source: draft-ietf-httpbis/16/draft-ietf-httpbis-p1-messaging-16.xml @ 1500

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<rfc obsoletes="2145,2616" updates="2817" category="std" ipr="pre5378Trust200902" docName="draft-ietf-httpbis-p1-messaging-16">
<front>

  <title abbrev="HTTP/1.1, Part 1">HTTP/1.1, part 1: URIs, Connections, and Message Parsing</title>

  <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
    <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
    <address>
      <postal>
        <street>345 Park Ave</street>
        <city>San Jose</city>
        <region>CA</region>
        <code>95110</code>
        <country>USA</country>
      </postal>
      <email>fielding@gbiv.com</email>
      <uri>http://roy.gbiv.com/</uri>
    </address>
  </author>

  <author initials="J." surname="Gettys" fullname="Jim Gettys">
    <organization abbrev="Alcatel-Lucent">Alcatel-Lucent Bell Labs</organization>
    <address>
      <postal>
        <street>21 Oak Knoll Road</street>
        <city>Carlisle</city>
        <region>MA</region>
        <code>01741</code>
        <country>USA</country>
      </postal>
      <email>jg@freedesktop.org</email>
      <uri>http://gettys.wordpress.com/</uri>
    </address>
  </author>
  
  <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
    <organization abbrev="HP">Hewlett-Packard Company</organization>
    <address>
      <postal>
        <street>HP Labs, Large Scale Systems Group</street>
        <street>1501 Page Mill Road, MS 1177</street>
        <city>Palo Alto</city>
        <region>CA</region>
        <code>94304</code>
        <country>USA</country>
      </postal>
      <email>JeffMogul@acm.org</email>
    </address>
  </author>

  <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
    <organization abbrev="Microsoft">Microsoft Corporation</organization>
    <address>
      <postal>
        <street>1 Microsoft Way</street>
        <city>Redmond</city>
        <region>WA</region>
        <code>98052</code>
        <country>USA</country>
      </postal>
      <email>henrikn@microsoft.com</email>
    </address>
  </author>

  <author initials="L." surname="Masinter" fullname="Larry Masinter">
    <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
    <address>
      <postal>
        <street>345 Park Ave</street>
        <city>San Jose</city>
        <region>CA</region>
        <code>95110</code>
        <country>USA</country>
      </postal>
      <email>LMM@acm.org</email>
      <uri>http://larry.masinter.net/</uri>
    </address>
  </author>
  
  <author initials="P." surname="Leach" fullname="Paul J. Leach">
    <organization abbrev="Microsoft">Microsoft Corporation</organization>
    <address>
      <postal>
        <street>1 Microsoft Way</street>
        <city>Redmond</city>
        <region>WA</region>
        <code>98052</code>
      </postal>
      <email>paulle@microsoft.com</email>
    </address>
  </author>
   
  <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
    <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
    <address>
      <postal>
        <street>MIT Computer Science and Artificial Intelligence Laboratory</street>
        <street>The Stata Center, Building 32</street>
        <street>32 Vassar Street</street>
        <city>Cambridge</city>
        <region>MA</region>
        <code>02139</code>
        <country>USA</country>
      </postal>
      <email>timbl@w3.org</email>
      <uri>http://www.w3.org/People/Berners-Lee/</uri>
    </address>
  </author>

  <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
    <organization abbrev="W3C">World Wide Web Consortium</organization>
    <address>
      <postal>
        <street>W3C / ERCIM</street>
        <street>2004, rte des Lucioles</street>
        <city>Sophia-Antipolis</city>
        <region>AM</region>
        <code>06902</code>
        <country>France</country>
      </postal>
      <email>ylafon@w3.org</email>
      <uri>http://www.raubacapeu.net/people/yves/</uri>
    </address>
  </author>

  <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
    <organization abbrev="greenbytes">greenbytes GmbH</organization>
    <address>
      <postal>
        <street>Hafenweg 16</street>
        <city>Muenster</city><region>NW</region><code>48155</code>
        <country>Germany</country>
      </postal>
      <phone>+49 251 2807760</phone>
      <facsimile>+49 251 2807761</facsimile>
      <email>julian.reschke@greenbytes.de</email>
      <uri>http://greenbytes.de/tech/webdav/</uri>
    </address>
  </author>

  <date month="August" year="2011" day="24"/>
  <workgroup>HTTPbis Working Group</workgroup>

<abstract>
<t>
   The Hypertext Transfer Protocol (HTTP) is an application-level protocol for
   distributed, collaborative, hypertext information systems. HTTP has been in
   use by the World Wide Web global information initiative since 1990. This
   document is Part 1 of the seven-part specification that defines the protocol
   referred to as "HTTP/1.1" and, taken together, obsoletes
   RFC 2616 and moves it to historic
   status, along with its predecessor RFC
   2068.
</t>
<t>
   Part 1 provides an overview of HTTP and its associated terminology, defines
   the "http" and "https" Uniform Resource Identifier (URI) schemes, defines
   the generic message syntax and parsing requirements for HTTP message frames,
   and describes general security concerns for implementations.
</t>
<t>
   This part also obsoletes RFCs 2145
   (on HTTP version numbers) and 2817
   (on using CONNECT for TLS upgrades) and moves them to historic status.
</t>
</abstract>

<note title="Editorial Note (To be removed by RFC Editor)">
  <t>
    Discussion of this draft should take place on the HTTPBIS working group
    mailing list (ietf-http-wg@w3.org), which is archived at
    <eref target="http://lists.w3.org/Archives/Public/ietf-http-wg/"/>.
  </t>
  <t>
    The current issues list is at
    <eref target="http://tools.ietf.org/wg/httpbis/trac/report/3"/> and related
    documents (including fancy diffs) can be found at
    <eref target="http://tools.ietf.org/wg/httpbis/"/>.
  </t>
  <t>
    The changes in this draft are summarized in <xref target="changes.since.15"/>.
  </t>
</note>
</front>
<middle>
<section title="Introduction" anchor="introduction">
<t>
   The Hypertext Transfer Protocol (HTTP) is an application-level
   request/response protocol that uses extensible semantics and MIME-like
   message payloads for flexible interaction with network-based hypertext
   information systems. HTTP relies upon the Uniform Resource Identifier (URI)
   standard <xref target="RFC3986"/> to indicate the target resource and
   relationships between resources.
   Messages are passed in a format similar to that used by Internet mail
   <xref target="RFC5322"/> and the Multipurpose Internet Mail Extensions
   (MIME) <xref target="RFC2045"/> (see Appendix A of <xref target="Part3"/> for the differences
   between HTTP and MIME messages).
</t>
<t>
   HTTP is a generic interface protocol for information systems. It is
   designed to hide the details of how a service is implemented by presenting
   a uniform interface to clients that is independent of the types of
   resources provided. Likewise, servers do not need to be aware of each
   client's purpose: an HTTP request can be considered in isolation rather
   than being associated with a specific type of client or a predetermined
   sequence of application steps. The result is a protocol that can be used
   effectively in many different contexts and for which implementations can
   evolve independently over time.
</t>
<t>
   HTTP is also designed for use as an intermediation protocol for translating
   communication to and from non-HTTP information systems.
   HTTP proxies and gateways can provide access to alternative information
   services by translating their diverse protocols into a hypertext
   format that can be viewed and manipulated by clients in the same way
   as HTTP services.
</t>
<t>
   One consequence of HTTP flexibility is that the protocol cannot be
   defined in terms of what occurs behind the interface. Instead, we
   are limited to defining the syntax of communication, the intent
   of received communication, and the expected behavior of recipients.
   If the communication is considered in isolation, then successful
   actions ought to be reflected in corresponding changes to the
   observable interface provided by servers. However, since multiple
   clients might act in parallel and perhaps at cross-purposes, we
   cannot require that such changes be observable beyond the scope
   of a single response.
</t>
<t>
   This document is Part 1 of the seven-part specification of HTTP,
   defining the protocol referred to as "HTTP/1.1", obsoleting
   <xref target="RFC2616"/> and <xref target="RFC2145"/>.
   Part 1 describes the architectural elements that are used or
   referred to in HTTP, defines the "http" and "https" URI schemes,
   describes overall network operation and connection management,
   and defines HTTP message framing and forwarding requirements.
   Our goal is to define all of the mechanisms necessary for HTTP message
   handling that are independent of message semantics, thereby defining the
   complete set of requirements for message parsers and
   message-forwarding intermediaries.
</t>

<section title="Requirements" anchor="intro.requirements">
<t>
   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in <xref target="RFC2119"/>.
</t>
<t>
   An implementation is not compliant if it fails to satisfy one or more
   of the "MUST" or "REQUIRED" level requirements for the protocols it
   implements. An implementation that satisfies all the "MUST" or "REQUIRED"
   level and all the "SHOULD" level requirements for its protocols is said
   to be "unconditionally compliant"; one that satisfies all the "MUST"
   level requirements but not all the "SHOULD" level requirements for its
   protocols is said to be "conditionally compliant".
</t>
</section>

<section title="Syntax Notation" anchor="notation">
<iref primary="true" item="Grammar" subitem="ALPHA"/>
<iref primary="true" item="Grammar" subitem="CR"/>
<iref primary="true" item="Grammar" subitem="CRLF"/>
<iref primary="true" item="Grammar" subitem="CTL"/>
<iref primary="true" item="Grammar" subitem="DIGIT"/>
<iref primary="true" item="Grammar" subitem="DQUOTE"/>
<iref primary="true" item="Grammar" subitem="HEXDIG"/>
<iref primary="true" item="Grammar" subitem="LF"/>
<iref primary="true" item="Grammar" subitem="OCTET"/>
<iref primary="true" item="Grammar" subitem="SP"/>
<iref primary="true" item="Grammar" subitem="VCHAR"/>
<iref primary="true" item="Grammar" subitem="WSP"/>
<t>
   This specification uses the Augmented Backus-Naur Form (ABNF) notation
   of <xref target="RFC5234"/>.
</t>
<t anchor="core.rules">
  
  
  
  
  
  
  
  
  
  
  
  
   The following core rules are included by
   reference, as defined in <xref target="RFC5234"/>, Appendix B.1:
   ALPHA (letters), CR (carriage return), CRLF (CR LF), CTL (controls),
   DIGIT (decimal 0-9), DQUOTE (double quote),
   HEXDIG (hexadecimal 0-9/A-F/a-f), LF (line feed),
   OCTET (any 8-bit sequence of data), SP (space),
   VCHAR (any visible <xref target="USASCII"/> character),
   and WSP (whitespace).
</t>
<t>
   As a syntactic convention, ABNF rule names prefixed with "obs-" denote
   "obsolete" grammar rules that appear for historical reasons.
</t>

<section title="ABNF Extension: #rule" anchor="notation.abnf">
<t>
  The #rule extension to the ABNF rules of <xref target="RFC5234"/> is used to
  improve readability. 
</t>
<t>
  A construct "#" is defined, similar to "*", for defining comma-delimited
  lists of elements. The full form is "&lt;n&gt;#&lt;m&gt;element" indicating
  at least &lt;n&gt; and at most &lt;m&gt; elements, each separated by a single
  comma (",") and optional whitespace (OWS, <xref target="basic.rules"/>).   
</t>
<figure><preamble>
  Thus,
</preamble><artwork type="example"><![CDATA[
  1#element => element *( OWS "," OWS element )
]]></artwork></figure>
<figure><preamble>
  and:
</preamble><artwork type="example"><![CDATA[
  #element => [ 1#element ]
]]></artwork></figure>
<figure><preamble>
  and for n &gt;= 1 and m &gt; 1:
</preamble><artwork type="example"><![CDATA[
  <n>#<m>element => element <n-1>*<m-1>( OWS "," OWS element )
]]></artwork></figure>
<t>
  For compatibility with legacy list rules, recipients SHOULD accept empty
  list elements. In other words, consumers would follow the list productions:
</t>
<figure><artwork type="example"><![CDATA[
  #element => [ ( "," / element ) *( OWS "," [ OWS element ] ) ]
  
  1#element => *( "," OWS ) element *( OWS "," [ OWS element ] )
]]></artwork></figure>
<t>
  Note that empty elements do not contribute to the count of elements present,
  though.
</t>
<t>
  For example, given these ABNF productions: 
</t>
<figure><artwork type="example"><![CDATA[
  example-list      = 1#example-list-elmt
  example-list-elmt = token ; see Section 3.2.3 
]]></artwork></figure>
<t>
  Then these are valid values for example-list (not including the double
  quotes, which are present for delimitation only):
</t>
<figure><artwork type="example"><![CDATA[
  "foo,bar"
  " foo ,bar,"
  "  foo , ,bar,charlie   "
  "foo ,bar,   charlie "
]]></artwork></figure>
<t>
  But these values would be invalid, as at least one non-empty element is
  required:
</t>
<figure><artwork type="example"><![CDATA[
  ""
  ","
  ",   ,"
]]></artwork></figure>
<t>
  <xref target="collected.abnf"/> shows the collected ABNF, with the list rules
  expanded as explained above.
</t>
</section>

<section title="Basic Rules" anchor="basic.rules">
<t anchor="rule.LWS">
   This specification uses three rules to denote the use of linear
   whitespace: OWS (optional whitespace), RWS (required whitespace), and
   BWS ("bad" whitespace). 
</t>
<t anchor="rule.OWS">
   The OWS rule is used where zero or more linear whitespace octets might
   appear. OWS SHOULD either not be produced or be produced as a single
   SP. Multiple OWS octets that occur within field-content SHOULD either
   be replaced with a single SP or transformed to all SP octets (each WSP
   octet other than SP replaced with SP) before interpreting the field value
   or forwarding the message downstream.
</t>
<t anchor="rule.RWS">
   RWS is used when at least one linear whitespace octet is required to
   separate field tokens. RWS SHOULD be produced as a single SP.
   Multiple RWS octets octets that occur within field-content SHOULD either
   be replaced with a single SP or transformed to all SP octets (each WSP
   octet other than SP replaced with SP) before interpreting the field value
   or forwarding the message downstream.
</t>
<t anchor="rule.BWS">
   BWS is used where the grammar allows optional whitespace for historical
   reasons but senders SHOULD NOT produce it in messages. HTTP/1.1
   recipients MUST accept such bad optional whitespace and remove it before
   interpreting the field value or forwarding the message downstream.
</t>
<t anchor="rule.whitespace">
  
  
  
  
</t>
<figure><iref primary="true" item="Grammar" subitem="OWS"/><iref primary="true" item="Grammar" subitem="RWS"/><iref primary="true" item="Grammar" subitem="BWS"/><artwork type="abnf2616"><![CDATA[
  OWS            = *( [ obs-fold ] WSP )
                 ; "optional" whitespace
  RWS            = 1*( [ obs-fold ] WSP )
                 ; "required" whitespace
  BWS            = OWS
                 ; "bad" whitespace
  obs-fold       = CRLF
                 ; see Section 3.2
]]></artwork></figure>
</section>
</section>
</section>

<section title="HTTP-related architecture" anchor="architecture">
<t>
   HTTP was created for the World Wide Web architecture
   and has evolved over time to support the scalability needs of a worldwide
   hypertext system. Much of that architecture is reflected in the terminology
   and syntax productions used to define HTTP.
</t>

