   <ttcol>Reference</ttcol>
3832   <c>HTTP</c>
3833   <c>Hypertext Transfer Protocol</c>
3834   <c>any DIGIT.DIGIT (e.g, "2.0")</c>
3835   <c><xref target="http.version"/></c>
3838   The responsible party is: "IETF ( - Internet Engineering Task Force".
3845<section title="Security Considerations" anchor="security.considerations">
3847   This section is meant to inform developers, information providers, and
3848   users of known security considerations relevant to HTTP message syntax,
3849   parsing, and routing. Security considerations about HTTP semantics and
3850   payloads are addressed in &semantics;.
3853<section title="DNS-related Attacks" anchor="dns.related.attacks">
3855   HTTP clients rely heavily on the Domain Name Service (DNS), and are thus
3856   generally prone to security attacks based on the deliberate misassociation
3857   of IP addresses and DNS names not protected by DNSSEC. Clients need to be
3858   cautious in assuming the validity of an IP number/DNS name association unless
3859   the response is protected by DNSSEC (<xref target="RFC4033"/>).
3863<section title="Intermediaries and Caching" anchor="attack.intermediaries">
3865   By their very nature, HTTP intermediaries are men-in-the-middle, and
3866   represent an opportunity for man-in-the-middle attacks. Compromise of
3867   the systems on which the intermediaries run can result in serious security
3868   and privacy problems. Intermediaries have access to security-related
3869   information, personal information about individual users and
3870   organizations, and proprietary information belonging to users and
3871   content providers. A compromised intermediary, or an intermediary
3872   implemented or configured without regard to security and privacy
3873   considerations, might be used in the commission of a wide range of
3874   potential attacks.
3877   Intermediaries that contain a shared cache are especially vulnerable
3878   to cache poisoning attacks.
3881   Implementers need to consider the privacy and security
3882   implications of their design and coding decisions, and of the
3883   configuration options they provide to operators (especially the
3884   default configuration).
3887   Users need to be aware that intermediaries are no more trustworthy than
3888   the people who run them; HTTP itself cannot solve this problem.
3892<section title="Buffer Overflows" anchor="attack.protocol.element.size.overflows">
3894   Because HTTP uses mostly textual, character-delimited fields, attackers can
3895   overflow buffers in implementations, and/or perform a Denial of Service
3896   against implementations that accept fields with unlimited lengths.
3899   To promote interoperability, this specification makes specific
3900   recommendations for minimum size limits on request-line
3901   (<xref target="request.line"/>)
3902   and header fields (<xref target="header.fields"/>). These are
3903   minimum recommendations, chosen to be supportable even by implementations
3904   with limited resources; it is expected that most implementations will
3905   choose substantially higher limits.
3908   This specification also provides a way for servers to reject messages that
3909   have request-targets that are too long (&status-414;) or request entities
3910   that are too large (&status-4xx;). Additional status codes related to
3911   capacity limits have been defined by extensions to HTTP
3912   <xref target="RFC6585"/>.
3915   Recipients ought to carefully limit the extent to which they read other
3916   fields, including (but not limited to) request methods, response status
3917   phrases, header field-names, and body chunks, so as to avoid denial of
3918   service attacks without impeding interoperability.
3922<section title="Message Integrity" anchor="message.integrity">
3924   HTTP does not define a specific mechanism for ensuring message integrity,
3925   instead relying on the error-detection ability of underlying transport
3926   protocols and the use of length or chunk-delimited framing to detect
3927   completeness. Additional integrity mechanisms, such as hash functions or
3928   digital signatures applied to the content, can be selectively added to
3929   messages via extensible metadata header fields. Historically, the lack of
3930   a single integrity mechanism has been justified by the informal nature of
3931   most HTTP communication.  However, the prevalence of HTTP as an information
3932   access mechanism has resulted in its increasing use within environments
3933   where verification of message integrity is crucial.
3936   User agents are encouraged to implement configurable means for detecting
3937   and reporting failures of message integrity such that those means can be
3938   enabled within environments for which integrity is necessary. For example,
3939   a browser being used to view medical history or drug interaction
3940   information needs to indicate to the user when such information is detected
3941   by the protocol to be incomplete, expired, or corrupted during transfer.
3942   Such mechanisms might be selectively enabled via user agent extensions or
3943   the presence of message integrity metadata in a response.
3944   At a minimum, user agents ought to provide some indication that allows a
3945   user to distinguish between a complete and incomplete response message
3946   (<xref target="incomplete.messages"/>) when such verification is desired.
3950<section title="Server Log Information" anchor="abuse.of.server.log.information">
3952   A server is in the position to save personal data about a user's requests
3953   over time, which might identify their reading patterns or subjects of
3954   interest.  In particular, log information gathered at an intermediary
3955   often contains a history of user agent interaction, across a multitude
3956   of sites, that can be traced to individual users.
3959   HTTP log information is confidential in nature; its handling is often
3960   constrained by laws and regulations.  Log information needs to be securely
3961   stored and appropriate guidelines followed for its analysis.
3962   Anonymization of personal information within individual entries helps,
3963   but is generally not sufficient to prevent real log traces from being
3964   re-identified based on correlation with other access characteristics.
3965   As such, access traces that are keyed to a specific client are unsafe to
3966   publish even if the key is pseudonymous.
3969   To minimize the risk of theft or accidental publication, log information
3970   ought to be purged of personally identifiable information, including
3971   user identifiers, IP addresses, and user-provided query parameters,
3972   as soon as that information is no longer necessary to support operational
3973   needs for security, auditing, or fraud control.
3978<section title="Acknowledgments" anchor="acks">
3980   This edition of HTTP/1.1 builds on the many contributions that went into
3981   <xref target="RFC1945" format="none">RFC 1945</xref>,
3982   <xref target="RFC2068" format="none">RFC 2068</xref>,
3983   <xref target="RFC2145" format="none">RFC 2145</xref>, and
3984   <xref target="RFC2616" format="none">RFC 2616</xref>, including
3985   substantial contributions made by the previous authors, editors, and
3986   working group chairs: Tim Berners-Lee, Ari Luotonen, Roy T. Fielding,
3987   Henrik Frystyk Nielsen, Jim Gettys, Jeffrey C. Mogul, Larry Masinter,
3988   and Paul J. Leach. Mark Nottingham oversaw this effort as working group chair.