<section title="Client/Server Messaging" anchor="operation">
<iref primary="true" item="client"/>
<iref primary="true" item="server"/>
<iref primary="true" item="connection"/>
<t>
   HTTP is a stateless request/response protocol that operates by exchanging
   messages across a reliable transport or session-layer
   "connection". An HTTP "client" is a
   program that establishes a connection to a server for the purpose of
   sending one or more HTTP requests.  An HTTP "server" is a
   program that accepts connections in order to service HTTP requests by
   sending HTTP responses.
</t>
<iref primary="true" item="user agent"/>
<iref primary="true" item="origin server"/>
<iref primary="true" item="browser"/>
<iref primary="true" item="spider"/>
<iref primary="true" item="sender"/>
<iref primary="true" item="recipient"/>
<t>
   Note that the terms client and server refer only to the roles that
   these programs perform for a particular connection.  The same program
   might act as a client on some connections and a server on others.  We use
   the term "user agent" to refer to the program that initiates a request,
   such as a WWW browser, editor, or spider (web-traversing robot), and
   the term "origin server" to refer to the program that can originate
   authoritative responses to a request.  For general requirements, we use
   the term "sender" to refer to whichever component sent a given message
   and the term "recipient" to refer to any component that receives the
   message.
</t>
<t>
   Most HTTP communication consists of a retrieval request (GET) for
   a representation of some resource identified by a URI.  In the
   simplest case, this might be accomplished via a single bidirectional
   connection (===) between the user agent (UA) and the origin server (O).
</t>
<figure><artwork type="drawing"><![CDATA[
         request   >
    UA ======================================= O
                                <   response
]]></artwork></figure>
<iref primary="true" item="message"/>
<iref primary="true" item="request"/>
<iref primary="true" item="response"/>
<t>
   A client sends an HTTP request to the server in the form of a request
   message (<xref target="request"/>), beginning with a method, URI, and
   protocol version, followed by MIME-like header fields containing
   request modifiers, client information, and payload metadata, an empty
   line to indicate the end of the header section, and finally the payload
   body (if any).
</t>
<t>
   A server responds to the client's request by sending an HTTP response
   message (<xref target="response"/>), beginning with a status line that
   includes the protocol version, a success or error code, and textual
   reason phrase, followed by MIME-like header fields containing server
   information, resource metadata, and payload metadata, an empty line to
   indicate the end of the header section, and finally the payload body (if any).
</t>
<t>
   The following example illustrates a typical message exchange for a
   GET request on the URI "http://www.example.com/hello.txt":
</t>
<figure><preamble>
client request:
</preamble><artwork type="message/http; msgtype=&#34;request&#34;"><![CDATA[
  GET /hello.txt HTTP/1.1
  User-Agent: curl/7.16.3 libcurl/7.16.3 OpenSSL/0.9.7l zlib/1.2.3
  Host: www.example.com
  Accept: */*
  
  ]]></artwork></figure>
<figure><preamble>
server response:
</preamble><artwork type="message/http; msgtype=&#34;response&#34;"><![CDATA[
  HTTP/1.1 200 OK
  Date: Mon, 27 Jul 2009 12:28:53 GMT
  Server: Apache
  Last-Modified: Wed, 22 Jul 2009 19:15:56 GMT
  ETag: "34aa387-d-1568eb00"
  Accept-Ranges: bytes
  Content-Length: 14
  Vary: Accept-Encoding
  Content-Type: text/plain
  
  Hello World!
  ]]></artwork></figure>
</section>

<section title="Message Orientation and Buffering" anchor="message-orientation-and-buffering">
<t>
   Fundamentally, HTTP is a message-based protocol. Although message bodies can
   be chunked (<xref target="chunked.encoding"/>) and implementations often
   make parts of a message available progressively, this is not required, and
   some widely-used implementations only make a message available when it is
   complete. Furthermore, while most proxies will progressively stream messages,
   some amount of buffering will take place, and some proxies might buffer
   messages to perform transformations, check content or provide other services.
</t>
<t>
   Therefore, extensions to and uses of HTTP cannot rely on the availability of
   a partial message, or assume that messages will not be buffered. There are
   strategies that can be used to test for buffering in a given connection, but
   it should be understood that behaviors can differ across connections, and
   between requests and responses. 
</t>
<t>
   Recipients MUST consider every message in a connection in isolation;
   because HTTP is a stateless protocol, it cannot be assumed that two requests
   on the same connection are from the same client or share any other common
   attributes. In particular, intermediaries might mix requests from different
   clients into a single server connection. Note that some existing HTTP
   extensions (e.g., <xref target="RFC4559"/>) violate this requirement, thereby
   potentially causing interoperability and security problems.
</t>
</section>

<section title="Connections and Transport Independence" anchor="transport-independence">
<t>
   HTTP messaging is independent of the underlying transport or
   session-layer connection protocol(s).  HTTP only presumes a reliable
   transport with in-order delivery of requests and the corresponding
   in-order delivery of responses.  The mapping of HTTP request and
   response structures onto the data units of the underlying transport
   protocol is outside the scope of this specification.
</t>
<t>
   The specific connection protocols to be used for an interaction
   are determined by client configuration and the target resource's URI.
   For example, the "http" URI scheme
   (<xref target="http.uri"/>) indicates a default connection of TCP
   over IP, with a default TCP port of 80, but the client might be
   configured to use a proxy via some other connection port or protocol
   instead of using the defaults.
</t>
<t>
   A connection might be used for multiple HTTP request/response exchanges,
   as defined in <xref target="persistent.connections"/>.
</t>
</section>

<section title="Intermediaries" anchor="intermediaries">
<iref primary="true" item="intermediary"/>
<t>
   HTTP enables the use of intermediaries to satisfy requests through
   a chain of connections.  There are three common forms of HTTP
   intermediary: proxy, gateway, and tunnel.  In some cases,
   a single intermediary might act as an origin server, proxy, gateway,
   or tunnel, switching behavior based on the nature of each request.
</t>
<figure><artwork type="drawing"><![CDATA[
         >             >             >             >
    UA =========== A =========== B =========== C =========== O
               <             <             <             <
]]></artwork></figure>
<t>
   The figure above shows three intermediaries (A, B, and C) between the
   user agent and origin server. A request or response message that
   travels the whole chain will pass through four separate connections.
   Some HTTP communication options
   might apply only to the connection with the nearest, non-tunnel
   neighbor, only to the end-points of the chain, or to all connections
   along the chain. Although the diagram is linear, each participant might
   be engaged in multiple, simultaneous communications. For example, B
   might be receiving requests from many clients other than A, and/or
   forwarding requests to servers other than C, at the same time that it
   is handling A's request.
</t>
<t>
<iref primary="true" item="upstream"/><iref primary="true" item="downstream"/>
<iref primary="true" item="inbound"/><iref primary="true" item="outbound"/>
   We use the terms "upstream" and "downstream"
   to describe various requirements in relation to the directional flow of a
   message: all messages flow from upstream to downstream.
   Likewise, we use the terms inbound and outbound to refer to
   directions in relation to the request path:
   "inbound" means toward the origin server and
   "outbound" means toward the user agent.
</t>
<t><iref primary="true" item="proxy"/>
   A "proxy" is a message forwarding agent that is selected by the
   client, usually via local configuration rules, to receive requests
   for some type(s) of absolute URI and attempt to satisfy those
   requests via translation through the HTTP interface.  Some translations
   are minimal, such as for proxy requests for "http" URIs, whereas
   other requests might require translation to and from entirely different
   application-layer protocols. Proxies are often used to group an
   organization's HTTP requests through a common intermediary for the
   sake of security, annotation services, or shared caching.
</t>
<t>
<iref primary="true" item="transforming proxy"/>
<iref primary="true" item="non-transforming proxy"/>
   An HTTP-to-HTTP proxy is called a "transforming proxy" if it is designed
   or configured to modify request or response messages in a semantically
   meaningful way (i.e., modifications, beyond those required by normal
   HTTP processing, that change the message in a way that would be
   significant to the original sender or potentially significant to
   downstream recipients).  For example, a transforming proxy might be
   acting as a shared annotation server (modifying responses to include
   references to a local annotation database), a malware filter, a
   format transcoder, or an intranet-to-Internet privacy filter.  Such
   transformations are presumed to be desired by the client (or client
   organization) that selected the proxy and are beyond the scope of
   this specification.  However, when a proxy is not intended to transform
   a given message, we use the term "non-transforming proxy" to target
   requirements that preserve HTTP message semantics. See Section 8.2.4 of <xref target="Part2"/> and
   Section 3.6 of <xref target="Part6"/> for status and warning codes related to transformations.
</t>
<t><iref primary="true" item="gateway"/><iref primary="true" item="reverse proxy"/>
<iref primary="true" item="accelerator"/>
   A "gateway" (a.k.a., "reverse proxy")
   is a receiving agent that acts
   as a layer above some other server(s) and translates the received
   requests to the underlying server's protocol.  Gateways are often
   used to encapsulate legacy or untrusted information services, to
   improve server performance through "accelerator" caching, and to
   enable partitioning or load-balancing of HTTP services across
   multiple machines.
</t>
<t>
   A gateway behaves as an origin server on its outbound connection and
   as a user agent on its inbound connection.
   All HTTP requirements applicable to an origin server
   also apply to the outbound communication of a gateway.
   A gateway communicates with inbound servers using any protocol that
   it desires, including private extensions to HTTP that are outside
   the scope of this specification.  However, an HTTP-to-HTTP gateway
   that wishes to interoperate with third-party HTTP servers MUST
   comply with HTTP user agent requirements on the gateway's inbound
   connection and MUST implement the Connection
   (<xref target="header.connection"/>) and Via (<xref target="header.via"/>)
   header fields for both connections.
</t>
<t><iref primary="true" item="tunnel"/>
   A "tunnel" acts as a blind relay between two connections
   without changing the messages. Once active, a tunnel is not
   considered a party to the HTTP communication, though the tunnel might
   have been initiated by an HTTP request. A tunnel ceases to exist when
   both ends of the relayed connection are closed. Tunnels are used to
   extend a virtual connection through an intermediary, such as when
   transport-layer security is used to establish private communication
   through a shared firewall proxy.
</t>
<t><iref primary="true" item="interception proxy"/><iref primary="true" item="transparent proxy"/>
<iref primary="true" item="captive portal"/>
   In addition, there may exist network intermediaries that are not
   considered part of the HTTP communication but nevertheless act as
   filters or redirecting agents (usually violating HTTP semantics,
   causing security problems, and otherwise making a mess of things).
   Such a network intermediary, often referred to as an "interception proxy"
   <xref target="RFC3040"/>, "transparent proxy" <xref target="RFC1919"/>,
   or "captive portal",
   differs from an HTTP proxy because it has not been selected by the client.
   Instead, the network intermediary redirects outgoing TCP port 80 packets
   (and occasionally other common port traffic) to an internal HTTP server.
   Interception proxies are commonly found on public network access points,
   as a means of enforcing account subscription prior to allowing use of
   non-local Internet services, and within corporate firewalls to enforce
   network usage policies.
   They are indistinguishable from a man-in-the-middle attack.
</t>
</section>

<section title="Caches" anchor="caches">
<iref primary="true" item="cache"/>
<t>
   A "cache" is a local store of previous response messages and the
   subsystem that controls its message storage, retrieval, and deletion.
   A cache stores cacheable responses in order to reduce the response
   time and network bandwidth consumption on future, equivalent
   requests. Any client or server MAY employ a cache, though a cache
   cannot be used by a server while it is acting as a tunnel.
</t>
<t>
   The effect of a cache is that the request/response chain is shortened
   if one of the participants along the chain has a cached response
   applicable to that request. The following illustrates the resulting
   chain if B has a cached copy of an earlier response from O (via C)
   for a request which has not been cached by UA or A.
</t>
<figure><artwork type="drawing"><![CDATA[
            >             >
       UA =========== A =========== B - - - - - - C - - - - - - O
                  <             <
]]></artwork></figure>
<t><iref primary="true" item="cacheable"/>
   A response is "cacheable" if a cache is allowed to store a copy of
   the response message for use in answering subsequent requests.
   Even when a response is cacheable, there might be additional
   constraints placed by the client or by the origin server on when
   that cached response can be used for a particular request. HTTP
   requirements for cache behavior and cacheable responses are
   defined in Section 2 of <xref target="Part6"/>.  
</t>
<t>
   There are a wide variety of architectures and configurations
   of caches and proxies deployed across the World Wide Web and
   inside large organizations. These systems include national hierarchies
   of proxy caches to save transoceanic bandwidth, systems that
   broadcast or multicast cache entries, organizations that distribute
   subsets of cached data via optical media, and so on.
</t>
</section>

<section title="Protocol Versioning" anchor="http.version">
  
  
<t>
   HTTP uses a "&lt;major&gt;.&lt;minor&gt;" numbering scheme to indicate
   versions of the protocol. This specification defines version "1.1".
   The protocol version as a whole indicates the sender's compliance
   with the set of requirements laid out in that version's corresponding
   specification of HTTP.
</t>
<t>
   The version of an HTTP message is indicated by an HTTP-Version field
   in the first line of the message. HTTP-Version is case-sensitive.
</t>
<figure><iref primary="true" item="Grammar" subitem="HTTP-Version"/><iref primary="true" item="Grammar" subitem="HTTP-Prot-Name"/><artwork type="abnf2616"><![CDATA[
  HTTP-Version   = HTTP-Prot-Name "/" DIGIT "." DIGIT
  HTTP-Prot-Name = %x48.54.54.50 ; "HTTP", case-sensitive 
]]></artwork></figure>
<t>
   The HTTP version number consists of two decimal digits separated by a "."
   (period or decimal point).  The first digit ("major version") indicates the
   HTTP messaging syntax, whereas the second digit ("minor version") indicates
   the highest minor version to which the sender is at least conditionally
   compliant and able to understand for future communication.  The minor
   version advertises the sender's communication capabilities even when the
   sender is only using a backwards-compatible subset of the protocol,
   thereby letting the recipient know that more advanced features can
   be used in response (by servers) or in future requests (by clients).
</t>
<t>
   When an HTTP/1.1 message is sent to an HTTP/1.0 recipient
   <xref target="RFC1945"/> or a recipient whose version is unknown,
   the HTTP/1.1 message is constructed such that it can be interpreted
   as a valid HTTP/1.0 message if all of the newer features are ignored.
   This specification places recipient-version requirements on some
   new features so that a compliant sender will only use compatible
   features until it has determined, through configuration or the
   receipt of a message, that the recipient supports HTTP/1.1.
</t>
<t>
   The interpretation of an HTTP header field does not change
   between minor versions of the same major version, though the default
   behavior of a recipient in the absence of such a field can change.
   Unless specified otherwise, header fields defined in HTTP/1.1 are
   defined for all versions of HTTP/1.x.  In particular, the Host and
   Connection header fields ought to be implemented by all HTTP/1.x
   implementations whether or not they advertise compliance with HTTP/1.1.
</t>
<t>
   New header fields can be defined such that, when they are
   understood by a recipient, they might override or enhance the
   interpretation of previously defined header fields.  When an
   implementation receives an unrecognized header field, the recipient
   MUST ignore that header field for local processing regardless of
   the message's HTTP version.  An unrecognized header field received
   by a proxy MUST be forwarded downstream unless the header field's
   field-name is listed in the message's Connection header-field
   (see <xref target="header.connection"/>).
   These requirements allow HTTP's functionality to be enhanced without
   requiring prior update of all compliant intermediaries.
</t>
<t>
   Intermediaries that process HTTP messages (i.e., all intermediaries
   other than those acting as a tunnel) MUST send their own HTTP-Version
   in forwarded messages.  In other words, they MUST NOT blindly
   forward the first line of an HTTP message without ensuring that the
   protocol version matches what the intermediary understands, and
   is at least conditionally compliant to, for both the receiving and
   sending of messages.  Forwarding an HTTP message without rewriting
   the HTTP-Version might result in communication errors when downstream
   recipients use the message sender's version to determine what features
   are safe to use for later communication with that sender.
</t>
<t>
   An HTTP client SHOULD send a request version equal to the highest
   version for which the client is at least conditionally compliant and
   whose major version is no higher than the highest version supported
   by the server, if this is known.  An HTTP client MUST NOT send a
   version for which it is not at least conditionally compliant.
</t>
<t>
   An HTTP client MAY send a lower request version if it is known that
   the server incorrectly implements the HTTP specification, but only
   after the client has attempted at least one normal request and determined
   from the response status or header fields (e.g., Server) that the
   server improperly handles higher request versions.
</t>
<t>
   An HTTP server SHOULD send a response version equal to the highest
   version for which the server is at least conditionally compliant and
   whose major version is less than or equal to the one received in the
   request.  An HTTP server MUST NOT send a version for which it is not
   at least conditionally compliant.  A server MAY send a 505 (HTTP
   Version Not Supported) response if it cannot send a response using the
   major version used in the client's request.
</t>
<t>
   An HTTP server MAY send an HTTP/1.0 response to an HTTP/1.0 request
   if it is known or suspected that the client incorrectly implements the
   HTTP specification and is incapable of correctly processing later
   version responses, such as when a client fails to parse the version
   number correctly or when an intermediary is known to blindly forward
   the HTTP-Version even when it doesn't comply with the given minor
   version of the protocol. Such protocol downgrades SHOULD NOT be
   performed unless triggered by specific client attributes, such as when
   one or more of the request header fields (e.g., User-Agent) uniquely
   match the values sent by a client known to be in error.
</t>
<t>
   The intention of HTTP's versioning design is that the major number
   will only be incremented if an incompatible message syntax is
   introduced, and that the minor number will only be incremented when
   changes made to the protocol have the effect of adding to the message
   semantics or implying additional capabilities of the sender.  However,
   the minor version was not incremented for the changes introduced between
   <xref target="RFC2068"/> and <xref target="RFC2616"/>, and this revision
   is specifically avoiding any such changes to the protocol.
</t>
</section>