3991   Since 1999, the following contributors have helped improve the HTTP
3992   specification by reporting bugs, asking smart questions, drafting or
3993   reviewing text, and evaluating open issues:
3995<?BEGININC acks ?>
3996<t>Adam Barth,
3997Adam Roach,
3998Addison Phillips,
3999Adrian Chadd,
4000Adrian Cole,
4001Adrien W. de Croy,
4002Alan Ford,
4003Alan Ruttenberg,
4004Albert Lunde,
4005Alek Storm,
4006Alex Rousskov,
4007Alexandre Morgaut,
4008Alexey Melnikov,
4009Alisha Smith,
4010Amichai Rothman,
4011Amit Klein,
4012Amos Jeffries,
4013Andreas Maier,
4014Andreas Petersson,
4015Andrei Popov,
4016Anil Sharma,
4017Anne van Kesteren,
4018Anthony Bryan,
4019Asbjorn Ulsberg,
4020Ashok Kumar,
4021Balachander Krishnamurthy,
4022Barry Leiba,
4023Ben Laurie,
4024Benjamin Carlyle,
4025Benjamin Niven-Jenkins,
4026Benoit Claise,
4027Bil Corry,
4028Bill Burke,
4029Bjoern Hoehrmann,
4030Bob Scheifler,
4031Boris Zbarsky,
4032Brett Slatkin,
4033Brian Kell,
4034Brian McBarron,
4035Brian Pane,
4036Brian Raymor,
4037Brian Smith,
4038Bruce Perens,
4039Bryce Nesbitt,
4040Cameron Heavon-Jones,
4041Carl Kugler,
4042Carsten Bormann,
4043Charles Fry,
4044Chris Burdess,
4045Chris Newman,
4046Christian Huitema,
4047Cyrus Daboo,
4048Dale Robert Anderson,
4049Dan Wing,
4050Dan Winship,
4051Daniel Stenberg,
4052Darrel Miller,
4053Dave Cridland,
4054Dave Crocker,
4055Dave Kristol,
4056Dave Thaler,
4057David Booth,
4058David Singer,
4059David W. Morris,
4060Diwakar Shetty,
4061Dmitry Kurochkin,
4062Drummond Reed,
4063Duane Wessels,
4064Edward Lee,
4065Eitan Adler,
4066Eliot Lear,
4067Emile Stephan,
4068Eran Hammer-Lahav,
4069Eric D. Williams,
4070Eric J. Bowman,
4071Eric Lawrence,
4072Eric Rescorla,
4073Erik Aronesty,
4074EungJun Yi,
4075Evan Prodromou,
4076Felix Geisendoerfer,
4077Florian Weimer,
4078Frank Ellermann,
4079Fred Akalin,
4080Fred Bohle,
4081Frederic Kayser,
4082Gabor Molnar,
4083Gabriel Montenegro,
4084Geoffrey Sneddon,
4085Gervase Markham,
4086Gili Tzabari,
4087Grahame Grieve,
4088Greg Slepak,
4089Greg Wilkins,
4090Grzegorz Calkowski,
4091Harald Tveit Alvestrand,
4092Harry Halpin,
4093Helge Hess,
4094Henrik Nordstrom,
4095Henry S. Thompson,
4096Henry Story,
4097Herbert van de Sompel,
4098Herve Ruellan,
4099Howard Melman,
4100Hugo Haas,
4101Ian Fette,
4102Ian Hickson,
4103Ido Safruti,
4104Ilari Liusvaara,
4105Ilya Grigorik,
4106Ingo Struck,
4107J. Ross Nicoll,
4108James Cloos,
4109James H. Manger,
4110James Lacey,
4111James M. Snell,
4112Jamie Lokier,
4113Jan Algermissen,
4114Jari Arkko,
4115Jeff Hodges (who came up with the term 'effective Request-URI'),
4116Jeff Pinner,
4117Jeff Walden,
4118Jim Luther,
4119Jitu Padhye,
4120Joe D. Williams,
4121Joe Gregorio,
4122Joe Orton,
4123Joel Jaeggli,
4124John C. Klensin,
4125John C. Mallery,
4126John Cowan,
4127John Kemp,
4128John Panzer,
4129John Schneider,
4130John Stracke,
4131John Sullivan,
4132Jonas Sicking,
4133Jonathan A. Rees,
4134Jonathan Billington,
4135Jonathan Moore,
4136Jonathan Silvera,
4137Jordi Ros,
4138Joris Dobbelsteen,
4139Josh Cohen,
4140Julien Pierre,
4141Jungshik Shin,
4142Justin Chapweske,
4143Justin Erenkrantz,
4144Justin James,
4145Kalvinder Singh,
4146Karl Dubost,
4147Kathleen Moriarty,
4148Keith Hoffman,
4149Keith Moore,
4150Ken Murchison,
4151Koen Holtman,
4152Konstantin Voronkov,
4153Kris Zyp,
4154Leif Hedstrom,
4155Lionel Morand,
4156Lisa Dusseault,
4157Maciej Stachowiak,
4158Manu Sporny,
4159Marc Schneider,
4160Marc Slemko,
4161Mark Baker,
4162Mark Pauley,
4163Mark Watson,
4164Markus Isomaki,
4165Markus Lanthaler,
4166Martin J. Duerst,
4167Martin Musatov,
4168Martin Nilsson,
4169Martin Thomson,
4170Matt Lynch,
4171Matthew Cox,
4172Matthew Kerwin,
4173Max Clark,
4174Menachem Dodge,
4175Meral Shirazipour,
4176Michael Burrows,
4177Michael Hausenblas,
4178Michael Scharf,
4179Michael Sweet,
4180Michael Tuexen,
4181Michael Welzl,
4182Mike Amundsen,
4183Mike Belshe,
4184Mike Bishop,
4185Mike Kelly,
4186Mike Schinkel,
4187Miles Sabin,
4188Murray S. Kucherawy,
4189Mykyta Yevstifeyev,
4190Nathan Rixham,
4191Nicholas Shanks,
4192Nico Williams,
4193Nicolas Alvarez,
4194Nicolas Mailhot,
4195Noah Slater,
4196Osama Mazahir,
4197Pablo Castro,
4198Pat Hayes,
4199Patrick R. McManus,
4200Paul E. Jones,
4201Paul Hoffman,
4202Paul Marquess,
4203Pete Resnick,
4204Peter Lepeska,
4205Peter Occil,
4206Peter Saint-Andre,
4207Peter Watkins,
4208Phil Archer,
4209Phil Hunt,
4210Philippe Mougin,
4211Phillip Hallam-Baker,
4212Piotr Dobrogost,
4213Poul-Henning Kamp,
4214Preethi Natarajan,
4215Rajeev Bector,
4216Ray Polk,
4217Reto Bachmann-Gmuer,
4218Richard Barnes,
4219Richard Cyganiak,
4220Rob Trace,
4221Robby Simpson,
4222Robert Brewer,
4223Robert Collins,
4224Robert Mattson,
4225Robert O'Callahan,
4226Robert Olofsson,
4227Robert Sayre,
4228Robert Siemer,
4229Robert de Wilde,
4230Roberto Javier Godoy,
4231Roberto Peon,
4232Roland Zink,
4233Ronny Widjaja,
4234Ryan Hamilton,
4235S. Mike Dierken,
4236Salvatore Loreto,
4237Sam Johnston,
4238Sam Pullara,
4239Sam Ruby,
4240Saurabh Kulkarni,
4241Scott Lawrence (who maintained the original issues list),
4242Sean B. Palmer,
4243Sean Turner,
4244Sebastien Barnoud,
4245Shane McCarron,
4246Shigeki Ohtsu,
4247Simon Yarde,
4248Stefan Eissing,
4249Stefan Tilkov,
4250Stefanos Harhalakis,
4251Stephane Bortzmeyer,
4252Stephen Farrell,
4253Stephen Kent,
4254Stephen Ludin,
4255Stuart Williams,
4256Subbu Allamaraju,
4257Subramanian Moonesamy,
4258Susan Hares,
4259Sylvain Hellegouarch,
4260Tapan Divekar,
4261Tatsuhiro Tsujikawa,
4262Tatsuya Hayashi,
4263Ted Hardie,
4264Ted Lemon,
4265Thomas Broyer,
4266Thomas Fossati,
4267Thomas Maslen,
4268Thomas Nadeau,
4269Thomas Nordin,
4270Thomas Roessler,
4271Tim Bray,
4272Tim Morgan,
4273Tim Olsen,
4274Tom Zhou,
4275Travis Snoozy,
4276Tyler Close,
4277Vincent Murphy,
4278Wenbo Zhu,
4279Werner Baumann,
4280Wilbur Streett,
4281Wilfredo Sanchez Vega,
4282William A. Rowe Jr.,
4283William Chan,
4284Willy Tarreau,
4285Xiaoshu Wang,
4286Yaron Goland,
4287Yngve Nysaeter Pettersen,
4288Yoav Nir,
4289Yogesh Bang,
4290Yuchung Cheng,
4291Yutaka Oiwa,
4292Yves Lafon (long-time member of the editor team),
4293Zed A. Shaw, and
4294Zhong Yu.
4296<?ENDINC acks ?>
4298   See <xref target="RFC2616" x:fmt="of" x:sec="16"/> for additional
4299   acknowledgements from prior revisions.