<section title="Uniform Resource Identifiers" anchor="uri">
<iref primary="true" item="resource"/>
<t>
   Uniform Resource Identifiers (URIs) <xref target="RFC3986"/> are used
   throughout HTTP as the means for identifying resources. URI references
   are used to target requests, indicate redirects, and define relationships.
   HTTP does not limit what a resource might be; it merely defines an interface
   that can be used to interact with a resource via HTTP. More information on
   the scope of URIs and resources can be found in <xref target="RFC3986"/>.
</t>
  
  
  
  
  
  
  
  
  
  
<t>
   This specification adopts the definitions of "URI-reference",
   "absolute-URI", "relative-part", "port", "host",
   "path-abempty", "path-absolute", "query", and "authority" from the
   URI generic syntax <xref target="RFC3986"/>.
   In addition, we define a partial-URI rule for protocol elements
   that allow a relative URI but not a fragment.
</t>
<figure><iref primary="true" item="Grammar" subitem="URI-reference"/><iref primary="true" item="Grammar" subitem="absolute-URI"/><iref primary="true" item="Grammar" subitem="authority"/><iref primary="true" item="Grammar" subitem="path-absolute"/><iref primary="true" item="Grammar" subitem="port"/><iref primary="true" item="Grammar" subitem="query"/><iref primary="true" item="Grammar" subitem="uri-host"/><artwork type="abnf2616"><![CDATA[
  URI-reference = <URI-reference, defined in [RFC3986], Section 4.1>
  absolute-URI  = <absolute-URI, defined in [RFC3986], Section 4.3>
  relative-part = <relative-part, defined in [RFC3986], Section 4.2>
  authority     = <authority, defined in [RFC3986], Section 3.2>
  path-abempty  = <path-abempty, defined in [RFC3986], Section 3.3>
  path-absolute = <path-absolute, defined in [RFC3986], Section 3.3>
  port          = <port, defined in [RFC3986], Section 3.2.3>
  query         = <query, defined in [RFC3986], Section 3.4>
  uri-host      = <host, defined in [RFC3986], Section 3.2.2>
  
  partial-URI   = relative-part [ "?" query ]
]]></artwork></figure>
<t>
   Each protocol element in HTTP that allows a URI reference will indicate
   in its ABNF production whether the element allows any form of reference
   (URI-reference), only a URI in absolute form (absolute-URI), only the
   path and optional query components, or some combination of the above.
   Unless otherwise indicated, URI references are parsed relative to the
   effective request URI, which defines the default base URI for references
   in both the request and its corresponding response.
</t>

<section title="http URI scheme" anchor="http.uri">
  
  <iref item="http URI scheme" primary="true"/>
  <iref item="URI scheme" subitem="http" primary="true"/>
<t>
   The "http" URI scheme is hereby defined for the purpose of minting
   identifiers according to their association with the hierarchical
   namespace governed by a potential HTTP origin server listening for
   TCP connections on a given port.
</t>
<figure><iref primary="true" item="Grammar" subitem="http-URI"/><artwork type="abnf2616"><![CDATA[
  http-URI = "http:" "//" authority path-abempty [ "?" query ]
]]></artwork></figure>
<t>
   The HTTP origin server is identified by the generic syntax's 
   <xref target="uri" format="none">authority</xref> component, which includes a host identifier
   and optional TCP port (<xref target="RFC3986"/>, Section 3.2.2).
   The remainder of the URI, consisting of both the hierarchical path
   component and optional query component, serves as an identifier for
   a potential resource within that origin server's name space.
</t>
<t>
   If the host identifier is provided as an IP literal or IPv4 address,
   then the origin server is any listener on the indicated TCP port at
   that IP address. If host is a registered name, then that name is
   considered an indirect identifier and the recipient might use a name
   resolution service, such as DNS, to find the address of a listener
   for that host.
   The host MUST NOT be empty; if an "http" URI is received with an
   empty host, then it MUST be rejected as invalid. 
   If the port subcomponent is empty or not given, then TCP port 80 is
   assumed (the default reserved port for WWW services).
</t>
<t>
   Regardless of the form of host identifier, access to that host is not
   implied by the mere presence of its name or address. The host might or might
   not exist and, even when it does exist, might or might not be running an
   HTTP server or listening to the indicated port. The "http" URI scheme
   makes use of the delegated nature of Internet names and addresses to
   establish a naming authority (whatever entity has the ability to place
   an HTTP server at that Internet name or address) and allows that
   authority to determine which names are valid and how they might be used.
</t>
<t>
   When an "http" URI is used within a context that calls for access to the
   indicated resource, a client MAY attempt access by resolving
   the host to an IP address, establishing a TCP connection to that address
   on the indicated port, and sending an HTTP request message to the server
   containing the URI's identifying data as described in <xref target="request"/>.
   If the server responds to that request with a non-interim HTTP response
   message, as described in <xref target="response"/>, then that response
   is considered an authoritative answer to the client's request.
</t>
<t>
   Although HTTP is independent of the transport protocol, the "http"
   scheme is specific to TCP-based services because the name delegation
   process depends on TCP for establishing authority.
   An HTTP service based on some other underlying connection protocol
   would presumably be identified using a different URI scheme, just as
   the "https" scheme (below) is used for servers that require an SSL/TLS
   transport layer on a connection. Other protocols might also be used to
   provide access to "http" identified resources — it is only the
   authoritative interface used for mapping the namespace that is
   specific to TCP.
</t>
<t>
   The URI generic syntax for authority also includes a deprecated
   userinfo subcomponent (<xref target="RFC3986"/>, Section 3.2.1)
   for including user authentication information in the URI.  Some
   implementations make use of the userinfo component for internal
   configuration of authentication information, such as within command
   invocation options, configuration files, or bookmark lists, even
   though such usage might expose a user identifier or password.
   Senders MUST NOT include a userinfo subcomponent (and its "@"
   delimiter) when transmitting an "http" URI in a message.  Recipients
   of HTTP messages that contain a URI reference SHOULD parse for the
   existence of userinfo and treat its presence as an error, likely
   indicating that the deprecated subcomponent is being used to obscure
   the authority for the sake of phishing attacks.
</t>
</section>

<section title="https URI scheme" anchor="https.uri">
   
   <iref item="https URI scheme"/>
   <iref item="URI scheme" subitem="https"/>
<t>
   The "https" URI scheme is hereby defined for the purpose of minting
   identifiers according to their association with the hierarchical
   namespace governed by a potential HTTP origin server listening for
   SSL/TLS-secured connections on a given TCP port.
</t>
<t>
   All of the requirements listed above for the "http" scheme are also
   requirements for the "https" scheme, except that a default TCP port
   of 443 is assumed if the port subcomponent is empty or not given,
   and the TCP connection MUST be secured for privacy through the
   use of strong encryption prior to sending the first HTTP request.
</t>
<figure><iref primary="true" item="Grammar" subitem="https-URI"/><artwork type="abnf2616"><![CDATA[
  https-URI = "https:" "//" authority path-abempty [ "?" query ]
]]></artwork></figure>
<t>
   Unlike the "http" scheme, responses to "https" identified requests
   are never "public" and thus MUST NOT be reused for shared caching.
   They can, however, be reused in a private cache if the message is
   cacheable by default in HTTP or specifically indicated as such by
   the Cache-Control header field (Section 3.2 of <xref target="Part6"/>).
</t>
<t>
   Resources made available via the "https" scheme have no shared
   identity with the "http" scheme even if their resource identifiers
   indicate the same authority (the same host listening to the same
   TCP port).  They are distinct name spaces and are considered to be
   distinct origin servers.  However, an extension to HTTP that is
   defined to apply to entire host domains, such as the Cookie protocol 
   <xref target="RFC6265"/>, can allow information
   set by one service to impact communication with other services
   within a matching group of host domains.
</t>
<t>
   The process for authoritative access to an "https" identified
   resource is defined in <xref target="RFC2818"/>.
</t>
</section>

<section title="http and https URI Normalization and Comparison" anchor="uri.comparison">
<t>
   Since the "http" and "https" schemes conform to the URI generic syntax,
   such URIs are normalized and compared according to the algorithm defined
   in <xref target="RFC3986"/>, Section 6, using the defaults
   described above for each scheme.
</t>
<t>
   If the port is equal to the default port for a scheme, the normal
   form is to elide the port subcomponent. Likewise, an empty path
   component is equivalent to an absolute path of "/", so the normal
   form is to provide a path of "/" instead. The scheme and host
   are case-insensitive and normally provided in lowercase; all
   other components are compared in a case-sensitive manner.
   Characters other than those in the "reserved" set are equivalent
   to their percent-encoded octets (see <xref target="RFC3986"/>, Section 2.1): the normal form is to not encode them.
</t>
<t>
   For example, the following three URIs are equivalent:
</t>
<figure><artwork type="example"><![CDATA[
   http://example.com:80/~smith/home.html
   http://EXAMPLE.com/%7Esmith/home.html
   http://EXAMPLE.com:/%7esmith/home.html
]]></artwork></figure>
</section>
</section>
</section>

<section title="Message Format" anchor="http.message">




<iref item="header section"/>
<iref item="headers"/>
<iref item="header field"/>
<t>
   All HTTP/1.1 messages consist of a start-line followed by a sequence of
   octets in a format similar to the Internet Message Format
   <xref target="RFC5322"/>: zero or more header fields (collectively
   referred to as the "headers" or the "header section"), an empty line
   indicating the end of the header section, and an optional message-body.
</t>
<t>
   An HTTP message can either be a request from client to server or a
   response from server to client.  Syntactically, the two types of message
   differ only in the start-line, which is either a Request-Line (for requests)
   or a Status-Line (for responses), and in the algorithm for determining
   the length of the message-body (<xref target="message.body"/>).
   In theory, a client could receive requests and a server could receive
   responses, distinguishing them by their different start-line formats,
   but in practice servers are implemented to only expect a request
   (a response is interpreted as an unknown or invalid request method)
   and clients are implemented to only expect a response.
</t>
<figure><iref primary="true" item="Grammar" subitem="HTTP-message"/><artwork type="abnf2616"><![CDATA[
  HTTP-message    = start-line
                    *( header-field CRLF )
                    CRLF
                    [ message-body ]
  start-line      = Request-Line / Status-Line
]]></artwork></figure>
<t>
   Implementations MUST NOT send whitespace between the start-line and
   the first header field. The presence of such whitespace in a request
   might be an attempt to trick a server into ignoring that field or
   processing the line after it as a new request, either of which might
   result in a security vulnerability if other implementations within
   the request chain interpret the same message differently.
   Likewise, the presence of such whitespace in a response might be
   ignored by some clients or cause others to cease parsing.
</t>

<section title="Message Parsing and Robustness" anchor="message.robustness">
<t>
   The normal procedure for parsing an HTTP message is to read the
   start-line into a structure, read each header field into a hash
   table by field name until the empty line, and then use the parsed
   data to determine if a message-body is expected.  If a message-body
   has been indicated, then it is read as a stream until an amount
   of octets equal to the message-body length is read or the connection
   is closed.
</t>
<t>
   Care must be taken to parse an HTTP message as a sequence
   of octets in an encoding that is a superset of US-ASCII.  Attempting
   to parse HTTP as a stream of Unicode characters in a character encoding
   like UTF-16 might introduce security flaws due to the differing ways
   that such parsers interpret invalid characters.
</t>
<t>
   Older HTTP/1.0 client implementations might send an extra CRLF
   after a POST request as a lame workaround for some early server
   applications that failed to read message-body content that was
   not terminated by a line-ending. An HTTP/1.1 client MUST NOT
   preface or follow a request with an extra CRLF.  If terminating
   the request message-body with a line-ending is desired, then the
   client MUST include the terminating CRLF octets as part of the
   message-body length. 
</t>
<t>
   In the interest of robustness, servers SHOULD ignore at least one
   empty line received where a Request-Line is expected. In other words, if
   the server is reading the protocol stream at the beginning of a
   message and receives a CRLF first, it SHOULD ignore the CRLF.
   Likewise, although the line terminator for the start-line and header
   fields is the sequence CRLF, we recommend that recipients recognize a
   single LF as a line terminator and ignore any CR.
</t>
<t>
   When a server listening only for HTTP request messages, or processing
   what appears from the start-line to be an HTTP request message,
   receives a sequence of octets that does not match the HTTP-message
   grammar aside from the robustness exceptions listed above, the
   server MUST respond with an HTTP/1.1 400 (Bad Request) response.  
</t>
</section>

<section title="Header Fields" anchor="header.fields">
  
  
  
  
  
<t>
   Each HTTP header field consists of a case-insensitive field name
   followed by a colon (":"), optional whitespace, and the field value.
</t>
<figure><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"/><artwork type="abnf2616"><![CDATA[
  header-field   = field-name ":" OWS [ field-value ] OWS
  field-name     = token
  field-value    = *( field-content / OWS )
  field-content  = *( WSP / VCHAR / obs-text )
]]></artwork></figure>
<t>
   The field-name token labels the corresponding field-value as having the
   semantics defined by that header field.  For example, the Date header field
   is defined in <xref target="header.date"/> as containing the origination
   timestamp for the message in which it appears.
</t>
<t>
   HTTP header fields are fully extensible: there is no limit on the
   introduction of new field names, each presumably defining new semantics,
   or on the number of header fields used in a given message.  Existing
   fields are defined in each part of this specification and in many other
   specifications outside the standards process.
   New header fields can be introduced without changing the protocol version
   if their defined semantics allow them to be safely ignored by recipients
   that do not recognize them.
</t>
<t>
   New HTTP header fields SHOULD be registered with IANA according
   to the procedures in <xref target="header.field.registration"/>.
   Unrecognized header fields MUST be forwarded by a proxy unless the
   field-name is listed in the Connection header field
   (<xref target="header.connection"/>) or the proxy is specifically
   configured to block or otherwise transform such fields.
   Unrecognized header fields SHOULD be ignored by other recipients.
</t>
<t>
   The order in which header fields with differing field names are
   received is not significant. However, it is "good practice" to send
   header fields that contain control data first, such as Host on
   requests and Date on responses, so that implementations can decide
   when not to handle a message as early as possible.  A server MUST
   wait until the entire header section is received before interpreting
   a request message, since later header fields might include conditionals,
   authentication credentials, or deliberately misleading duplicate
   header fields that would impact request processing.
</t>
<t>
   Multiple header fields with the same field name MUST NOT be
   sent in a message unless the entire field value for that
   header field is defined as a comma-separated list [i.e., #(values)].
   Multiple header fields with the same field name can be combined into
   one "field-name: field-value" pair, without changing the semantics of the
   message, by appending each subsequent field value to the combined
   field value in order, separated by a comma. The order in which
   header fields with the same field name are received is therefore
   significant to the interpretation of the combined field value;
   a proxy MUST NOT change the order of these field values when
   forwarding a message.
</t>
<t><list>
  <t>
   Note: The "Set-Cookie" header field as implemented in
   practice can occur multiple times, but does not use the list syntax, and
   thus cannot be combined into a single line (<xref target="RFC6265"/>). (See Appendix A.2.3 of <xref target="Kri2001"/>
   for details.) Also note that the Set-Cookie2 header field specified in
   <xref target="RFC2965"/> does not share this problem. 
  </t>
</list></t>

<section title="Field Parsing" anchor="field.parsing">
<t>
   No whitespace is allowed between the header field-name and colon.
   In the past, differences in the handling of such whitespace have led to
   security vulnerabilities in request routing and response handling.
   Any received request message that contains whitespace between a header
   field-name and colon MUST be rejected with a response code of 400
   (Bad Request).  A proxy MUST remove any such whitespace from a response
   message before forwarding the message downstream.
</t>
<t>
   A field value MAY be preceded by optional whitespace (OWS); a single SP is
   preferred. The field value does not include any leading or trailing white
   space: OWS occurring before the first non-whitespace octet of the
   field value or after the last non-whitespace octet of the field value
   is ignored and SHOULD be removed before further processing (as this does
   not change the meaning of the header field). 
</t>
<t>
   Historically, HTTP header field values could be extended over multiple
   lines by preceding each extra line with at least one space or horizontal
   tab octet (line folding). This specification deprecates such line
   folding except within the message/http media type
   (<xref target="internet.media.type.message.http"/>).
   HTTP senders MUST NOT produce messages that include line folding
   (i.e., that contain any field-content that matches the obs-fold rule) unless
   the message is intended for packaging within the message/http media type.
   HTTP recipients SHOULD accept line folding and replace any embedded
   obs-fold whitespace with either a single SP or a matching number of SP
   octets (to avoid buffer copying) prior to interpreting the field value or
   forwarding the message downstream.
</t>
<t>
   Historically, HTTP has allowed field content with text in the ISO-8859-1
   <xref target="ISO-8859-1"/> character encoding and supported other
   character sets only through use of <xref target="RFC2047"/> encoding.
   In practice, most HTTP header field values use only a subset of the
   US-ASCII character encoding <xref target="USASCII"/>. Newly defined
   header fields SHOULD limit their field values to US-ASCII octets.
   Recipients SHOULD treat other (obs-text) octets in field content as
   opaque data.
</t>
</section>

<section title="Field Length" anchor="field.length">
<t>
   HTTP does not place a pre-defined limit on the length of header fields,
   either in isolation or as a set. A server MUST be prepared to receive
   request header fields of unbounded length and respond with a 4xx status
   code if the received header field(s) would be longer than the server wishes
   to handle.
</t>
<t>
   A client that receives response headers that are longer than it wishes to
   handle can only treat it as a server error.
</t>
<t>
   Various ad-hoc limitations on header length are found in practice. It is
   RECOMMENDED that all HTTP senders and recipients support messages whose
   combined header fields have 4000 or more octets.
</t>
</section>

<section title="Common Field ABNF Rules" anchor="field.rules">
<t anchor="rule.token.separators">
  
  
  
  
   Many HTTP/1.1 header field values consist of words (token or quoted-string)
   separated by whitespace or special characters. These special characters
   MUST be in a quoted string to be used within a parameter value (as defined
   in <xref target="transfer.codings"/>).
</t>
<figure><iref primary="true" item="Grammar" subitem="word"/><iref primary="true" item="Grammar" subitem="token"/><iref primary="true" item="Grammar" subitem="tchar"/><iref primary="true" item="Grammar" subitem="special"/><artwork type="abnf2616"><![CDATA[
  word           = token / quoted-string

  token          = 1*tchar

  tchar          = "!" / "#" / "$" / "%" / "&" / "'" / "*"
                 / "+" / "-" / "." / "^" / "_" / "`" / "|" / "~" 
                 / DIGIT / ALPHA
                 ; any VCHAR, except special

  special        = "(" / ")" / "<" / ">" / "@" / ","
                 / ";" / ":" / "\" / DQUOTE / "/" / "[" 
                 / "]" / "?" / "=" / "{" / "}"
]]></artwork></figure>
<t anchor="rule.quoted-string">
  
  
  
   A string of text is parsed as a single word if it is quoted using
   double-quote marks.
</t>
<figure><iref primary="true" item="Grammar" subitem="quoted-string"/><iref primary="true" item="Grammar" subitem="qdtext"/><iref primary="true" item="Grammar" subitem="obs-text"/><artwork type="abnf2616"><![CDATA[
  quoted-string  = DQUOTE *( qdtext / quoted-pair ) DQUOTE
  qdtext         = OWS / %x21 / %x23-5B / %x5D-7E / obs-text
  obs-text       = %x80-FF
]]></artwork></figure>
<t anchor="rule.quoted-pair">
  
   The backslash octet ("\") can be used as a single-octet
   quoting mechanism within quoted-string constructs:
</t>
<figure><iref primary="true" item="Grammar" subitem="quoted-pair"/><artwork type="abnf2616"><![CDATA[
  quoted-pair    = "\" ( WSP / VCHAR / obs-text ) 
]]></artwork></figure>
<t>
   Recipients that process the value of the quoted-string MUST handle a
   quoted-pair as if it were replaced by the octet following the backslash. 
</t>
<t>
   Senders SHOULD NOT escape octets in quoted-strings that do not require
   escaping (i.e., other than DQUOTE and the backslash octet).
</t>
<t anchor="rule.comment">
  
  
   Comments can be included in some HTTP header fields by surrounding
   the comment text with parentheses. Comments are only allowed in
   fields containing "comment" as part of their field value definition.
</t>
<figure><iref primary="true" item="Grammar" subitem="comment"/><iref primary="true" item="Grammar" subitem="ctext"/><artwork type="abnf2616"><![CDATA[
  comment        = "(" *( ctext / quoted-cpair / comment ) ")"
  ctext          = OWS / %x21-27 / %x2A-5B / %x5D-7E / obs-text
]]></artwork></figure>
<t anchor="rule.quoted-cpair">
  
   The backslash octet ("\") can be used as a single-octet
   quoting mechanism within comment constructs:
</t>
<figure><iref primary="true" item="Grammar" subitem="quoted-cpair"/><artwork type="abnf2616"><![CDATA[
  quoted-cpair    = "\" ( WSP / VCHAR / obs-text ) 
]]></artwork></figure>
<t>
   Senders SHOULD NOT escape octets in comments that do not require escaping
   (i.e., other than the backslash octet "\" and the parentheses "(" and ")").
</t>
</section>
</section>

<section title="Message Body" anchor="message.body">
  
<t>
   The message-body (if any) of an HTTP message is used to carry the
   payload body associated with the request or response.
</t>
<figure><iref primary="true" item="Grammar" subitem="message-body"/><artwork type="abnf2616"><![CDATA[
  message-body = *OCTET
]]></artwork></figure>
<t>
   The message-body differs from the payload body only when a transfer-coding
   has been applied, as indicated by the Transfer-Encoding header field
   (<xref target="header.transfer-encoding"/>).  If more than one
   Transfer-Encoding header field is present in a message, the multiple
   field-values MUST be combined into one field-value, according to the
   algorithm defined in <xref target="header.fields"/>, before determining
   the message-body length.
</t>
<t>
   When one or more transfer-codings are applied to a payload in order to
   form the message-body, the Transfer-Encoding header field MUST contain
   the list of transfer-codings applied. Transfer-Encoding is a property of
   the message, not of the payload, and thus MAY be added or removed by
   any implementation along the request/response chain under the constraints
   found in <xref target="transfer.codings"/>.
</t>
<t>
   If a message is received that has multiple Content-Length header fields
   (<xref target="header.content-length"/>) with field-values consisting
   of the same decimal value, or a single Content-Length header field with
   a field value containing a list of identical decimal values (e.g.,
   "Content-Length: 42, 42"), indicating that duplicate Content-Length
   header fields have been generated or combined by an upstream message
   processor, then the recipient MUST either reject the message as invalid
   or replace the duplicated field-values with a single valid Content-Length
   field containing that decimal value prior to determining the message-body
   length.
</t>
<t>
   The rules for when a message-body is allowed in a message differ for
   requests and responses.
</t>
<t>
   The presence of a message-body in a request is signaled by the
   inclusion of a Content-Length or Transfer-Encoding header field in
   the request's header fields, even if the request method does not
   define any use for a message-body.  This allows the request
   message framing algorithm to be independent of method semantics.
</t>
<t>
   For response messages, whether or not a message-body is included with
   a message is dependent on both the request method and the response
   status code (<xref target="status.code.and.reason.phrase"/>).
   Responses to the HEAD request method never include a message-body
   because the associated response header fields (e.g., Transfer-Encoding,
   Content-Length, etc.) only indicate what their values would have been
   if the request method had been GET.  All 1xx (Informational), 204 (No Content),
   and 304 (Not Modified) responses MUST NOT include a message-body.
   All other responses do include a message-body, although the body
   MAY be of zero length.
</t>
<t>
   The length of the message-body is determined by one of the following
   (in order of precedence):
</t>
<t>
  <list style="numbers">
    <t>
     Any response to a HEAD request and any response with a status
     code of 100-199, 204, or 304 is always terminated by the first
     empty line after the header fields, regardless of the header
     fields present in the message, and thus cannot contain a message-body.
    </t>
    <t>
     If a Transfer-Encoding header field is present
     and the "chunked" transfer-coding (<xref target="transfer.codings"/>)
     is the final encoding, the message-body length is determined by reading
     and decoding the chunked data until the transfer-coding indicates the
     data is complete.
    <vspace blankLines="1"/>
     If a Transfer-Encoding header field is present in a response and the
     "chunked" transfer-coding is not the final encoding, the message-body
     length is determined by reading the connection until it is closed by
     the server.
     If a Transfer-Encoding header field is present in a request and the
     "chunked" transfer-coding is not the final encoding, the message-body
     length cannot be determined reliably; the server MUST respond with
     the 400 (Bad Request) status code and then close the connection.
    <vspace blankLines="1"/>
     If a message is received with both a Transfer-Encoding header field
     and a Content-Length header field, the Transfer-Encoding overrides
     the Content-Length.
     Such a message might indicate an attempt to perform request or response
     smuggling (bypass of security-related checks on message routing or content)
     and thus ought to be handled as an error.  The provided Content-Length MUST
     be removed, prior to forwarding the message downstream, or replaced with
     the real message-body length after the transfer-coding is decoded.
    </t>
    <t>
     If a message is received without Transfer-Encoding and with either
     multiple Content-Length header fields having differing field-values or
     a single Content-Length header field having an invalid value, then the
     message framing is invalid and MUST be treated as an error to
     prevent request or response smuggling.
     If this is a request message, the server MUST respond with
     a 400 (Bad Request) status code and then close the connection.
     If this is a response message received by a proxy, the proxy
     MUST discard the received response, send a 502 (Bad Gateway)
     status code as its downstream response, and then close the connection.
     If this is a response message received by a user-agent, it MUST be
     treated as an error by discarding the message and closing the connection.
    </t>
    <t>
     If a valid Content-Length header field 
     is present without Transfer-Encoding, its decimal value defines the
     message-body length in octets.  If the actual number of octets sent in
     the message is less than the indicated Content-Length, the recipient
     MUST consider the message to be incomplete and treat the connection
     as no longer usable.
     If the actual number of octets sent in the message is more than the indicated
     Content-Length, the recipient MUST only process the message-body up to the
     field value's number of octets; the remainder of the message MUST either
     be discarded or treated as the next message in a pipeline.  For the sake of
     robustness, a user-agent MAY attempt to detect and correct such an error
     in message framing if it is parsing the response to the last request on
     a connection and the connection has been closed by the server.
    </t>
    <t>
     If this is a request message and none of the above are true, then the
     message-body length is zero (no message-body is present).
    </t>
    <t>
     Otherwise, this is a response message without a declared message-body
     length, so the message-body length is determined by the number of octets
     received prior to the server closing the connection.
    </t>
  </list>
</t>
<t>
   Since there is no way to distinguish a successfully completed,
   close-delimited message from a partially-received message interrupted
   by network failure, implementations SHOULD use encoding or
   length-delimited messages whenever possible.  The close-delimiting
   feature exists primarily for backwards compatibility with HTTP/1.0.
</t>
<t>
   A server MAY reject a request that contains a message-body but
   not a Content-Length by responding with 411 (Length Required).
</t>
<t>
   Unless a transfer-coding other than "chunked" has been applied,
   a client that sends a request containing a message-body SHOULD
   use a valid Content-Length header field if the message-body length
   is known in advance, rather than the "chunked" encoding, since some
   existing services respond to "chunked" with a 411 (Length Required)
   status code even though they understand the chunked encoding.  This
   is typically because such services are implemented via a gateway that
   requires a content-length in advance of being called and the server
   is unable or unwilling to buffer the entire request before processing.
</t>
<t>
   A client that sends a request containing a message-body MUST include a
   valid Content-Length header field if it does not know the server will
   handle HTTP/1.1 (or later) requests; such knowledge can be in the form
   of specific user configuration or by remembering the version of a prior
   received response.
</t>
</section>

<section anchor="incomplete.messages" title="Incomplete Messages">
<t>
   Request messages that are prematurely terminated, possibly due to a
   cancelled connection or a server-imposed time-out exception, MUST
   result in closure of the connection; sending an HTTP/1.1 error response
   prior to closing the connection is OPTIONAL.
</t>
<t>
   Response messages that are prematurely terminated, usually by closure
   of the connection prior to receiving the expected number of octets or by
   failure to decode a transfer-encoded message-body, MUST be recorded
   as incomplete.  A response that terminates in the middle of the header
   block (before the empty line is received) cannot be assumed to convey the
   full semantics of the response and MUST be treated as an error.
</t>
<t>
   A message-body that uses the chunked transfer encoding is
   incomplete if the zero-sized chunk that terminates the encoding has not
   been received.  A message that uses a valid Content-Length is incomplete
   if the size of the message-body received (in octets) is less than the
   value given by Content-Length.  A response that has neither chunked
   transfer encoding nor Content-Length is terminated by closure of the
   connection, and thus is considered complete regardless of the number of
   message-body octets received, provided that the header block was received
   intact.
</t>
<t>
   A user agent MUST NOT render an incomplete response message-body as if
   it were complete (i.e., some indication must be given to the user that an
   error occurred).  Cache requirements for incomplete responses are defined
   in Section 2.1 of <xref target="Part6"/>.
</t>
<t>
   A server MUST read the entire request message-body or close
   the connection after sending its response, since otherwise the
   remaining data on a persistent connection would be misinterpreted
   as the next request.  Likewise,
   a client MUST read the entire response message-body if it intends
   to reuse the same connection for a subsequent request.  Pipelining
   multiple requests on a connection is described in <xref target="pipelining"/>.
</t>
</section>

<section title="General Header Fields" anchor="general.header.fields">
  
<t>
   There are a few header fields which have general applicability for
   both request and response messages, but which do not apply to the
   payload being transferred. These header fields apply only to the
   message being transmitted.
</t>
<texttable align="left">
  <ttcol>Header Field Name</ttcol>
  <ttcol>Defined in...</ttcol>
  
  <c>Connection</c> <c><xref target="header.connection"/></c>
  <c>Date</c> <c><xref target="header.date"/></c>
  <c>Trailer</c> <c><xref target="header.trailer"/></c>
  <c>Transfer-Encoding</c> <c><xref target="header.transfer-encoding"/></c>
  <c>Upgrade</c> <c><xref target="header.upgrade"/></c>
  <c>Via</c> <c><xref target="header.via"/></c>
</texttable>
</section>
</section>

<section title="Request" anchor="request">
  
<t>
   A request message from a client to a server begins with a
   Request-Line, followed by zero or more header fields, an empty
   line signifying the end of the header block, and an optional
   message body.
</t>
<!--                 Host                      ; should be moved here eventually -->
<figure><iref primary="true" item="Grammar" subitem="Request"/><artwork type="abnf2616"><![CDATA[
  Request       = Request-Line              ; Section 4.1
                  *( header-field CRLF )    ; Section 3.2
                  CRLF
                  [ message-body ]          ; Section 3.3
]]></artwork></figure>