4306<references title="Normative References">
4308<reference anchor="Part2">
4309  <front>
4310    <title>Hypertext Transfer Protocol (HTTP/1.1): Semantics and Content</title>
4311    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
4312      <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
4313      <address><email></email></address>
4314    </author>
4315    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
4316      <organization abbrev="greenbytes">greenbytes GmbH</organization>
4317      <address><email></email></address>
4318    </author>
4319    <date month="&ID-MONTH;" year="&ID-YEAR;"/>
4320  </front>
4321  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p2-semantics-&ID-VERSION;"/>
4322  <x:source href="p2-semantics.xml" basename="p2-semantics">
4323    <x:defines>1xx (Informational)</x:defines>
4324    <x:defines>1xx</x:defines>
4325    <x:defines>100 (Continue)</x:defines>
4326    <x:defines>101 (Switching Protocols)</x:defines>
4327    <x:defines>2xx (Successful)</x:defines>
4328    <x:defines>2xx</x:defines>
4329    <x:defines>200 (OK)</x:defines>
4330    <x:defines>203 (Non-Authoritative Information)</x:defines>
4331    <x:defines>204 (No Content)</x:defines>
4332    <x:defines>3xx (Redirection)</x:defines>
4333    <x:defines>3xx</x:defines>
4334    <x:defines>301 (Moved Permanently)</x:defines>
4335    <x:defines>4xx (Client Error)</x:defines>
4336    <x:defines>4xx</x:defines>
4337    <x:defines>400 (Bad Request)</x:defines>
4338    <x:defines>411 (Length Required)</x:defines>
4339    <x:defines>414 (URI Too Long)</x:defines>
4340    <x:defines>417 (Expectation Failed)</x:defines>
4341    <x:defines>426 (Upgrade Required)</x:defines>
4342    <x:defines>501 (Not Implemented)</x:defines>
4343    <x:defines>502 (Bad Gateway)</x:defines>
4344    <x:defines>505 (HTTP Version Not Supported)</x:defines>
4345    <x:defines>Accept-Encoding</x:defines>
4346    <x:defines>Allow</x:defines>
4347    <x:defines>Content-Encoding</x:defines>
4348    <x:defines>Content-Location</x:defines>
4349    <x:defines>Content-Type</x:defines>
4350    <x:defines>Date</x:defines>
4351    <x:defines>Expect</x:defines>
4352    <x:defines>Location</x:defines>
4353    <x:defines>Server</x:defines>
4354    <x:defines>User-Agent</x:defines>
4355  </x:source>
4358<reference anchor="Part4">
4359  <front>
4360    <title>Hypertext Transfer Protocol (HTTP/1.1): Conditional Requests</title>
4361    <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
4362      <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
4363      <address><email></email></address>
4364    </author>
4365    <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
4366      <organization abbrev="greenbytes">greenbytes GmbH</organization>
4367      <address><email></email></address>
4368    </author>
4369    <date month="&ID-MONTH;" year="&ID-YEAR;" />
4370  </front>
4371  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p4-conditional-&ID-VERSION;" />
4372  <x:source basename="p4-conditional" href="p4-conditional.xml">
4373    <x:defines>304 (Not Modified)</x:defines>
4374    <x:defines>ETag</x:defines>
4375    <x:defines>Last-Modified</x:defines>
4376  </x:source>
4379<reference anchor="Part5">
4380  <front>
4381    <title>Hypertext Transfer Protocol (HTTP/1.1): Range Requests</title>
4382    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
4383      <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
4384      <address><email></email></address>
4385    </author>
4386    <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
4387      <organization abbrev="W3C">World Wide Web Consortium</organization>
4388      <address><email></email></address>
4389    </author>
4390    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
4391      <organization abbrev="greenbytes">greenbytes GmbH</organization>
4392      <address><email></email></address>
4393    </author>
4394    <date month="&ID-MONTH;" year="&ID-YEAR;"/>
4395  </front>
4396  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p5-range-&ID-VERSION;"/>
4397  <x:source href="p5-range.xml" basename="p5-range">
4398    <x:defines>Content-Range</x:defines>
4399  </x:source>
4402<reference anchor="Part6">
4403  <front>
4404    <title>Hypertext Transfer Protocol (HTTP/1.1): Caching</title>
4405    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
4406      <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
4407      <address><email></email></address>
4408    </author>
4409    <author initials="M." surname="Nottingham" fullname="Mark Nottingham" role="editor">
4410      <organization>Akamai</organization>
4411      <address><email></email></address>
4412    </author>
4413    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
4414      <organization abbrev="greenbytes">greenbytes GmbH</organization>
4415      <address><email></email></address>
4416    </author>
4417    <date month="&ID-MONTH;" year="&ID-YEAR;"/>
4418  </front>
4419  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p6-cache-&ID-VERSION;"/>
4420  <x:source href="p6-cache.xml" basename="p6-cache">
4421    <x:defines>Cache-Control</x:defines>
4422    <x:defines>Expires</x:defines>
4423  </x:source>
4426<reference anchor="Part7">
4427  <front>
4428    <title>Hypertext Transfer Protocol (HTTP/1.1): Authentication</title>
4429    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
4430      <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
4431      <address><email></email></address>
4432    </author>
4433    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
4434      <organization abbrev="greenbytes">greenbytes GmbH</organization>
4435      <address><email></email></address>
4436    </author>
4437    <date month="&ID-MONTH;" year="&ID-YEAR;"/>
4438  </front>
4439  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p7-auth-&ID-VERSION;"/>
4440  <x:source href="p7-auth.xml" basename="p7-auth">
4441    <x:defines>Proxy-Authenticate</x:defines>
4442    <x:defines>Proxy-Authorization</x:defines>
4443  </x:source>
4446<reference anchor="RFC5234">
4447  <front>
4448    <title abbrev="ABNF for Syntax Specifications">Augmented BNF for Syntax Specifications: ABNF</title>
4449    <author initials="D." surname="Crocker" fullname="Dave Crocker" role="editor">
4450      <organization>Brandenburg InternetWorking</organization>
4451      <address>
4452        <email></email>
4453      </address> 
4454    </author>
4455    <author initials="P." surname="Overell" fullname="Paul Overell">
4456      <organization>THUS plc.</organization>
4457      <address>
4458        <email></email>
4459      </address>
4460    </author>
4461    <date month="January" year="2008"/>
4462  </front>
4463  <seriesInfo name="STD" value="68"/>
4464  <seriesInfo name="RFC" value="5234"/>
4467<reference anchor="RFC2119">
4468  <front>
4469    <title>Key words for use in RFCs to Indicate Requirement Levels</title>
4470    <author initials="S." surname="Bradner" fullname="Scott Bradner">
4471      <organization>Harvard University</organization>
4472      <address><email></email></address>
4473    </author>
4474    <date month="March" year="1997"/>
4475  </front>
4476  <seriesInfo name="BCP" value="14"/>
4477  <seriesInfo name="RFC" value="2119"/>
4480<reference anchor="RFC3986">
4481 <front>
4482  <title abbrev='URI Generic Syntax'>Uniform Resource Identifier (URI): Generic Syntax</title>
4483  <author initials='T.' surname='Berners-Lee' fullname='Tim Berners-Lee'>
4484    <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
4485    <address>
4486       <email></email>
4487       <uri></uri>
4488    </address>
4489  </author>
4490  <author initials='R.' surname='Fielding' fullname='Roy T. Fielding'>
4491    <organization abbrev="Day Software">Day Software</organization>
4492    <address>
4493      <email></email>
4494      <uri></uri>
4495    </address>
4496  </author>
4497  <author initials='L.' surname='Masinter' fullname='Larry Masinter'>
4498    <organization abbrev="Adobe Systems">Adobe Systems Incorporated</organization>
4499    <address>
4500      <email></email>
4501      <uri></uri>
4502    </address>
4503  </author>
4504  <date month='January' year='2005'></date>
4505 </front>
4506 <seriesInfo name="STD" value="66"/>
4507 <seriesInfo name="RFC" value="3986"/>
4510<reference anchor="RFC0793">
4511  <front>
4512    <title>Transmission Control Protocol</title>
4513    <author initials='J.' surname='Postel' fullname='Jon Postel'>
4514      <organization>University of Southern California (USC)/Information Sciences Institute</organization>
4515    </author>
4516    <date year='1981' month='September' />
4517  </front>
4518  <seriesInfo name='STD' value='7' />
4519  <seriesInfo name='RFC' value='793' />
4522<reference anchor="USASCII">
4523  <front>
4524    <title>Coded Character Set -- 7-bit American Standard Code for Information Interchange</title>
4525    <author>
4526      <organization>American National Standards Institute</organization>
4527    </author>
4528    <date year="1986"/>
4529  </front>
4530  <seriesInfo name="ANSI" value="X3.4"/>
4533<reference anchor="RFC1950">
4534  <front>
4535    <title>ZLIB Compressed Data Format Specification version 3.3</title>
4536    <author initials="L.P." surname="Deutsch" fullname="L. Peter Deutsch">
4537      <organization>Aladdin Enterprises</organization>
4538      <address><email></email></address>
4539    </author>
4540    <author initials="J-L." surname="Gailly" fullname="Jean-Loup Gailly"/>
4541    <date month="May" year="1996"/>
4542  </front>
4543  <seriesInfo name="RFC" value="1950"/>
4544  <!--<annotation>
4545    RFC 1950 is an Informational RFC, thus it might be less stable than
4546    this specification. On the other hand, this downward reference was
4547    present since the publication of <xref target="RFC2068" x:fmt="none">RFC 2068</xref> in 1997,
4548    therefore it is unlikely to cause problems in practice. See also
4549    <xref target="BCP97"/>.
4550  </annotation>-->
4553<reference anchor="RFC1951">
4554  <front>
4555    <title>DEFLATE Compressed Data Format Specification version 1.3</title>
4556    <author initials="P." surname="Deutsch" fullname="L. Peter Deutsch">
4557      <organization>Aladdin Enterprises</organization>
4558      <address><email></email></address>
4559    </author>
4560    <date month="May" year="1996"/>
4561  </front>
4562  <seriesInfo name="RFC" value="1951"/>
4563  <!--<annotation>
4564    RFC 1951 is an Informational RFC, thus it might be less stable than
4565    this specification. On the other hand, this downward reference was
4566    present since the publication of <xref target="RFC2068" x:fmt="none">RFC 2068</xref> in 1997,
4567    therefore it is unlikely to cause problems in practice. See also
4568    <xref target="BCP97"/>.
4569  </annotation>-->
4572<reference anchor="RFC1952">
4573  <front>
4574    <title>GZIP file format specification version 4.3</title>
4575    <author initials="P." surname="Deutsch" fullname="L. Peter Deutsch">
4576      <organization>Aladdin Enterprises</organization>
4577      <address><email></email></address>
4578    </author>
4579    <author initials="J-L." surname="Gailly" fullname="Jean-Loup Gailly">
4580      <address><email></email></address>
4581    </author>
4582    <author initials="M." surname="Adler" fullname="Mark Adler">
4583      <address><email></email></address>
4584    </author>
4585    <author initials="L.P." surname="Deutsch" fullname="L. Peter Deutsch">
4586      <address><email></email></address>
4587    </author>
4588    <author initials="G." surname="Randers-Pehrson" fullname="Glenn Randers-Pehrson">
4589      <address><email></email></address>
4590    </author>
4591    <date month="May" year="1996"/>
4592  </front>
4593  <seriesInfo name="RFC" value="1952"/>
4594  <!--<annotation>
4595    RFC 1952 is an Informational RFC, thus it might be less stable than
4596    this specification. On the other hand, this downward reference was
4597    present since the publication of <xref target="RFC2068" x:fmt="none">RFC 2068</xref> in 1997,
4598    therefore it is unlikely to cause problems in practice. See also
4599    <xref target="BCP97"/>.
4600  </annotation>-->
4603<reference anchor="Welch">
4604  <front>
4605    <title>A Technique for High Performance Data Compression</title>
4606    <author initials="T.A." surname="Welch" fullname="Terry A. Welch"/>
4607    <date month="June" year="1984"/>
4608  </front>
4609  <seriesInfo name="IEEE Computer" value="17(6)"/>
4614<references title="Informative References">
4616<reference anchor="ISO-8859-1">
4617  <front>
4618    <title>
4619     Information technology -- 8-bit single-byte coded graphic character sets -- Part 1: Latin alphabet No. 1
4620    </title>
4621    <author>
4622      <organization>International Organization for Standardization</organization>
4623    </author>
4624    <date year="1998"/>
4625  </front>
4626  <seriesInfo name="ISO/IEC" value="8859-1:1998"/>
4629<reference anchor='RFC1919'>
4630  <front>
4631    <title>Classical versus Transparent IP Proxies</title>
4632    <author initials='M.' surname='Chatel' fullname='Marc Chatel'>
4633      <address><email></email></address>
4634    </author>
4635    <date year='1996' month='March' />
4636  </front>
4637  <seriesInfo name='RFC' value='1919' />
4640<reference anchor="RFC1945">
4641  <front>
4642    <title abbrev="HTTP/1.0">Hypertext Transfer Protocol -- HTTP/1.0</title>
4643    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
4644      <organization>MIT, Laboratory for Computer Science</organization>
4645      <address><email></email></address>
4646    </author>
4647    <author initials="R.T." surname="Fielding" fullname="Roy T. Fielding">
4648      <organization>University of California, Irvine, Department of Information and Computer Science</organization>
4649      <address><email></email></address>
4650    </author>
4651    <author initials="H.F." surname="Nielsen" fullname="Henrik Frystyk Nielsen">
4652      <organization>W3 Consortium, MIT Laboratory for Computer Science</organization>
4653      <address><email></email></address>
4654    </author>
4655    <date month="May" year="1996"/>
4656  </front>
4657  <seriesInfo name="RFC" value="1945"/>
4660<reference anchor="RFC2045">
4661  <front>
4662    <title abbrev="Internet Message Bodies">Multipurpose Internet Mail Extensions (MIME) Part One: Format of Internet Message Bodies</title>
4663    <author initials="N." surname="Freed" fullname="Ned Freed">
4664      <organization>Innosoft International, Inc.</organization>
4665      <address><email></email></address>
4666    </author>
4667    <author initials="N.S." surname="Borenstein" fullname="Nathaniel S. Borenstein">
4668      <organization>First Virtual Holdings</organization>
4669      <address><email></email></address>
4670    </author>
4671    <date month="November" year="1996"/>
4672  </front>
4673  <seriesInfo name="RFC" value="2045"/>
4676<reference anchor="RFC2047">
4677  <front>
4678    <title abbrev="Message Header Extensions">MIME (Multipurpose Internet Mail Extensions) Part Three: Message Header Extensions for Non-ASCII Text</title>
4679    <author initials="K." surname="Moore" fullname="Keith Moore">
4680      <organization>University of Tennessee</organization>
4681      <address><email></email></address>
4682    </author>
4683    <date month="November" year="1996"/>
4684  </front>
4685  <seriesInfo name="RFC" value="2047"/>
4688<reference anchor="RFC2068">
4689  <front>
4690    <title>Hypertext Transfer Protocol -- HTTP/1.1</title>
4691    <author initials="R." surname="Fielding" fullname="Roy T. Fielding">
4692      <organization>University of California, Irvine, Department of Information and Computer Science</organization>
4693      <address><email></email></address>
4694    </author>
4695    <author initials="J." surname="Gettys" fullname="Jim Gettys">
4696      <organization>MIT Laboratory for Computer Science</organization>
4697      <address><email></email></address>
4698    </author>
4699    <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
4700      <organization>Digital Equipment Corporation, Western Research Laboratory</organization>
4701      <address><email></email></address>
4702    </author>
4703    <author initials="H." surname="Nielsen" fullname="Henrik Frystyk Nielsen">
4704      <organization>MIT Laboratory for Computer Science</organization>
4705      <address><email></email></address>
4706    </author>
4707    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
4708      <organization>MIT Laboratory for Computer Science</organization>
4709      <address><email></email></address>
4710    </author>
4711    <date month="January" year="1997"/>
4712  </front>
4713  <seriesInfo name="RFC" value="2068"/>
4716<reference anchor="RFC2145">
4717  <front>
4718    <title abbrev="HTTP Version Numbers">Use and Interpretation of HTTP Version Numbers</title>
4719    <author initials="J.C." surname="Mogul" fullname="Jeffrey C. Mogul">
4720      <organization>Western Research Laboratory</organization>
4721      <address><email></email></address>
4722    </author>
4723    <author initials="R.T." surname="Fielding" fullname="Roy T. Fielding">
4724      <organization>Department of Information and Computer Science</organization>
4725      <address><email></email></address>
4726    </author>
4727    <author initials="J." surname="Gettys" fullname="Jim Gettys">
4728      <organization>MIT Laboratory for Computer Science</organization>
4729      <address><email></email></address>
4730    </author>