<section title="Request-Line" anchor="request-line">
  
<t>
   The Request-Line begins with a method token, followed by a single
   space (SP), the request-target, another single space (SP), the
   protocol version, and ending with CRLF.
</t>
<figure><iref primary="true" item="Grammar" subitem="Request-Line"/><artwork type="abnf2616"><![CDATA[
  Request-Line   = Method SP request-target SP HTTP-Version CRLF
]]></artwork></figure>

<section title="Method" anchor="method">
  
<t>
   The Method token indicates the request method to be performed on the
   target resource. The request method is case-sensitive.
</t>
<figure><iref primary="true" item="Grammar" subitem="Method"/><artwork type="abnf2616"><![CDATA[
  Method         = token
]]></artwork></figure>
</section>

<section title="request-target" anchor="request-target">
  
<t>
   The request-target identifies the target resource upon which to apply
   the request.  In most cases, the user agent is provided a URI reference
   from which it determines an absolute URI for identifying the target
   resource.  When a request to the resource is initiated, all or part
   of that URI is used to construct the HTTP request-target.
</t>
<figure><iref primary="true" item="Grammar" subitem="request-target"/><artwork type="abnf2616"><![CDATA[
  request-target = "*"
                 / absolute-URI
                 / ( path-absolute [ "?" query ] )
                 / authority
]]></artwork></figure>
<t>
   The four options for request-target are dependent on the nature of the
   request.
</t>   
<t><iref item="asterisk form (of request-target)"/>
   The asterisk "*" form of request-target, which MUST NOT be used
   with any request method other than OPTIONS, means that the request
   applies to the server as a whole (the listening process) rather than
   to a specific named resource at that server.  For example,
</t>
<figure><artwork type="message/http; msgtype=&#34;request&#34;"><![CDATA[
  OPTIONS * HTTP/1.1
  ]]></artwork></figure>
<t><iref item="absolute-URI form (of request-target)"/>
   The "absolute-URI" form is REQUIRED when the request is being made to a
   proxy. The proxy is requested to either forward the request or service it
   from a valid cache, and then return the response. Note that the proxy MAY
   forward the request on to another proxy or directly to the server
   specified by the absolute-URI. In order to avoid request loops, a
   proxy that forwards requests to other proxies MUST be able to
   recognize and exclude all of its own server names, including
   any aliases, local variations, and the numeric IP address. An example
   Request-Line would be:
</t>
<figure><artwork type="message/http; msgtype=&#34;request&#34;"><![CDATA[
  GET http://www.example.org/pub/WWW/TheProject.html HTTP/1.1
  ]]></artwork></figure>
<t>
   To allow for transition to absolute-URIs in all requests in future
   versions of HTTP, all HTTP/1.1 servers MUST accept the absolute-URI
   form in requests, even though HTTP/1.1 clients will only generate
   them in requests to proxies.
</t>
<t>
   If a proxy receives a host name that is not a fully qualified domain
   name, it MAY add its domain to the host name it received. If a proxy
   receives a fully qualified domain name, the proxy MUST NOT change
   the host name.
</t>
<t><iref item="authority form (of request-target)"/>
   The "authority form" is only used by the CONNECT request method (Section 7.9 of <xref target="Part2"/>).
</t>
<t><iref item="origin form (of request-target)"/>
   The most common form of request-target is that used when making
   a request to an origin server ("origin form").
   In this case, the absolute path and query components of the URI
   MUST be transmitted as the request-target, and the authority component
   MUST be transmitted in a Host header field. For example, a client wishing
   to retrieve a representation of the resource, as identified above,
   directly from the origin server would open (or reuse) a TCP connection
   to port 80 of the host "www.example.org" and send the lines:
</t>
<figure><artwork type="message/http; msgtype=&#34;request&#34;"><![CDATA[
  GET /pub/WWW/TheProject.html HTTP/1.1
  Host: www.example.org
  ]]></artwork></figure>
<t>
   followed by the remainder of the Request. Note that the origin form
   of request-target always starts with an absolute path; if the target
   resource's URI path is empty, then an absolute path of "/" MUST be
   provided in the request-target.
</t>
<t>
   If a proxy receives an OPTIONS request with an absolute-URI form of
   request-target in which the URI has an empty path and no query component,
   then the last proxy on the request chain MUST use a request-target
   of "*" when it forwards the request to the indicated origin server.
</t>
<figure><preamble>   
   For example, the request
</preamble><artwork type="message/http; msgtype=&#34;request&#34;"><![CDATA[
  OPTIONS http://www.example.org:8001 HTTP/1.1
  ]]></artwork></figure>
<figure><preamble>   
  would be forwarded by the final proxy as
</preamble><artwork type="message/http; msgtype=&#34;request&#34;"><![CDATA[
  OPTIONS * HTTP/1.1
  Host: www.example.org:8001
  ]]></artwork>
<postamble>
   after connecting to port 8001 of host "www.example.org".
</postamble>
</figure>
<t>
   The request-target is transmitted in the format specified in 
   <xref target="http.uri"/>. If the request-target is percent-encoded
   (<xref target="RFC3986"/>, Section 2.1), the origin server
   MUST decode the request-target in order to
   properly interpret the request. Servers SHOULD respond to invalid
   request-targets with an appropriate status code.
</t>
<t>
   A non-transforming proxy MUST NOT rewrite the "path-absolute" part of the
   received request-target when forwarding it to the next inbound server,
   except as noted above to replace a null path-absolute with "/" or "*".
</t>
<t><list>
  <t>
    Note: The "no rewrite" rule prevents the proxy from changing the
    meaning of the request when the origin server is improperly using
    a non-reserved URI character for a reserved purpose.  Implementors
    need to be aware that some pre-HTTP/1.1 proxies have been known to
    rewrite the request-target.
  </t>
</list></t>
<t>
   HTTP does not place a pre-defined limit on the length of a request-target.
   A server MUST be prepared to receive URIs of unbounded length and
   respond with the 414 (URI Too Long) status code if the received
   request-target would be longer than the server wishes to handle
   (see Section 8.4.15 of <xref target="Part2"/>).
</t>
<t>
   Various ad-hoc limitations on request-target length are found in practice.
   It is RECOMMENDED that all HTTP senders and recipients support
   request-target lengths of 8000 or more octets.
</t>
<t><list>
  <t>
    Note: Fragments (<xref target="RFC3986"/>, Section 3.5)
    are not part of the request-target and thus will not be transmitted
    in an HTTP request.
  </t>
</list></t>
</section>
</section>

<section title="The Resource Identified by a Request" anchor="the.resource.identified.by.a.request">
<t>
   The exact resource identified by an Internet request is determined by
   examining both the request-target and the Host header field.
</t>
<t>
   An origin server that does not allow resources to differ by the
   requested host MAY ignore the Host header field value when
   determining the resource identified by an HTTP/1.1 request. (But see
   <xref target="changes.to.simplify.multi-homed.web.servers.and.conserve.ip.addresses"/>
   for other requirements on Host support in HTTP/1.1.)
</t>
<t>
   An origin server that does differentiate resources based on the host
   requested (sometimes referred to as virtual hosts or vanity host
   names) MUST use the following rules for determining the requested
   resource on an HTTP/1.1 request:
  <list style="numbers">
    <t>If request-target is an absolute-URI, the host is part of the
     request-target. Any Host header field value in the request MUST be
     ignored.</t>
    <t>If the request-target is not an absolute-URI, and the request includes
     a Host header field, the host is determined by the Host header
     field value.</t>
    <t>If the host as determined by rule 1 or 2 is not a valid host on
     the server, the response MUST be a 400 (Bad Request) error message.</t>
  </list>
</t>
<t>
   Recipients of an HTTP/1.0 request that lacks a Host header field MAY
   attempt to use heuristics (e.g., examination of the URI path for
   something unique to a particular host) in order to determine what
   exact resource is being requested.
</t>
</section>

<section title="Effective Request URI" anchor="effective.request.uri">
  <iref primary="true" item="effective request URI"/>
  <iref primary="true" item="target resource"/>
<t>
   HTTP requests often do not carry the absolute URI (<xref target="RFC3986"/>, Section 4.3)
   for the target resource; instead, the URI needs to be inferred from the
   request-target, Host header field, and connection context. The result of
   this process is called the "effective request URI".  The "target resource"
   is the resource identified by the effective request URI.
</t>
<t>
   If the request-target is an absolute-URI, then the effective request URI is
   the request-target.
</t>
<t>
   If the request-target uses the path-absolute form or the asterisk form,
   and the Host header field is present, then the effective request URI is
   constructed by concatenating
</t>
<t>
  <list style="symbols">
    <t>
      the scheme name: "http" if the request was received over an insecure
      TCP connection, or "https" when received over a SSL/TLS-secured TCP
      connection,
    </t>
    <t>
      the octet sequence "://",
    </t>
    <t>
      the authority component, as specified in the Host header field
      (<xref target="header.host"/>), and
    </t>
    <t>
      the request-target obtained from the Request-Line, unless the
      request-target is just the asterisk "*".
    </t>
  </list>
</t>
<t>
   If the request-target uses the path-absolute form or the asterisk form,
   and the Host header field is not present, then the effective request URI is
   undefined.
</t>
<t>
   Otherwise, when request-target uses the authority form, the effective
   request URI is undefined.
</t>
<figure>
<preamble>
   Example 1: the effective request URI for the message
</preamble> 
<artwork type="example"><![CDATA[
  GET /pub/WWW/TheProject.html HTTP/1.1
  Host: www.example.org:8080
  ]]></artwork>
<postamble>
  (received over an insecure TCP connection) is "http", plus "://", plus the
  authority component "www.example.org:8080", plus the request-target
  "/pub/WWW/TheProject.html", thus
  "http://www.example.org:8080/pub/WWW/TheProject.html".
</postamble>
</figure>
<figure>
<preamble>
   Example 2: the effective request URI for the message
</preamble> 
<artwork type="example"><![CDATA[
  OPTIONS * HTTP/1.1
  Host: www.example.org
  ]]></artwork>
<postamble>
  (received over an SSL/TLS secured TCP connection) is "https", plus "://", plus the
  authority component "www.example.org", thus "https://www.example.org".
</postamble>
</figure>
<t>
   Effective request URIs are compared using the rules described in 
   <xref target="uri.comparison"/>, except that empty path components MUST NOT
   be treated as equivalent to an absolute path of "/".
</t>  
</section>

</section>


<section title="Response" anchor="response">
  
<t>
   After receiving and interpreting a request message, a server responds
   with an HTTP response message.
</t>
<figure><iref primary="true" item="Grammar" subitem="Response"/><artwork type="abnf2616"><![CDATA[
  Response      = Status-Line               ; Section 5.1
                  *( header-field CRLF )    ; Section 3.2
                  CRLF
                  [ message-body ]          ; Section 3.3
]]></artwork></figure>

<section title="Status-Line" anchor="status-line">
  
<t>
   The first line of a Response message is the Status-Line, consisting
   of the protocol version, a space (SP), the status code, another space,
   a possibly-empty textual phrase describing the status code, and
   ending with CRLF.
</t>
<figure><iref primary="true" item="Grammar" subitem="Status-Line"/><artwork type="abnf2616"><![CDATA[
  Status-Line = HTTP-Version SP Status-Code SP Reason-Phrase CRLF
]]></artwork></figure>

<section title="Status Code and Reason Phrase" anchor="status.code.and.reason.phrase">
  
  
<t>
   The Status-Code element is a 3-digit integer result code of the
   attempt to understand and satisfy the request. These codes are fully
   defined in Section 8 of <xref target="Part2"/>.  The Reason Phrase exists for the sole
   purpose of providing a textual description associated with the numeric
   status code, out of deference to earlier Internet application protocols
   that were more frequently used with interactive text clients.
   A client SHOULD ignore the content of the Reason Phrase.
</t>
<t>
   The first digit of the Status-Code defines the class of response. The
   last two digits do not have any categorization role. There are 5
   values for the first digit:
  <list style="symbols">
    <t>
      1xx: Informational - Request received, continuing process
    </t>
    <t>
      2xx: Success - The action was successfully received,
        understood, and accepted
    </t>
    <t>
      3xx: Redirection - Further action must be taken in order to
        complete the request
    </t>
    <t>
      4xx: Client Error - The request contains bad syntax or cannot
        be fulfilled
    </t>
    <t>
      5xx: Server Error - The server failed to fulfill an apparently
        valid request
    </t>
  </list>
</t>
<figure><iref primary="true" item="Grammar" subitem="Status-Code"/><iref primary="true" item="Grammar" subitem="Reason-Phrase"/><artwork type="abnf2616"><![CDATA[
  Status-Code    = 3DIGIT
  Reason-Phrase  = *( WSP / VCHAR / obs-text )
]]></artwork></figure>
</section>
</section>

</section>


<section title="Protocol Parameters" anchor="protocol.parameters">

<section title="Date/Time Formats: Full Date" anchor="date.time.formats.full.date">
  
<t>
   HTTP applications have historically allowed three different formats
   for date/time stamps. However, the preferred format is a fixed-length subset
   of that defined by <xref target="RFC1123"/>:
</t>
<figure><artwork type="example"><![CDATA[
  Sun, 06 Nov 1994 08:49:37 GMT  ; RFC 1123
  ]]></artwork></figure>
<t>
   The other formats are described here only for compatibility with obsolete
   implementations.
</t>
<figure><artwork type="example"><![CDATA[
  Sunday, 06-Nov-94 08:49:37 GMT ; obsolete RFC 850 format
  Sun Nov  6 08:49:37 1994       ; ANSI C's asctime() format
  ]]></artwork></figure>
<t>
   HTTP/1.1 clients and servers that parse a date value MUST accept
   all three formats (for compatibility with HTTP/1.0), though they MUST
   only generate the RFC 1123 format for representing HTTP-date values
   in header fields.
</t>
<t>
   All HTTP date/time stamps MUST be represented in Greenwich Mean Time
   (GMT), without exception. For the purposes of HTTP, GMT is exactly
   equal to UTC (Coordinated Universal Time). This is indicated in the
   first two formats by the inclusion of "GMT" as the three-letter
   abbreviation for time zone, and MUST be assumed when reading the
   asctime format. HTTP-date is case sensitive and MUST NOT include
   additional whitespace beyond that specifically included as SP in the
   grammar.
</t>
<figure><iref primary="true" item="Grammar" subitem="HTTP-date"/><artwork type="abnf2616"><![CDATA[
  HTTP-date    = rfc1123-date / obs-date
]]></artwork></figure>
<t anchor="preferred.date.format">
  
  
  
  
  
  
  
  
  
  
  Preferred format:
</t>
<figure><iref primary="true" item="Grammar" subitem="rfc1123-date"/><iref primary="true" item="Grammar" subitem="date1"/><iref primary="true" item="Grammar" subitem="time-of-day"/><iref primary="true" item="Grammar" subitem="hour"/><iref primary="true" item="Grammar" subitem="minute"/><iref primary="true" item="Grammar" subitem="second"/><iref primary="true" item="Grammar" subitem="day-name"/><iref primary="true" item="Grammar" subitem="day-name-l"/><iref primary="true" item="Grammar" subitem="day"/><iref primary="true" item="Grammar" subitem="month"/><iref primary="true" item="Grammar" subitem="year"/><iref primary="true" item="Grammar" subitem="GMT"/><artwork type="abnf2616"><![CDATA[
  rfc1123-date = day-name "," SP date1 SP time-of-day SP GMT
  ; fixed length subset of the format defined in
  ; Section 5.2.14 of [RFC1123]
  
  day-name     = %x4D.6F.6E ; "Mon", case-sensitive
               / %x54.75.65 ; "Tue", case-sensitive
               / %x57.65.64 ; "Wed", case-sensitive
               / %x54.68.75 ; "Thu", case-sensitive
               / %x46.72.69 ; "Fri", case-sensitive
               / %x53.61.74 ; "Sat", case-sensitive
               / %x53.75.6E ; "Sun", case-sensitive
               
  date1        = day SP month SP year
               ; e.g., 02 Jun 1982

  day          = 2DIGIT
  month        = %x4A.61.6E ; "Jan", case-sensitive
               / %x46.65.62 ; "Feb", case-sensitive
               / %x4D.61.72 ; "Mar", case-sensitive
               / %x41.70.72 ; "Apr", case-sensitive
               / %x4D.61.79 ; "May", case-sensitive
               / %x4A.75.6E ; "Jun", case-sensitive
               / %x4A.75.6C ; "Jul", case-sensitive
               / %x41.75.67 ; "Aug", case-sensitive
               / %x53.65.70 ; "Sep", case-sensitive
               / %x4F.63.74 ; "Oct", case-sensitive
               / %x4E.6F.76 ; "Nov", case-sensitive
               / %x44.65.63 ; "Dec", case-sensitive
  year         = 4DIGIT