4731    <author initials="H.F." surname="Nielsen" fullname="Henrik Frystyk Nielsen">
4732      <organization>W3 Consortium</organization>
4733      <address><email></email></address>
4734    </author>
4735    <date month="May" year="1997"/>
4736  </front>
4737  <seriesInfo name="RFC" value="2145"/>
4740<reference anchor="RFC2616">
4741  <front>
4742    <title>Hypertext Transfer Protocol -- HTTP/1.1</title>
4743    <author initials="R." surname="Fielding" fullname="R. Fielding">
4744      <organization>University of California, Irvine</organization>
4745      <address><email></email></address>
4746    </author>
4747    <author initials="J." surname="Gettys" fullname="J. Gettys">
4748      <organization>W3C</organization>
4749      <address><email></email></address>
4750    </author>
4751    <author initials="J." surname="Mogul" fullname="J. Mogul">
4752      <organization>Compaq Computer Corporation</organization>
4753      <address><email></email></address>
4754    </author>
4755    <author initials="H." surname="Frystyk" fullname="H. Frystyk">
4756      <organization>MIT Laboratory for Computer Science</organization>
4757      <address><email></email></address>
4758    </author>
4759    <author initials="L." surname="Masinter" fullname="L. Masinter">
4760      <organization>Xerox Corporation</organization>
4761      <address><email></email></address>
4762    </author>
4763    <author initials="P." surname="Leach" fullname="P. Leach">
4764      <organization>Microsoft Corporation</organization>
4765      <address><email></email></address>
4766    </author>
4767    <author initials="T." surname="Berners-Lee" fullname="T. Berners-Lee">
4768      <organization>W3C</organization>
4769      <address><email></email></address>
4770    </author>
4771    <date month="June" year="1999"/>
4772  </front>
4773  <seriesInfo name="RFC" value="2616"/>
4776<reference anchor='RFC2817'>
4777  <front>
4778    <title>Upgrading to TLS Within HTTP/1.1</title>
4779    <author initials='R.' surname='Khare' fullname='R. Khare'>
4780      <organization>4K Associates / UC Irvine</organization>
4781      <address><email></email></address>
4782    </author>
4783    <author initials='S.' surname='Lawrence' fullname='S. Lawrence'>
4784      <organization>Agranat Systems, Inc.</organization>
4785      <address><email></email></address>
4786    </author>
4787    <date year='2000' month='May' />
4788  </front>
4789  <seriesInfo name='RFC' value='2817' />
4792<reference anchor='RFC2818'>
4793  <front>
4794    <title>HTTP Over TLS</title>
4795    <author initials='E.' surname='Rescorla' fullname='Eric Rescorla'>
4796      <organization>RTFM, Inc.</organization>
4797      <address><email></email></address>
4798    </author>
4799    <date year='2000' month='May' />
4800  </front>
4801  <seriesInfo name='RFC' value='2818' />
4804<reference anchor='RFC3040'>
4805  <front>
4806    <title>Internet Web Replication and Caching Taxonomy</title>
4807    <author initials='I.' surname='Cooper' fullname='I. Cooper'>
4808      <organization>Equinix, Inc.</organization>
4809    </author>
4810    <author initials='I.' surname='Melve' fullname='I. Melve'>
4811      <organization>UNINETT</organization>
4812    </author>
4813    <author initials='G.' surname='Tomlinson' fullname='G. Tomlinson'>
4814      <organization>CacheFlow Inc.</organization>
4815    </author>
4816    <date year='2001' month='January' />
4817  </front>
4818  <seriesInfo name='RFC' value='3040' />
4821<reference anchor='BCP90'>
4822  <front>
4823    <title>Registration Procedures for Message Header Fields</title>
4824    <author initials='G.' surname='Klyne' fullname='G. Klyne'>
4825      <organization>Nine by Nine</organization>
4826      <address><email></email></address>
4827    </author>
4828    <author initials='M.' surname='Nottingham' fullname='M. Nottingham'>
4829      <organization>BEA Systems</organization>
4830      <address><email></email></address>
4831    </author>
4832    <author initials='J.' surname='Mogul' fullname='J. Mogul'>
4833      <organization>HP Labs</organization>
4834      <address><email></email></address>
4835    </author>
4836    <date year='2004' month='September' />
4837  </front>
4838  <seriesInfo name='BCP' value='90' />
4839  <seriesInfo name='RFC' value='3864' />
4842<reference anchor='RFC4033'>
4843  <front>
4844    <title>DNS Security Introduction and Requirements</title>
4845    <author initials='R.' surname='Arends' fullname='R. Arends'/>
4846    <author initials='R.' surname='Austein' fullname='R. Austein'/>
4847    <author initials='M.' surname='Larson' fullname='M. Larson'/>
4848    <author initials='D.' surname='Massey' fullname='D. Massey'/>
4849    <author initials='S.' surname='Rose' fullname='S. Rose'/>
4850    <date year='2005' month='March' />
4851  </front>
4852  <seriesInfo name='RFC' value='4033' />
4855<reference anchor="BCP13">
4856  <front>
4857    <title>Media Type Specifications and Registration Procedures</title>
4858    <author initials="N." surname="Freed" fullname="Ned Freed">
4859      <organization>Oracle</organization>
4860      <address>
4861        <email></email>
4862      </address>
4863    </author>
4864    <author initials="J." surname="Klensin" fullname="John C. Klensin">
4865      <address>
4866        <email></email>
4867      </address>
4868    </author>
4869    <author initials="T." surname="Hansen" fullname="Tony Hansen">
4870      <organization>AT&amp;T Laboratories</organization>
4871      <address>
4872        <email></email>
4873      </address>
4874    </author>
4875    <date year="2013" month="January"/>
4876  </front>
4877  <seriesInfo name="BCP" value="13"/>
4878  <seriesInfo name="RFC" value="6838"/>
4881<reference anchor='BCP115'>
4882  <front>
4883    <title>Guidelines and Registration Procedures for New URI Schemes</title>
4884    <author initials='T.' surname='Hansen' fullname='T. Hansen'>
4885      <organization>AT&amp;T Laboratories</organization>
4886      <address>
4887        <email></email>
4888      </address>
4889    </author>
4890    <author initials='T.' surname='Hardie' fullname='T. Hardie'>
4891      <organization>Qualcomm, Inc.</organization>
4892      <address>
4893        <email></email>
4894      </address>
4895    </author>
4896    <author initials='L.' surname='Masinter' fullname='L. Masinter'>
4897      <organization>Adobe Systems</organization>
4898      <address>
4899        <email></email>
4900      </address>
4901    </author>
4902    <date year='2006' month='February' />
4903  </front>
4904  <seriesInfo name='BCP' value='115' />
4905  <seriesInfo name='RFC' value='4395' />
4908<reference anchor='RFC4559'>
4909  <front>
4910    <title>SPNEGO-based Kerberos and NTLM HTTP Authentication in Microsoft Windows</title>
4911    <author initials='K.' surname='Jaganathan' fullname='K. Jaganathan'/>
4912    <author initials='L.' surname='Zhu' fullname='L. Zhu'/>
4913    <author initials='J.' surname='Brezak' fullname='J. Brezak'/>
4914    <date year='2006' month='June' />
4915  </front>
4916  <seriesInfo name='RFC' value='4559' />
4919<reference anchor='RFC5226'>
4920  <front>
4921    <title>Guidelines for Writing an IANA Considerations Section in RFCs</title>
4922    <author initials='T.' surname='Narten' fullname='T. Narten'>
4923      <organization>IBM</organization>
4924      <address><email></email></address>
4925    </author>
4926    <author initials='H.' surname='Alvestrand' fullname='H. Alvestrand'>
4927      <organization>Google</organization>
4928      <address><email></email></address>
4929    </author>
4930    <date year='2008' month='May' />
4931  </front>
4932  <seriesInfo name='BCP' value='26' />
4933  <seriesInfo name='RFC' value='5226' />
4936<reference anchor='RFC5246'>
4937   <front>
4938      <title>The Transport Layer Security (TLS) Protocol Version 1.2</title>
4939      <author initials='T.' surname='Dierks' fullname='T. Dierks'/>
4940      <author initials='E.' surname='Rescorla' fullname='E. Rescorla'>
4941         <organization>RTFM, Inc.</organization>
4942      </author>
4943      <date year='2008' month='August' />
4944   </front>
4945   <seriesInfo name='RFC' value='5246' />
4948<reference anchor="RFC5322">
4949  <front>
4950    <title>Internet Message Format</title>
4951    <author initials="P." surname="Resnick" fullname="P. Resnick">
4952      <organization>Qualcomm Incorporated</organization>
4953    </author>
4954    <date year="2008" month="October"/>
4955  </front>
4956  <seriesInfo name="RFC" value="5322"/>
4959<reference anchor="RFC6265">
4960  <front>
4961    <title>HTTP State Management Mechanism</title>
4962    <author initials="A." surname="Barth" fullname="Adam Barth">
4963      <organization abbrev="U.C. Berkeley">
4964        University of California, Berkeley
4965      </organization>
4966      <address><email></email></address>
4967    </author>
4968    <date year="2011" month="April" />
4969  </front>
4970  <seriesInfo name="RFC" value="6265"/>
4973<reference anchor='RFC6585'>
4974  <front>
4975    <title>Additional HTTP Status Codes</title>
4976    <author initials='M.' surname='Nottingham' fullname='M. Nottingham'>
4977      <organization>Rackspace</organization>
4978    </author>
4979    <author initials='R.' surname='Fielding' fullname='R. Fielding'>
4980      <organization>Adobe</organization>
4981    </author>
4982    <date year='2012' month='April' />
4983   </front>
4984   <seriesInfo name='RFC' value='6585' />
4987<!--<reference anchor='BCP97'>
4988  <front>
4989    <title>Handling Normative References to Standards-Track Documents</title>
4990    <author initials='J.' surname='Klensin' fullname='J. Klensin'>
4991      <address>
4992        <email></email>
4993      </address>
4994    </author>
4995    <author initials='S.' surname='Hartman' fullname='S. Hartman'>
4996      <organization>MIT</organization>
4997      <address>
4998        <email></email>
4999      </address>
5000    </author>
5001    <date year='2007' month='June' />
5002  </front>
5003  <seriesInfo name='BCP' value='97' />
5004  <seriesInfo name='RFC' value='4897' />
5007<reference anchor="Kri2001" target="">
5008  <front>
5009    <title>HTTP Cookies: Standards, Privacy, and Politics</title>
5010    <author initials="D." surname="Kristol" fullname="David M. Kristol"/>
5011    <date year="2001" month="November"/>
5012  </front>
5013  <seriesInfo name="ACM Transactions on Internet Technology" value="1(2)"/>
5019<section title="HTTP Version History" anchor="compatibility">
5021   HTTP has been in use since 1990. The first version, later referred to as
5022   HTTP/0.9, was a simple protocol for hypertext data transfer across the
5023   Internet, using only a single request method (GET) and no metadata.
5024   HTTP/1.0, as defined by <xref target="RFC1945"/>, added a range of request
5025   methods and MIME-like messaging, allowing for metadata to be transferred
5026   and modifiers placed on the request/response semantics. However,
5027   HTTP/1.0 did not sufficiently take into consideration the effects of
5028   hierarchical proxies, caching, the need for persistent connections, or
5029   name-based virtual hosts. The proliferation of incompletely-implemented
5030   applications calling themselves "HTTP/1.0" further necessitated a
5031   protocol version change in order for two communicating applications
5032   to determine each other's true capabilities.
5035   HTTP/1.1 remains compatible with HTTP/1.0 by including more stringent
5036   requirements that enable reliable implementations, adding only
5037   those features that can either be safely ignored by an HTTP/1.0
5038   recipient or only sent when communicating with a party advertising
5039   conformance with HTTP/1.1.
5042   HTTP/1.1 has been designed to make supporting previous versions easy.
5043   A general-purpose HTTP/1.1 server ought to be able to understand any valid
5044   request in the format of HTTP/1.0, responding appropriately with an
5045   HTTP/1.1 message that only uses features understood (or safely ignored) by
5046   HTTP/1.0 clients. Likewise, an HTTP/1.1 client can be expected to
5047   understand any valid HTTP/1.0 response.
5050   Since HTTP/0.9 did not support header fields in a request, there is no
5051   mechanism for it to support name-based virtual hosts (selection of resource
5052   by inspection of the <x:ref>Host</x:ref> header field).
5053   Any server that implements name-based virtual hosts ought to disable
5054   support for HTTP/0.9. Most requests that appear to be HTTP/0.9 are, in
5055   fact, badly constructed HTTP/1.x requests caused by a client failing to
5056   properly encode the request-target.
5059<section title="Changes from HTTP/1.0" anchor="changes.from.1.0">
5061   This section summarizes major differences between versions HTTP/1.0
5062   and HTTP/1.1.
5065<section title="Multi-homed Web Servers" anchor="">
5067   The requirements that clients and servers support the <x:ref>Host</x:ref>
5068   header field (<xref target=""/>), report an error if it is
5069   missing from an HTTP/1.1 request, and accept absolute URIs (<xref target="request-target"/>)
5070   are among the most important changes defined by HTTP/1.1.
5073   Older HTTP/1.0 clients assumed a one-to-one relationship of IP
5074   addresses and servers; there was no other established mechanism for
5075   distinguishing the intended server of a request than the IP address
5076   to which that request was directed. The <x:ref>Host</x:ref> header field was
5077   introduced during the development of HTTP/1.1 and, though it was
5078   quickly implemented by most HTTP/1.0 browsers, additional requirements
5079   were placed on all HTTP/1.1 requests in order to ensure complete
5080   adoption.  At the time of this writing, most HTTP-based services
5081   are dependent upon the Host header field for targeting requests.
5085<section title="Keep-Alive Connections" anchor="compatibility.with.http.1.0.persistent.connections">
5087   In HTTP/1.0, each connection is established by the client prior to the
5088   request and closed by the server after sending the response. However, some
5089   implementations implement the explicitly negotiated ("Keep-Alive") version
5090   of persistent connections described in <xref x:sec="19.7.1" x:fmt="of"
5091   target="RFC2068"/>.
5094   Some clients and servers might wish to be compatible with these previous
5095   approaches to persistent connections, by explicitly negotiating for them
5096   with a "Connection: keep-alive" request header field. However, some
5097   experimental implementations of HTTP/1.0 persistent connections are faulty;
5098   for example, if an HTTP/1.0 proxy server doesn't understand
5099   <x:ref>Connection</x:ref>, it will erroneously forward that header field
5100   to the next inbound server, which would result in a hung connection.
5103   One attempted solution was the introduction of a Proxy-Connection header
5104   field, targeted specifically at proxies. In practice, this was also
5105   unworkable, because proxies are often deployed in multiple layers, bringing
5106   about the same problem discussed above.
5109   As a result, clients are encouraged not to send the Proxy-Connection header
5110   field in any requests.