  GMT   = %x47.4D.54 ; "GMT", case-sensitive

  time-of-day  = hour ":" minute ":" second
                 ; 00:00:00 - 23:59:59
                 
  hour         = 2DIGIT               
  minute       = 2DIGIT               
  second       = 2DIGIT               
]]></artwork></figure>
<t>
  The semantics of <xref target="preferred.date.format" format="none">day-name</xref>, <xref target="preferred.date.format" format="none">day</xref>,
  <xref target="preferred.date.format" format="none">month</xref>, <xref target="preferred.date.format" format="none">year</xref>, and <xref target="preferred.date.format" format="none">time-of-day</xref> are the
  same as those defined for the RFC 5322 constructs
  with the corresponding name (<xref target="RFC5322"/>, Section 3.3).
</t>
<t anchor="obsolete.date.formats">
  
  
  
  
  
  
  
  
  Obsolete formats:
</t>
<figure><iref primary="true" item="Grammar" subitem="obs-date"/><artwork type="abnf2616"><![CDATA[
  obs-date     = rfc850-date / asctime-date 
]]></artwork></figure>
<figure><iref primary="true" item="Grammar" subitem="rfc850-date"/><artwork type="abnf2616"><![CDATA[
  rfc850-date  = day-name-l "," SP date2 SP time-of-day SP GMT
  date2        = day "-" month "-" 2DIGIT
                 ; day-month-year (e.g., 02-Jun-82)

  day-name-l   = %x4D.6F.6E.64.61.79 ; "Monday", case-sensitive
         / %x54.75.65.73.64.61.79 ; "Tuesday", case-sensitive
         / %x57.65.64.6E.65.73.64.61.79 ; "Wednesday", case-sensitive
         / %x54.68.75.72.73.64.61.79 ; "Thursday", case-sensitive
         / %x46.72.69.64.61.79 ; "Friday", case-sensitive
         / %x53.61.74.75.72.64.61.79 ; "Saturday", case-sensitive
         / %x53.75.6E.64.61.79 ; "Sunday", case-sensitive
]]></artwork></figure>
<figure><iref primary="true" item="Grammar" subitem="asctime-date"/><artwork type="abnf2616"><![CDATA[
  asctime-date = day-name SP date3 SP time-of-day SP year
  date3        = month SP ( 2DIGIT / ( SP 1DIGIT ))
                 ; month day (e.g., Jun  2)
]]></artwork></figure>
<t><list>
  <t>
    Note: Recipients of date values are encouraged to be robust in
    accepting date values that might have been sent by non-HTTP
    applications, as is sometimes the case when retrieving or posting
    messages via proxies/gateways to SMTP or NNTP.
  </t>
</list></t>
<t><list>
  <t>
    Note: HTTP requirements for the date/time stamp format apply only
    to their usage within the protocol stream. Clients and servers are
    not required to use these formats for user presentation, request
    logging, etc.
  </t>
</list></t>
</section>

<section title="Transfer Codings" anchor="transfer.codings">
  
  
<t>
   Transfer-coding values are used to indicate an encoding
   transformation that has been, can be, or might need to be applied to a
   payload body in order to ensure "safe transport" through the network.
   This differs from a content coding in that the transfer-coding is a
   property of the message rather than a property of the representation
   that is being transferred.
</t>
<figure><iref primary="true" item="Grammar" subitem="transfer-coding"/><iref primary="true" item="Grammar" subitem="transfer-extension"/><artwork type="abnf2616"><![CDATA[
  transfer-coding         = "chunked" ; Section 6.2.1
                          / "compress" ; Section 6.2.2.1
                          / "deflate" ; Section 6.2.2.2
                          / "gzip" ; Section 6.2.2.3
                          / transfer-extension
  transfer-extension      = token *( OWS ";" OWS transfer-parameter )
]]></artwork></figure>
<t anchor="rule.parameter">
  
  
  
   Parameters are in the form of attribute/value pairs.
</t>
<figure><iref primary="true" item="Grammar" subitem="transfer-parameter"/><iref primary="true" item="Grammar" subitem="attribute"/><iref primary="true" item="Grammar" subitem="value"/><iref primary="true" item="Grammar" subitem="date2"/><iref primary="true" item="Grammar" subitem="date3"/><artwork type="abnf2616"><![CDATA[
  transfer-parameter      = attribute BWS "=" BWS value
  attribute               = token
  value                   = word
]]></artwork></figure>
<t>
   All transfer-coding values are case-insensitive. HTTP/1.1 uses
   transfer-coding values in the TE header field (<xref target="header.te"/>) and in
   the Transfer-Encoding header field (<xref target="header.transfer-encoding"/>).
</t>
<t>
   Transfer-codings are analogous to the Content-Transfer-Encoding values of
   MIME, which were designed to enable safe transport of binary data over a
   7-bit transport service (<xref target="RFC2045"/>, Section 6).
   However, safe transport
   has a different focus for an 8bit-clean transfer protocol. In HTTP,
   the only unsafe characteristic of message-bodies is the difficulty in
   determining the exact message body length (<xref target="message.body"/>),
   or the desire to encrypt data over a shared transport.
</t>
<t>
   A server that receives a request message with a transfer-coding it does
   not understand SHOULD respond with 501 (Not Implemented) and then
   close the connection. A server MUST NOT send transfer-codings to an HTTP/1.0
   client.
</t>

<section title="Chunked Transfer Coding" anchor="chunked.encoding">
  <iref item="chunked (Coding Format)"/>
  <iref item="Coding Format" subitem="chunked"/>
  
  
  
  
  
  
  
  
  
  
  
<t>
   The chunked encoding modifies the body of a message in order to
   transfer it as a series of chunks, each with its own size indicator,
   followed by an OPTIONAL trailer containing header fields. This
   allows dynamically produced content to be transferred along with the
   information necessary for the recipient to verify that it has
   received the full message.
</t>
<figure><iref primary="true" item="Grammar" subitem="Chunked-Body"/><iref primary="true" item="Grammar" subitem="chunk"/><iref primary="true" item="Grammar" subitem="chunk-size"/><iref primary="true" item="Grammar" subitem="last-chunk"/><iref primary="true" item="Grammar" subitem="chunk-ext"/><iref primary="true" item="Grammar" subitem="chunk-ext-name"/><iref primary="true" item="Grammar" subitem="chunk-ext-val"/><iref primary="true" item="Grammar" subitem="chunk-data"/><iref primary="true" item="Grammar" subitem="trailer-part"/><iref primary="true" item="Grammar" subitem="quoted-str-nf"/><iref primary="true" item="Grammar" subitem="qdtext-nf"/><artwork type="abnf2616"><![CDATA[
  Chunked-Body   = *chunk
                   last-chunk
                   trailer-part
                   CRLF
  
  chunk          = chunk-size *WSP [ chunk-ext ] CRLF
                   chunk-data CRLF
  chunk-size     = 1*HEXDIG
  last-chunk     = 1*("0") *WSP [ chunk-ext ] CRLF
  
  chunk-ext      = *( ";" *WSP chunk-ext-name
                      [ "=" chunk-ext-val ] *WSP )
  chunk-ext-name = token
  chunk-ext-val  = token / quoted-str-nf
  chunk-data     = 1*OCTET ; a sequence of chunk-size octets
  trailer-part   = *( header-field CRLF )
  
  quoted-str-nf  = DQUOTE *( qdtext-nf / quoted-pair ) DQUOTE
                 ; like quoted-string, but disallowing line folding
  qdtext-nf      = WSP / %x21 / %x23-5B / %x5D-7E / obs-text
                 ; WSP / <VCHAR except DQUOTE and "\"> / obs-text 
]]></artwork></figure>
<t>
   The chunk-size field is a string of hex digits indicating the size of
   the chunk-data in octets. The chunked encoding is ended by any chunk whose size is
   zero, followed by the trailer, which is terminated by an empty line.
</t>
<t>
   The trailer allows the sender to include additional HTTP header
   fields at the end of the message. The Trailer header field can be
   used to indicate which header fields are included in a trailer (see
   <xref target="header.trailer"/>).
</t>
<t>
   A server using chunked transfer-coding in a response MUST NOT use the
   trailer for any header fields unless at least one of the following is
   true:
  <list style="numbers">
    <t>the request included a TE header field that indicates "trailers" is
     acceptable in the transfer-coding of the  response, as described in
     <xref target="header.te"/>; or,</t>
     
    <t>the trailer fields consist entirely of optional metadata, and the
    recipient could use the message (in a manner acceptable to the server where
    the field originated) without receiving it. In other words, the server that
    generated the header (often but not always the origin server) is willing to
    accept the possibility that the trailer fields might be silently discarded
    along the path to the client.</t>
  </list>
</t>
<t>
   This requirement prevents an interoperability failure when the
   message is being received by an HTTP/1.1 (or later) proxy and
   forwarded to an HTTP/1.0 recipient. It avoids a situation where
   compliance with the protocol would have necessitated a possibly
   infinite buffer on the proxy.
</t>
<t>
   A process for decoding the "chunked" transfer-coding
   can be represented in pseudo-code as:
</t>
<figure><artwork type="code"><![CDATA[
  length := 0
  read chunk-size, chunk-ext (if any) and CRLF
  while (chunk-size > 0) {
     read chunk-data and CRLF
     append chunk-data to decoded-body
     length := length + chunk-size
     read chunk-size and CRLF
  }
  read header-field
  while (header-field not empty) {
     append header-field to existing header fields
     read header-field
  }
  Content-Length := length
  Remove "chunked" from Transfer-Encoding
]]></artwork></figure>
<t>
   All HTTP/1.1 applications MUST be able to receive and decode the
   "chunked" transfer-coding and MUST ignore chunk-ext extensions
   they do not understand.
</t>
<t>
   Since "chunked" is the only transfer-coding required to be understood
   by HTTP/1.1 recipients, it plays a crucial role in delimiting messages
   on a persistent connection.  Whenever a transfer-coding is applied to
   a payload body in a request, the final transfer-coding applied MUST
   be "chunked".  If a transfer-coding is applied to a response payload
   body, then either the final transfer-coding applied MUST be "chunked"
   or the message MUST be terminated by closing the connection. When the
   "chunked" transfer-coding is used, it MUST be the last transfer-coding
   applied to form the message-body. The "chunked" transfer-coding MUST NOT
   be applied more than once in a message-body.
</t>
</section>

<section title="Compression Codings" anchor="compression.codings">
<t>
   The codings defined below can be used to compress the payload of a
   message.
</t>
<t><list><t>
   Note: Use of program names for the identification of encoding formats
   is not desirable and is discouraged for future encodings. Their
   use here is representative of historical practice, not good
   design.
</t></list></t>
<t><list><t>
   Note: For compatibility with previous implementations of HTTP,
   applications SHOULD consider "x-gzip" and "x-compress" to be
   equivalent to "gzip" and "compress" respectively.
</t></list></t>

<section title="Compress Coding" anchor="compress.coding">
<iref item="compress (Coding Format)"/>
<iref item="Coding Format" subitem="compress"/>
<t>
   The "compress" format is produced by the common UNIX file compression
   program "compress". This format is an adaptive Lempel-Ziv-Welch
   coding (LZW).
</t>
</section>

<section title="Deflate Coding" anchor="deflate.coding">
<iref item="deflate (Coding Format)"/>
<iref item="Coding Format" subitem="deflate"/>
<t>
   The "deflate" format is defined as the "deflate" compression mechanism
   (described in <xref target="RFC1951"/>) used inside the "zlib"
   data format (<xref target="RFC1950"/>).
</t>
<t><list>
  <t>
    Note: Some incorrect implementations send the "deflate" 
    compressed data without the zlib wrapper.
   </t>
</list></t>
</section>

<section title="Gzip Coding" anchor="gzip.coding">
<iref item="gzip (Coding Format)"/>
<iref item="Coding Format" subitem="gzip"/>
<t>
   The "gzip" format is produced by the file compression program
   "gzip" (GNU zip), as described in <xref target="RFC1952"/>. This format is a
   Lempel-Ziv coding (LZ77) with a 32 bit CRC.
</t>
</section>

</section>

<section title="Transfer Coding Registry" anchor="transfer.coding.registry">
<t>
   The HTTP Transfer Coding Registry defines the name space for the transfer
   coding names.
</t>
<t>
   Registrations MUST include the following fields:
   <list style="symbols">
     <t>Name</t>
     <t>Description</t>
     <t>Pointer to specification text</t>
   </list>
</t>
<t>
   Names of transfer codings MUST NOT overlap with names of content codings
   (Section 2.2 of <xref target="Part3"/>), unless the encoding transformation is identical (as it
   is the case for the compression codings defined in
   <xref target="compression.codings"/>).
</t>
<t>
   Values to be added to this name space require a specification
   (see "Specification Required" in Section 4.1 of <xref target="RFC5226"/>), and MUST
   conform to the purpose of transfer coding defined in this section.
</t>
<t>
   The registry itself is maintained at
   <eref target="http://www.iana.org/assignments/http-parameters"/>.
</t>
</section>
</section>

<section title="Product Tokens" anchor="product.tokens">
  
  
<t>
   Product tokens are used to allow communicating applications to
   identify themselves by software name and version. Most fields using
   product tokens also allow sub-products which form a significant part
   of the application to be listed, separated by whitespace. By
   convention, the products are listed in order of their significance
   for identifying the application.
</t>
<figure><iref primary="true" item="Grammar" subitem="product"/><iref primary="true" item="Grammar" subitem="product-version"/><artwork type="abnf2616"><![CDATA[
  product         = token ["/" product-version]
  product-version = token
]]></artwork></figure>
<t>
   Examples:
</t>
<figure><artwork type="example"><![CDATA[
  User-Agent: CERN-LineMode/2.15 libwww/2.17b3
  Server: Apache/0.8.4
]]></artwork></figure>
<t>
   Product tokens SHOULD be short and to the point. They MUST NOT be
   used for advertising or other non-essential information. Although any
   token octet MAY appear in a product-version, this token SHOULD
   only be used for a version identifier (i.e., successive versions of
   the same product SHOULD only differ in the product-version portion of
   the product value).
</t>
</section>

<section title="Quality Values" anchor="quality.values">
  
<t>
   Both transfer codings (TE request header field, <xref target="header.te"/>)
   and content negotiation (Section 5 of <xref target="Part3"/>) use short "floating point"
   numbers to indicate the relative importance ("weight") of various
   negotiable parameters.  A weight is normalized to a real number in
   the range 0 through 1, where 0 is the minimum and 1 the maximum
   value. If a parameter has a quality value of 0, then content with
   this parameter is "not acceptable" for the client. HTTP/1.1
   applications MUST NOT generate more than three digits after the
   decimal point. User configuration of these values SHOULD also be
   limited in this fashion.
</t>
<figure><iref primary="true" item="Grammar" subitem="qvalue"/><artwork type="abnf2616"><![CDATA[
  qvalue         = ( "0" [ "." 0*3DIGIT ] )
                 / ( "1" [ "." 0*3("0") ] )
]]></artwork></figure>
<t><list>
  <t>
     Note: "Quality values" is a misnomer, since these values merely represent
     relative degradation in desired quality.
  </t>
</list></t>
</section>