5113   Clients are also encouraged to consider the use of Connection: keep-alive
5114   in requests carefully; while they can enable persistent connections with
5115   HTTP/1.0 servers, clients using them will need to monitor the
5116   connection for "hung" requests (which indicate that the client ought stop
5117   sending the header field), and this mechanism ought not be used by clients
5118   at all when a proxy is being used.
5122<section title="Introduction of Transfer-Encoding" anchor="introduction.of.transfer-encoding">
5124   HTTP/1.1 introduces the <x:ref>Transfer-Encoding</x:ref> header field
5125   (<xref target="header.transfer-encoding"/>).
5126   Transfer codings need to be decoded prior to forwarding an HTTP message
5127   over a MIME-compliant protocol.
5133<section title="Changes from RFC 2616" anchor="changes.from.rfc.2616">
5135  HTTP's approach to error handling has been explained.
5136  (<xref target="conformance" />)
5139  The HTTP-version ABNF production has been clarified to be case-sensitive.
5140  Additionally, version numbers has been restricted to single digits, due
5141  to the fact that implementations are known to handle multi-digit version
5142  numbers incorrectly.
5143  (<xref target="http.version"/>)
5146  Userinfo (i.e., username and password) are now disallowed in HTTP and
5147  HTTPS URIs, because of security issues related to their transmission on the
5148  wire.
5149  (<xref target="http.uri" />)
5152  The HTTPS URI scheme is now defined by this specification; previously,
5153  it was done in  <xref target="RFC2818" x:fmt="of" x:sec="2.4"/>.
5154  Furthermore, it implies end-to-end security.
5155  (<xref target="https.uri"/>)
5158  HTTP messages can be (and often are) buffered by implementations; despite
5159  it sometimes being available as a stream, HTTP is fundamentally a
5160  message-oriented protocol.
5161  Minimum supported sizes for various protocol elements have been
5162  suggested, to improve interoperability.
5163  (<xref target="http.message" />)
5166  Invalid whitespace around field-names is now required to be rejected,
5167  because accepting it represents a security vulnerability.
5168  The ABNF productions defining header fields now only list the field value.
5169  (<xref target="header.fields"/>)
5172  Rules about implicit linear whitespace between certain grammar productions
5173  have been removed; now whitespace is only allowed where specifically
5174  defined in the ABNF.
5175  (<xref target="whitespace"/>)
5178  Header fields that span multiple lines ("line folding") are deprecated.
5179  (<xref target="field.parsing" />)
5182  The NUL octet is no longer allowed in comment and quoted-string text, and
5183  handling of backslash-escaping in them has been clarified.
5184  The quoted-pair rule no longer allows escaping control characters other than
5185  HTAB.
5186  Non-ASCII content in header fields and the reason phrase has been obsoleted
5187  and made opaque (the TEXT rule was removed).
5188  (<xref target="field.components"/>)
5191  Bogus "<x:ref>Content-Length</x:ref>" header fields are now required to be
5192  handled as errors by recipients.
5193  (<xref target="header.content-length"/>)
5196  The algorithm for determining the message body length has been clarified
5197  to indicate all of the special cases (e.g., driven by methods or status
5198  codes) that affect it, and that new protocol elements cannot define such
5199  special cases.
5200  CONNECT is a new, special case in determining message body length.
5201  "multipart/byteranges" is no longer a way of determining message body length
5202  detection.
5203  (<xref target="message.body.length"/>)
5206  The "identity" transfer coding token has been removed.
5207  (Sections <xref format="counter" target="message.body"/> and
5208  <xref format="counter" target="transfer.codings"/>)
5211  Chunk length does not include the count of the octets in the
5212  chunk header and trailer.
5213  Line folding in chunk extensions is  disallowed.
5214  (<xref target="chunked.encoding"/>)
5217  The meaning of the "deflate" content coding has been clarified.
5218  (<xref target="deflate.coding" />)
5221  The segment + query components of RFC 3986 have been used to define the
5222  request-target, instead of abs_path from RFC 1808.
5223  The asterisk-form of the request-target is only allowed with the OPTIONS
5224  method.
5225  (<xref target="request-target"/>)
5228  The term "Effective Request URI" has been introduced.
5229  (<xref target="effective.request.uri" />)
5232  Gateways do not need to generate <x:ref>Via</x:ref> header fields anymore.
5233  (<xref target="header.via"/>)
5236  Exactly when "close" connection options have to be sent has been clarified.
5237  Also, "hop-by-hop" header fields are required to appear in the Connection header
5238  field; just because they're defined as hop-by-hop in this specification
5239  doesn't exempt them.
5240  (<xref target="header.connection"/>)
5243  The limit of two connections per server has been removed.
5244  An idempotent sequence of requests is no longer required to be retried.
5245  The requirement to retry requests under certain circumstances when the
5246  server prematurely closes the connection has been removed.
5247  Also, some extraneous requirements about when servers are allowed to close
5248  connections prematurely have been removed.
5249  (<xref target="persistent.connections"/>)
5252  The semantics of the <x:ref>Upgrade</x:ref> header field is now defined in
5253  responses other than 101 (this was incorporated from <xref
5254  target="RFC2817"/>). Furthermore, the ordering in the field value is now
5255  significant.
5256  (<xref target="header.upgrade"/>)
5259  Empty list elements in list productions (e.g., a list header field containing
5260  ", ,") have been deprecated.
5261  (<xref target="abnf.extension"/>)
5264  Registration of Transfer Codings now requires IETF Review
5265  (<xref target="transfer.coding.registry"/>)
5268  This specification now defines the Upgrade Token Registry, previously
5269  defined in <xref target="RFC2817" x:fmt="of" x:sec="7.2"/>.
5270  (<xref target="upgrade.token.registry"/>)
5273  The expectation to support HTTP/0.9 requests has been removed.
5274  (<xref target="compatibility"/>)
5277  Issues with the Keep-Alive and Proxy-Connection header fields in requests
5278  are pointed out, with use of the latter being discouraged altogether.