</section>

<section title="Connections" anchor="connections">

<section title="Persistent Connections" anchor="persistent.connections">

<section title="Purpose" anchor="persistent.purpose">
<t>
   Prior to persistent connections, a separate TCP connection was
   established for each request, increasing the load on HTTP servers
   and causing congestion on the Internet. The use of inline images and
   other associated data often requires a client to make multiple
   requests of the same server in a short amount of time. Analysis of
   these performance problems and results from a prototype
   implementation are available <xref target="Pad1995"/> <xref target="Spe"/>. Implementation experience and
   measurements of actual HTTP/1.1 implementations show good
   results <xref target="Nie1997"/>. Alternatives have also been explored, for example,
   T/TCP <xref target="Tou1998"/>.
</t>
<t>
   Persistent HTTP connections have a number of advantages:
  <list style="symbols">
      <t>
        By opening and closing fewer TCP connections, CPU time is saved
        in routers and hosts (clients, servers, proxies, gateways,
        tunnels, or caches), and memory used for TCP protocol control
        blocks can be saved in hosts.
      </t>
      <t>
        HTTP requests and responses can be pipelined on a connection.
        Pipelining allows a client to make multiple requests without
        waiting for each response, allowing a single TCP connection to
        be used much more efficiently, with much lower elapsed time.
      </t>
      <t>
        Network congestion is reduced by reducing the number of packets
        caused by TCP opens, and by allowing TCP sufficient time to
        determine the congestion state of the network.
      </t>
      <t>
        Latency on subsequent requests is reduced since there is no time
        spent in TCP's connection opening handshake.
      </t>
      <t>
        HTTP can evolve more gracefully, since errors can be reported
        without the penalty of closing the TCP connection. Clients using
        future versions of HTTP might optimistically try a new feature,
        but if communicating with an older server, retry with old
        semantics after an error is reported.
      </t>
    </list>
</t>
<t>
   HTTP implementations SHOULD implement persistent connections.
</t>
</section>

<section title="Overall Operation" anchor="persistent.overall">
<t>
   A significant difference between HTTP/1.1 and earlier versions of
   HTTP is that persistent connections are the default behavior of any
   HTTP connection. That is, unless otherwise indicated, the client
   SHOULD assume that the server will maintain a persistent connection,
   even after error responses from the server.
</t>
<t>
   Persistent connections provide a mechanism by which a client and a
   server can signal the close of a TCP connection. This signaling takes
   place using the Connection header field (<xref target="header.connection"/>). Once a close
   has been signaled, the client MUST NOT send any more requests on that
   connection.
</t>

<section title="Negotiation" anchor="persistent.negotiation">
<t>
   An HTTP/1.1 server MAY assume that a HTTP/1.1 client intends to
   maintain a persistent connection unless a Connection header field including
   the connection-token "close" was sent in the request. If the server
   chooses to close the connection immediately after sending the
   response, it SHOULD send a Connection header field including the
   connection-token "close".
</t>
<t>
   An HTTP/1.1 client MAY expect a connection to remain open, but would
   decide to keep it open based on whether the response from a server
   contains a Connection header field with the connection-token close. In case
   the client does not want to maintain a connection for more than that
   request, it SHOULD send a Connection header field including the
   connection-token close.
</t>
<t>
   If either the client or the server sends the close token in the
   Connection header field, that request becomes the last one for the
   connection.
</t>
<t>
   Clients and servers SHOULD NOT  assume that a persistent connection is
   maintained for HTTP versions less than 1.1 unless it is explicitly
   signaled. See <xref target="compatibility.with.http.1.0.persistent.connections"/> for more information on backward
   compatibility with HTTP/1.0 clients.
</t>
<t>
   In order to remain persistent, all messages on the connection MUST
   have a self-defined message length (i.e., one not defined by closure
   of the connection), as described in <xref target="message.body"/>.
</t>
</section>

<section title="Pipelining" anchor="pipelining">
<t>
   A client that supports persistent connections MAY "pipeline" its
   requests (i.e., send multiple requests without waiting for each
   response). A server MUST send its responses to those requests in the
   same order that the requests were received.
</t>
<t>
   Clients which assume persistent connections and pipeline immediately
   after connection establishment SHOULD be prepared to retry their
   connection if the first pipelined attempt fails. If a client does
   such a retry, it MUST NOT pipeline before it knows the connection is
   persistent. Clients MUST also be prepared to resend their requests if
   the server closes the connection before sending all of the
   corresponding responses.
</t>
<t>
   Clients SHOULD NOT pipeline requests using non-idempotent request methods or
   non-idempotent sequences of request methods (see Section 7.1.2 of <xref target="Part2"/>). Otherwise, a
   premature termination of the transport connection could lead to
   indeterminate results. A client wishing to send a non-idempotent
   request SHOULD wait to send that request until it has received the
   response status line for the previous request.
</t>
</section>
</section>

<section title="Proxy Servers" anchor="persistent.proxy">
<t>
   It is especially important that proxies correctly implement the
   properties of the Connection header field as specified in <xref target="header.connection"/>.
</t>
<t>
   The proxy server MUST signal persistent connections separately with
   its clients and the origin servers (or other proxy servers) that it
   connects to. Each persistent connection applies to only one transport
   link.
</t>
<t>
   A proxy server MUST NOT establish a HTTP/1.1 persistent connection
   with an HTTP/1.0 client (but see Section 19.7.1 of <xref target="RFC2068"/>
   for information and discussion of the problems with the Keep-Alive header field
   implemented by many HTTP/1.0 clients).
</t>

<section title="End-to-end and Hop-by-hop Header Fields" anchor="end-to-end.and.hop-by-hop.header-fields">
<!--<t>
  <cref anchor="TODO-end-to-end" source="jre">
    Restored from <eref target="http://tools.ietf.org/html/draft-ietf-httpbis-p6-cache-05#section-7.1"/>.
    See also <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/60"/>.
  </cref>
</t>-->
<t>
   For the purpose of defining the behavior of caches and non-caching
   proxies, we divide HTTP header fields into two categories:
  <list style="symbols">
      <t>End-to-end header fields, which are  transmitted to the ultimate
        recipient of a request or response. End-to-end header fields in
        responses MUST be stored as part of a cache entry and MUST be
        transmitted in any response formed from a cache entry.</t>

      <t>Hop-by-hop header fields, which are meaningful only for a single
        transport-level connection, and are not stored by caches or
        forwarded by proxies.</t>
  </list>
</t>
<t>
   The following HTTP/1.1 header fields are hop-by-hop header fields:
  <list style="symbols">
      <t>Connection</t>
      <t>Keep-Alive</t>
      <t>Proxy-Authenticate</t>
      <t>Proxy-Authorization</t>
      <t>TE</t>
      <t>Trailer</t>
      <t>Transfer-Encoding</t>
      <t>Upgrade</t>
  </list>
</t>
<t>
   All other header fields defined by HTTP/1.1 are end-to-end header fields.
</t>
<t>
   Other hop-by-hop header fields MUST be listed in a Connection header field
   (<xref target="header.connection"/>).
</t>
</section>

<section title="Non-modifiable Header Fields" anchor="non-modifiable.header-fields">
<!--<t>
  <cref anchor="TODO-non-mod-headers" source="jre">
    Restored from <eref target="http://tools.ietf.org/html/draft-ietf-httpbis-p6-cache-05#section-7.2"/>.
    See also <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/60"/>.
  </cref>
</t>-->
<t>
   Some features of HTTP/1.1, such as Digest Authentication, depend on the
   value of certain end-to-end header fields. A non-transforming proxy SHOULD NOT
   modify an end-to-end header field unless the definition of that header field requires
   or specifically allows that.
</t>
<t>
   A non-transforming proxy MUST NOT modify any of the following fields in a
   request or response, and it MUST NOT add any of these fields if not
   already present:
  <list style="symbols">
    <t>Allow</t>
    <t>Content-Location</t>
    <t>Content-MD5</t>
    <t>ETag</t>
    <t>Last-Modified</t>
    <t>Server</t>
  </list>
</t>
<t>
   A non-transforming proxy MUST NOT modify any of the following fields in a
   response:
  <list style="symbols">
    <t>Expires</t>
  </list>
</t>
<t>
   but it MAY add any of these fields if not already present. If an
   Expires header field is added, it MUST be given a field-value identical to
   that of the Date header field in that response.
</t>
<t>
   A proxy MUST NOT modify or add any of the following fields in a
   message that contains the no-transform cache-control directive, or in
   any request:
  <list style="symbols">
    <t>Content-Encoding</t>
    <t>Content-Range</t>
    <t>Content-Type</t>
  </list>
</t>
<t>
   A transforming proxy MAY modify or add these fields to a message
   that does not include no-transform, but if it does so, it MUST add a
   Warning 214 (Transformation applied) if one does not already appear
   in the message (see Section 3.6 of <xref target="Part6"/>).
</t>
<t><list>
  <t>
    Warning: Unnecessary modification of end-to-end header fields might
    cause authentication failures if stronger authentication
    mechanisms are introduced in later versions of HTTP. Such
    authentication mechanisms MAY rely on the values of header fields
    not listed here.
  </t>
</list></t>
<t>
   A non-transforming proxy MUST preserve the message payload (<xref target="Part3"/>),
   though it MAY change the message-body through application or removal
   of a transfer-coding (<xref target="transfer.codings"/>).
</t>
</section>

</section>

<section title="Practical Considerations" anchor="persistent.practical">
<t>
   Servers will usually have some time-out value beyond which they will
   no longer maintain an inactive connection. Proxy servers might make
   this a higher value since it is likely that the client will be making
   more connections through the same server. The use of persistent
   connections places no requirements on the length (or existence) of
   this time-out for either the client or the server.
</t>
<t>
   When a client or server wishes to time-out it SHOULD issue a graceful
   close on the transport connection. Clients and servers SHOULD both
   constantly watch for the other side of the transport close, and
   respond to it as appropriate. If a client or server does not detect
   the other side's close promptly it could cause unnecessary resource
   drain on the network.
</t>
<t>
   A client, server, or proxy MAY close the transport connection at any
   time. For example, a client might have started to send a new request
   at the same time that the server has decided to close the "idle"
   connection. From the server's point of view, the connection is being
   closed while it was idle, but from the client's point of view, a
   request is in progress.
</t>
<t>
   This means that clients, servers, and proxies MUST be able to recover
   from asynchronous close events. Client software SHOULD reopen the
   transport connection and retransmit the aborted sequence of requests
   without user interaction so long as the request sequence is
   idempotent (see Section 7.1.2 of <xref target="Part2"/>). Non-idempotent request methods or sequences
   MUST NOT be automatically retried, although user agents MAY offer a
   human operator the choice of retrying the request(s). Confirmation by
   user-agent software with semantic understanding of the application
   MAY substitute for user confirmation. The automatic retry SHOULD NOT 
   be repeated if the second sequence of requests fails.
</t>
<t>
   Servers SHOULD always respond to at least one request per connection,
   if at all possible. Servers SHOULD NOT  close a connection in the
   middle of transmitting a response, unless a network or client failure
   is suspected.
</t>
<t>
   Clients (including proxies) SHOULD limit the number of simultaneous
   connections that they maintain to a given server (including proxies).
</t>
<t>
   Previous revisions of HTTP gave a specific number of connections as a
   ceiling, but this was found to be impractical for many applications. As a
   result, this specification does not mandate a particular maximum number of
   connections, but instead encourages clients to be conservative when opening
   multiple connections.
</t>
<t>
   In particular, while using multiple connections avoids the "head-of-line
   blocking" problem (whereby a request that takes significant server-side
   processing and/or has a large payload can block subsequent requests on the
   same connection), each connection used consumes server resources (sometimes
   significantly), and furthermore using multiple connections can cause
   undesirable side effects in congested networks. 
</t>
<t>
   Note that servers might reject traffic that they deem abusive, including an
   excessive number of connections from a client.
</t>
</section>
</section>

<section title="Message Transmission Requirements" anchor="message.transmission.requirements">

<section title="Persistent Connections and Flow Control" anchor="persistent.flow">
<t>
   HTTP/1.1 servers SHOULD maintain persistent connections and use TCP's
   flow control mechanisms to resolve temporary overloads, rather than
   terminating connections with the expectation that clients will retry.
   The latter technique can exacerbate network congestion.
</t>
</section>

<section title="Monitoring Connections for Error Status Messages" anchor="persistent.monitor">
<t>
   An HTTP/1.1 (or later) client sending a message-body SHOULD monitor
   the network connection for an error status code while it is transmitting
   the request. If the client sees an error status code, it SHOULD
   immediately cease transmitting the body. If the body is being sent
   using a "chunked" encoding (<xref target="transfer.codings"/>), a zero length chunk and
   empty trailer MAY be used to prematurely mark the end of the message.
   If the body was preceded by a Content-Length header field, the client MUST
   close the connection.
</t>
</section>

<section title="Use of the 100 (Continue) Status" anchor="use.of.the.100.status">
<t>
   The purpose of the 100 (Continue) status code (see Section 8.1.1 of <xref target="Part2"/>) is to
   allow a client that is sending a request message with a request body
   to determine if the origin server is willing to accept the request
   (based on the request header fields) before the client sends the request
   body. In some cases, it might either be inappropriate or highly
   inefficient for the client to send the body if the server will reject
   the message without looking at the body.
</t>
<t>
   Requirements for HTTP/1.1 clients:
  <list style="symbols">
    <t>
        If a client will wait for a 100 (Continue) response before
        sending the request body, it MUST send an Expect header
        field (Section 9.2 of <xref target="Part2"/>) with the "100-continue" expectation.
    </t>
    <t>
        A client MUST NOT send an Expect header field (Section 9.2 of <xref target="Part2"/>)
        with the "100-continue" expectation if it does not intend
        to send a request body.
    </t>
  </list>
</t>
<t>
   Because of the presence of older implementations, the protocol allows
   ambiguous situations in which a client might send "Expect: 100-continue"
   without receiving either a 417 (Expectation Failed) 
   or a 100 (Continue) status code. Therefore, when a client sends this
   header field to an origin server (possibly via a proxy) from which it
   has never seen a 100 (Continue) status code, the client SHOULD NOT  
   wait for an indefinite period before sending the request body.
</t>
<t>
   Requirements for HTTP/1.1 origin servers:
  <list style="symbols">
    <t> Upon receiving a request which includes an Expect header
        field with the "100-continue" expectation, an origin server MUST
        either respond with 100 (Continue) status code and continue to read
        from the input stream, or respond with a final status code. The
        origin server MUST NOT wait for the request body before sending
        the 100 (Continue) response. If it responds with a final status
        code, it MAY close the transport connection or it MAY continue
        to read and discard the rest of the request.  It MUST NOT
        perform the request method if it returns a final status code.
    </t>
    <t> An origin server SHOULD NOT  send a 100 (Continue) response if
        the request message does not include an Expect header
        field with the "100-continue" expectation, and MUST NOT send a
        100 (Continue) response if such a request comes from an HTTP/1.0
        (or earlier) client. There is an exception to this rule: for
        compatibility with <xref target="RFC2068"/>, a server MAY send a 100 (Continue)
        status code in response to an HTTP/1.1 PUT or POST request that does
        not include an Expect header field with the "100-continue"
        expectation. This exception, the purpose of which is
        to minimize any client processing delays associated with an
        undeclared wait for 100 (Continue) status code, applies only to
        HTTP/1.1 requests, and not to requests with any other HTTP-version
        value.
    </t>
    <t> An origin server MAY omit a 100 (Continue) response if it has
        already received some or all of the request body for the
        corresponding request.
    </t>
    <t> An origin server that sends a 100 (Continue) response MUST
    ultimately send a final status code, once the request body is
        received and processed, unless it terminates the transport
        connection prematurely.
    </t>
    <t> If an origin server receives a request that does not include an
        Expect header field with the "100-continue" expectation,
        the request includes a request body, and the server responds
        with a final status code before reading the entire request body
        from the transport connection, then the server SHOULD NOT  close
        the transport connection until it has read the entire request,
        or until the client closes the connection. Otherwise, the client
        might not reliably receive the response message. However, this
        requirement is not be construed as preventing a server from
        defending itself against denial-of-service attacks, or from
        badly broken client implementations.
      </t>
    </list>
</t>
<t>
   Requirements for HTTP/1.1 proxies:
  <list style="symbols">
    <t> If a proxy receives a request that includes an Expect header
        field with the "100-continue" expectation, and the proxy
        either knows that the next-hop server complies with HTTP/1.1 or
        higher, or does not know the HTTP version of the next-hop
        server, it MUST forward the request, including the Expect header
        field.
    </t>
    <t> If the proxy knows that the version of the next-hop server is
        HTTP/1.0 or lower, it MUST NOT forward the request, and it MUST
        respond with a 417 (Expectation Failed) status code.
    </t>
    <t> Proxies SHOULD maintain a record of the HTTP version
        numbers received from recently-referenced next-hop servers.
    </t>
    <t> A proxy MUST NOT forward a 100 (Continue) response if the
        request message was received from an HTTP/1.0 (or earlier)
        client and did not include an Expect header field with
        the "100-continue" expectation. This requirement overrides the
        general rule for forwarding of 1xx responses (see Section 8.1 of <xref target="Part2"/>).
    </t>
  </list>
</t>
</section>