5279  (<xref target="compatibility.with.http.1.0.persistent.connections" />)
5284<?BEGININC p1-messaging.abnf-appendix ?>
5285<section xmlns:x="" title="Collected ABNF" anchor="collected.abnf">
5287<artwork type="abnf" name="p1-messaging.parsed-abnf">
5288<x:ref>BWS</x:ref> = OWS
5290<x:ref>Connection</x:ref> = *( "," OWS ) connection-option *( OWS "," [ OWS
5291 connection-option ] )
5292<x:ref>Content-Length</x:ref> = 1*DIGIT
5294<x:ref>HTTP-message</x:ref> = start-line *( header-field CRLF ) CRLF [ message-body
5295 ]
5296<x:ref>HTTP-name</x:ref> = %x48.54.54.50 ; HTTP
5297<x:ref>HTTP-version</x:ref> = HTTP-name "/" DIGIT "." DIGIT
5298<x:ref>Host</x:ref> = uri-host [ ":" port ]
5300<x:ref>OWS</x:ref> = *( SP / HTAB )
5302<x:ref>RWS</x:ref> = 1*( SP / HTAB )
5304<x:ref>TE</x:ref> = [ ( "," / t-codings ) *( OWS "," [ OWS t-codings ] ) ]
5305<x:ref>Trailer</x:ref> = *( "," OWS ) field-name *( OWS "," [ OWS field-name ] )
5306<x:ref>Transfer-Encoding</x:ref> = *( "," OWS ) transfer-coding *( OWS "," [ OWS
5307 transfer-coding ] )
5309<x:ref>URI-reference</x:ref> = &lt;URI-reference, defined in [RFC3986], Section 4.1&gt;
5310<x:ref>Upgrade</x:ref> = *( "," OWS ) protocol *( OWS "," [ OWS protocol ] )
5312<x:ref>Via</x:ref> = *( "," OWS ) ( received-protocol RWS received-by [ RWS comment
5313 ] ) *( OWS "," [ OWS ( received-protocol RWS received-by [ RWS
5314 comment ] ) ] )
5316<x:ref>absolute-URI</x:ref> = &lt;absolute-URI, defined in [RFC3986], Section 4.3&gt;
5317<x:ref>absolute-form</x:ref> = absolute-URI
5318<x:ref>absolute-path</x:ref> = 1*( "/" segment )
5319<x:ref>asterisk-form</x:ref> = "*"
5320<x:ref>authority</x:ref> = &lt;authority, defined in [RFC3986], Section 3.2&gt;
5321<x:ref>authority-form</x:ref> = authority
5323<x:ref>chunk</x:ref> = chunk-size [ chunk-ext ] CRLF chunk-data CRLF
5324<x:ref>chunk-data</x:ref> = 1*OCTET
5325<x:ref>chunk-ext</x:ref> = *( ";" chunk-ext-name [ "=" chunk-ext-val ] )
5326<x:ref>chunk-ext-name</x:ref> = token
5327<x:ref>chunk-ext-val</x:ref> = token / quoted-string
5328<x:ref>chunk-size</x:ref> = 1*HEXDIG
5329<x:ref>chunked-body</x:ref> = *chunk last-chunk trailer-part CRLF
5330<x:ref>comment</x:ref> = "(" *( ctext / quoted-pair / comment ) ")"
5331<x:ref>connection-option</x:ref> = token
5332<x:ref>ctext</x:ref> = HTAB / SP / %x21-27 ; '!'-'''
5333 / %x2A-5B ; '*'-'['
5334 / %x5D-7E ; ']'-'~'
5335 / obs-text
5337<x:ref>field-content</x:ref> = *( HTAB / SP / VCHAR / obs-text )
5338<x:ref>field-name</x:ref> = token
5339<x:ref>field-value</x:ref> = *( field-content / obs-fold )
5340<x:ref>fragment</x:ref> = &lt;fragment, defined in [RFC3986], Section 3.5&gt;
5342<x:ref>header-field</x:ref> = field-name ":" OWS field-value OWS
5343<x:ref>http-URI</x:ref> = "http://" authority path-abempty [ "?" query ] [ "#"
5344 fragment ]
5345<x:ref>https-URI</x:ref> = "https://" authority path-abempty [ "?" query ] [ "#"
5346 fragment ]
5348<x:ref>last-chunk</x:ref> = 1*"0" [ chunk-ext ] CRLF
5350<x:ref>message-body</x:ref> = *OCTET
5351<x:ref>method</x:ref> = token
5353<x:ref>obs-fold</x:ref> = CRLF ( SP / HTAB )
5354<x:ref>obs-text</x:ref> = %x80-FF
5355<x:ref>origin-form</x:ref> = absolute-path [ "?" query ]
5357<x:ref>partial-URI</x:ref> = relative-part [ "?" query ]
5358<x:ref>path-abempty</x:ref> = &lt;path-abempty, defined in [RFC3986], Section 3.3&gt;
5359<x:ref>port</x:ref> = &lt;port, defined in [RFC3986], Section 3.2.3&gt;
5360<x:ref>protocol</x:ref> = protocol-name [ "/" protocol-version ]
5361<x:ref>protocol-name</x:ref> = token
5362<x:ref>protocol-version</x:ref> = token
5363<x:ref>pseudonym</x:ref> = token
5365<x:ref>qdtext</x:ref> = HTAB / SP / "!" / %x23-5B ; '#'-'['
5366 / %x5D-7E ; ']'-'~'
5367 / obs-text
5368<x:ref>query</x:ref> = &lt;query, defined in [RFC3986], Section 3.4&gt;
5369<x:ref>quoted-pair</x:ref> = "\" ( HTAB / SP / VCHAR / obs-text )
5370<x:ref>quoted-string</x:ref> = DQUOTE *( qdtext / quoted-pair ) DQUOTE
5372<x:ref>rank</x:ref> = ( "0" [ "." *3DIGIT ] ) / ( "1" [ "." *3"0" ] )
5373<x:ref>reason-phrase</x:ref> = *( HTAB / SP / VCHAR / obs-text )
5374<x:ref>received-by</x:ref> = ( uri-host [ ":" port ] ) / pseudonym
5375<x:ref>received-protocol</x:ref> = [ protocol-name "/" ] protocol-version
5376<x:ref>relative-part</x:ref> = &lt;relative-part, defined in [RFC3986], Section 4.2&gt;
5377<x:ref>request-line</x:ref> = method SP request-target SP HTTP-version CRLF
5378<x:ref>request-target</x:ref> = origin-form / absolute-form / authority-form /
5379 asterisk-form
5381<x:ref>segment</x:ref> = &lt;segment, defined in [RFC3986], Section 3.3&gt;
5382<x:ref>start-line</x:ref> = request-line / status-line
5383<x:ref>status-code</x:ref> = 3DIGIT
5384<x:ref>status-line</x:ref> = HTTP-version SP status-code SP reason-phrase CRLF
5386<x:ref>t-codings</x:ref> = "trailers" / ( transfer-coding [ t-ranking ] )
5387<x:ref>t-ranking</x:ref> = OWS ";" OWS "q=" rank
5388<x:ref>tchar</x:ref> = "!" / "#" / "$" / "%" / "&amp;" / "'" / "*" / "+" / "-" / "." /
5389 "^" / "_" / "`" / "|" / "~" / DIGIT / ALPHA
5390<x:ref>token</x:ref> = 1*tchar
5391<x:ref>trailer-part</x:ref> = *( header-field CRLF )
5392<x:ref>transfer-coding</x:ref> = "chunked" / "compress" / "deflate" / "gzip" /
5393 transfer-extension
5394<x:ref>transfer-extension</x:ref> = token *( OWS ";" OWS transfer-parameter )
5395<x:ref>transfer-parameter</x:ref> = token BWS "=" BWS ( token / quoted-string )
5397<x:ref>uri-host</x:ref> = &lt;host, defined in [RFC3986], Section 3.2.2&gt;
5401<?ENDINC p1-messaging.abnf-appendix ?>
5403<section title="Change Log (to be removed by RFC Editor before publication)" anchor="change.log">
5405<section title="Since RFC 2616">
5407  Changes up to the IETF Last Call draft are summarized
5408  in <eref target=""/>.
5412<section title="Since draft-ietf-httpbis-p1-messaging-24" anchor="changes.since.24">
5414  Closed issues:
5415  <list style="symbols">
5416    <t>
5417      <eref target=""/>:
5418      "APPSDIR review of draft-ietf-httpbis-p1-messaging-24"
5419    </t>
5420    <t>
5421      <eref target=""/>:
5422      "integer value parsing"
5423    </t>
5424    <t>
5425      <eref target=""/>:
5426      "move IANA registrations to correct draft"
5427    </t>
5428  </list>
5432<section title="Since draft-ietf-httpbis-p1-messaging-25" anchor="changes.since.25">
5434  Closed issues:
5435  <list style="symbols">
5436    <t>
5437      <eref target=""/>:
5438      "check media type registration templates"
5439    </t>
5440    <t>
5441      <eref target=""/>:
5442      "Redundant rule quoted-str-nf"
5443    </t>
5444    <t>
5445      <eref target=""/>:
5446      "add 'stateless' to Abstract"
5447    </t>
5448    <t>
5449      <eref target=""/>:
5450      "clarify ABNF layering"
5451    </t>
5452    <t>
5453      <eref target=""/>:
5454      "use of 'word' ABNF production"
5455    </t>
5456    <t>
5457      <eref target=""/>:
5458      "improve introduction of list rule"
5459    </t>
5460    <t>
5461      <eref target=""/>:
5462      "moving 2616/2068/2145 to historic"
5463    </t>
5464    <t>
5465      <eref target=""/>:
5466      "augment security considerations with pointers to current research"
5467    </t>
5468    <t>
5469      <eref target=""/>:
5470      "intermediaries handling trailers"
5471    </t>
5472  </list>
5475  Partly resolved issues:
5476  <list style="symbols">
5477    <t>
5478      <eref target=""/>:
5479      "IESG ballot on draft-ietf-httpbis-p1-messaging-25"
5480    </t>
5481  </list>
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