<section title="Client Behavior if Server Prematurely Closes Connection" anchor="connection.premature">
<t>
   If an HTTP/1.1 client sends a request which includes a request body,
   but which does not include an Expect header field with the
   "100-continue" expectation, and if the client is not directly
   connected to an HTTP/1.1 origin server, and if the client sees the
   connection close before receiving a status line from the server, the
   client SHOULD retry the request.  If the client does retry this
   request, it MAY use the following "binary exponential backoff"
   algorithm to be assured of obtaining a reliable response:
  <list style="numbers">
    <t>
      Initiate a new connection to the server
    </t>
    <t>
      Transmit the request-line, header fields, and the CRLF that
      indicates the end of header fields.
    </t>
    <t>
      Initialize a variable R to the estimated round-trip time to the
         server (e.g., based on the time it took to establish the
         connection), or to a constant value of 5 seconds if the round-trip
         time is not available.
    </t>
    <t>
       Compute T = R * (2**N), where N is the number of previous
         retries of this request.
    </t>
    <t>
       Wait either for an error response from the server, or for T
         seconds (whichever comes first)
    </t>
    <t>
       If no error response is received, after T seconds transmit the
         body of the request.
    </t>
    <t>
       If client sees that the connection is closed prematurely,
         repeat from step 1 until the request is accepted, an error
         response is received, or the user becomes impatient and
         terminates the retry process.
    </t>
  </list>
</t>
<t>
   If at any point an error status code is received, the client
  <list style="symbols">
      <t>SHOULD NOT  continue and</t>

      <t>SHOULD close the connection if it has not completed sending the
        request message.</t>
    </list>
</t>
</section>
</section>
</section>


<section title="Miscellaneous notes that might disappear" anchor="misc">
<section title="Scheme aliases considered harmful" anchor="scheme.aliases">
<t>
   <cref anchor="TBD-aliases-harmful">describe why aliases like webcal are harmful.</cref>
</t>
</section>

<section title="Use of HTTP for proxy communication" anchor="http.proxy">
<t>
   <cref anchor="TBD-proxy-other">Configured to use HTTP to proxy HTTP or other protocols.</cref>
</t>
</section>

<section title="Interception of HTTP for access control" anchor="http.intercept">
<t>
   <cref anchor="TBD-intercept">Interception of HTTP traffic for initiating access control.</cref>
</t>
</section>

<section title="Use of HTTP by other protocols" anchor="http.others">
<t>
   <cref anchor="TBD-profiles">Profiles of HTTP defined by other protocol.
   Extensions of HTTP like WebDAV.</cref>
</t>

</section>
<section title="Use of HTTP by media type specification" anchor="http.media">
<t>
   <cref anchor="TBD-hypertext">Instructions on composing HTTP requests via hypertext formats.</cref>
</t>
</section>
</section>

<section title="Header Field Definitions" anchor="header.field.definitions">
<t>
   This section defines the syntax and semantics of HTTP header fields
   related to message framing and transport protocols.
</t>

<section title="Connection" anchor="header.connection">
  <iref primary="true" item="Connection header field"/>
  <iref primary="true" item="Header Fields" subitem="Connection"/>
  
  
<t>
   The "Connection" header field allows the sender to specify
   options that are desired only for that particular connection.
   Such connection options MUST be removed or replaced before the
   message can be forwarded downstream by a proxy or gateway.
   This mechanism also allows the sender to indicate which HTTP
   header fields used in the message are only intended for the
   immediate recipient ("hop-by-hop"), as opposed to all recipients
   on the chain ("end-to-end"), enabling the message to be
   self-descriptive and allowing future connection-specific extensions
   to be deployed in HTTP without fear that they will be blindly
   forwarded by previously deployed intermediaries.
</t>
<t>
   The Connection header field's value has the following grammar:
</t>
<figure><iref primary="true" item="Grammar" subitem="Connection"/><iref primary="true" item="Grammar" subitem="connection-token"/><artwork type="abnf2616"><![CDATA[
  Connection       = 1#connection-token
  connection-token = token
]]></artwork></figure>
<t>
   A proxy or gateway MUST parse a received Connection
   header field before a message is forwarded and, for each
   connection-token in this field, remove any header field(s) from
   the message with the same name as the connection-token, and then
   remove the Connection header field itself or replace it with the
   sender's own connection options for the forwarded message.
</t>
<t>
   A sender MUST NOT include field-names in the Connection header
   field-value for fields that are defined as expressing constraints
   for all recipients in the request or response chain, such as the
   Cache-Control header field (Section 3.2 of <xref target="Part6"/>).
</t>
<t>
   The connection options do not have to correspond to a header field
   present in the message, since a connection-specific header field
   might not be needed if there are no parameters associated with that
   connection option.  Recipients that trigger certain connection
   behavior based on the presence of connection options MUST do so
   based on the presence of the connection-token rather than only the
   presence of the optional header field.  In other words, if the
   connection option is received as a header field but not indicated
   within the Connection field-value, then the recipient MUST ignore
   the connection-specific header field because it has likely been
   forwarded by an intermediary that is only partially compliant.
</t>
<t>
   When defining new connection options, specifications ought to
   carefully consider existing deployed header fields and ensure
   that the new connection-token does not share the same name as
   an unrelated header field that might already be deployed.
   Defining a new connection-token essentially reserves that potential
   field-name for carrying additional information related to the
   connection option, since it would be unwise for senders to use
   that field-name for anything else.
</t>
<t>
   HTTP/1.1 defines the "close" connection option for the sender to
   signal that the connection will be closed after completion of the
   response. For example,
</t>
<figure><artwork type="example"><![CDATA[
  Connection: close
]]></artwork></figure>
<t>
   in either the request or the response header fields indicates that
   the connection SHOULD NOT  be considered "persistent" (<xref target="persistent.connections"/>)
   after the current request/response is complete.
</t>
<t>
   An HTTP/1.1 client that does not support persistent connections MUST
   include the "close" connection option in every request message.
</t>
<t>
   An HTTP/1.1 server that does not support persistent connections MUST
   include the "close" connection option in every response message that
   does not have a 1xx (Informational) status code.
</t>
</section>

<section title="Content-Length" anchor="header.content-length">
  <iref primary="true" item="Content-Length header field"/>
  <iref primary="true" item="Header Fields" subitem="Content-Length"/>
  
<t>
   The "Content-Length" header field indicates the size of the
   message-body, in decimal number of octets, for any message other than
   a response to a HEAD request or a response with a status code of 304.
   In the case of a response to a HEAD request, Content-Length indicates
   the size of the payload body (not including any potential transfer-coding)
   that would have been sent had the request been a GET.
   In the case of a 304 (Not Modified) response to a GET request,
   Content-Length indicates the size of the payload body (not including
   any potential transfer-coding) that would have been sent in a 200 (OK)
   response.
</t>
<figure><iref primary="true" item="Grammar" subitem="Content-Length"/><artwork type="abnf2616"><![CDATA[
  Content-Length = 1*DIGIT
]]></artwork></figure>
<t>
   An example is
</t>
<figure><artwork type="example"><![CDATA[
  Content-Length: 3495
]]></artwork></figure>
<t>
   Implementations SHOULD use this field to indicate the message-body
   length when no transfer-coding is being applied and the
   payload's body length can be determined prior to being transferred.
   <xref target="message.body"/> describes how recipients determine the length
   of a message-body.
</t>
<t>
   Any Content-Length greater than or equal to zero is a valid value.
</t>
<t>
   Note that the use of this field in HTTP is significantly different from
   the corresponding definition in MIME, where it is an optional field
   used within the "message/external-body" content-type.
</t>
</section>

<section title="Date" anchor="header.date">
  <iref primary="true" item="Date header field"/>
  <iref primary="true" item="Header Fields" subitem="Date"/>
  
<t>
   The "Date" header field represents the date and time at which
   the message was originated, having the same semantics as the Origination
   Date Field (orig-date) defined in Section 3.6.1 of <xref target="RFC5322"/>.
   The field value is an HTTP-date, as described in <xref target="date.time.formats.full.date"/>;
   it MUST be sent in rfc1123-date format.
</t>
<figure><iref primary="true" item="Grammar" subitem="Date"/><artwork type="abnf2616"><![CDATA[
  Date = HTTP-date
]]></artwork></figure>
<t>
   An example is
</t>
<figure><artwork type="example"><![CDATA[
  Date: Tue, 15 Nov 1994 08:12:31 GMT
]]></artwork></figure>
<t>
   Origin servers MUST include a Date header field in all responses,
   except in these cases:
  <list style="numbers">
      <t>If the response status code is 100 (Continue) or 101 (Switching
         Protocols), the response MAY include a Date header field, at
         the server's option.</t>

      <t>If the response status code conveys a server error, e.g., 500
         (Internal Server Error) or 503 (Service Unavailable), and it is
         inconvenient or impossible to generate a valid Date.</t>

      <t>If the server does not have a clock that can provide a
         reasonable approximation of the current time, its responses
         MUST NOT include a Date header field. In this case, the rules
         in <xref target="clockless.origin.server.operation"/> MUST be followed.</t>
  </list>
</t>
<t>
   A received message that does not have a Date header field MUST be
   assigned one by the recipient if the message will be cached by that
   recipient.
</t>
<t>
   Clients can use the Date header field as well; in order to keep request
   messages small, they are advised not to include it when it doesn't convey
   any useful information (as it is usually the case for requests that do not
   contain a payload).
</t>
<t>
   The HTTP-date sent in a Date header field SHOULD NOT  represent a date and
   time subsequent to the generation of the message. It SHOULD represent
   the best available approximation of the date and time of message
   generation, unless the implementation has no means of generating a
   reasonably accurate date and time. In theory, the date ought to
   represent the moment just before the payload is generated. In
   practice, the date can be generated at any time during the message
   origination without affecting its semantic value.
</t>

<section title="Clockless Origin Server Operation" anchor="clockless.origin.server.operation">
<t>
   Some origin server implementations might not have a clock available.
   An origin server without a clock MUST NOT assign Expires or Last-Modified
   values to a response, unless these values were associated
   with the resource by a system or user with a reliable clock. It MAY
   assign an Expires value that is known, at or before server
   configuration time, to be in the past (this allows "pre-expiration"
   of responses without storing separate Expires values for each
   resource).
</t>
</section>
</section>

<section title="Host" anchor="header.host">
  <iref primary="true" item="Host header field"/>
  <iref primary="true" item="Header Fields" subitem="Host"/>
  
<t>
   The "Host" header field in a request provides the host and port
   information from the target resource's URI, enabling the origin
   server to distinguish between resources while servicing requests
   for multiple host names on a single IP address.  Since the Host
   field-value is critical information for handling a request, it
   SHOULD be sent as the first header field following the Request-Line.
</t>
<figure><iref primary="true" item="Grammar" subitem="Host"/><artwork type="abnf2616"><![CDATA[
  Host = uri-host [ ":" port ] ; Section 2.7.1
]]></artwork></figure>
<t>
   A client MUST send a Host header field in all HTTP/1.1 request
   messages.  If the target resource's URI includes an authority
   component, then the Host field-value MUST be identical to that
   authority component after excluding any userinfo (<xref target="http.uri"/>).
   If the authority component is missing or undefined for the target
   resource's URI, then the Host header field MUST be sent with an
   empty field-value.
</t>
<t>
   For example, a GET request to the origin server for
   &lt;http://www.example.org/pub/WWW/&gt; would begin with:
</t>
<figure><artwork type="message/http; msgtype=&#34;request&#34;"><![CDATA[
  GET /pub/WWW/ HTTP/1.1
  Host: www.example.org
  ]]></artwork></figure>
<t>
   The Host header field MUST be sent in an HTTP/1.1 request even
   if the request-target is in the form of an absolute-URI, since this
   allows the Host information to be forwarded through ancient HTTP/1.0
   proxies that might not have implemented Host.
</t>
<t>
   When an HTTP/1.1 proxy receives a request with a request-target in
   the form of an absolute-URI, the proxy MUST ignore the received
   Host header field (if any) and instead replace it with the host
   information of the request-target.  When a proxy forwards a request,
   it MUST generate the Host header field based on the received
   absolute-URI rather than the received Host.
</t>
<t>
   Since the Host header field acts as an application-level routing
   mechanism, it is a frequent target for malware seeking to poison
   a shared cache or redirect a request to an unintended server.
   An interception proxy is particularly vulnerable if it relies on
   the Host header field value for redirecting requests to internal
   servers, or for use as a cache key in a shared cache, without
   first verifying that the intercepted connection is targeting a
   valid IP address for that host.
</t>
<t>
   A server MUST respond with a 400 (Bad Request) status code to
   any HTTP/1.1 request message that lacks a Host header field and
   to any request message that contains more than one Host header field
   or a Host header field with an invalid field-value.
</t>
<t>
   See Sections <xref target="the.resource.identified.by.a.request" format="counter"/>
   and <xref target="changes.to.simplify.multi-homed.web.servers.and.conserve.ip.addresses" format="counter"/>
   for other requirements relating to Host.
</t>
</section>

<section title="TE" anchor="header.te">
  <iref primary="true" item="TE header field"/>
  <iref primary="true" item="Header Fields" subitem="TE"/>
  
  
  
  
<t>
   The "TE" header field indicates what extension transfer-codings
   it is willing to accept in the response, and whether or not it is
   willing to accept trailer fields in a chunked transfer-coding.
</t>
<t>
   Its value consists of the keyword "trailers" and/or a comma-separated
   list of extension transfer-coding names with optional accept
   parameters (as described in <xref target="transfer.codings"/>).
</t>
<figure><iref primary="true" item="Grammar" subitem="TE"/><iref primary="true" item="Grammar" subitem="t-codings"/><iref primary="true" item="Grammar" subitem="te-params"/><iref primary="true" item="Grammar" subitem="te-ext"/><artwork type="abnf2616"><![CDATA[
  TE        = #t-codings
  t-codings = "trailers" / ( transfer-extension [ te-params ] )
  te-params = OWS ";" OWS "q=" qvalue *( te-ext )
  te-ext    = OWS ";" OWS token [ "=" word ]
]]></artwork></figure>
<t>
   The presence of the keyword "trailers" indicates that the client is
   willing to accept trailer fields in a chunked transfer-coding, as
   defined in <xref target="chunked.encoding"/>. This keyword is reserved for use with
   transfer-coding values even though it does not itself represent a
   transfer-coding.
</t>
<t>
   Examples of its use are:
</t>
<figure><artwork type="example"><![CDATA[
  TE: deflate
  TE:
  TE: trailers, deflate;q=0.5
]]></artwork></figure>
<t>
   The TE header field only applies to the immediate connection.
   Therefore, the keyword MUST be supplied within a Connection header
   field (<xref target="header.connection"/>) whenever TE is present in an HTTP/1.1 message.
</t>
<t>
   A server tests whether a transfer-coding is acceptable, according to
   a TE field, using these rules:
  <list style="numbers">
    <t>The "chunked" transfer-coding is always acceptable. If the
         keyword "trailers" is listed, the client indicates that it is
         willing to accept trailer fields in the chunked response on
         behalf of itself and any downstream clients. The implication is
         that, if given, the client is stating that either all
         downstream clients are willing to accept trailer fields in the
         forwarded response, or that it will attempt to buffer the
         response on behalf of downstream recipients.
      <vspace blankLines="1"/>
         Note: HTTP/1.1 does not define any means to limit the size of a
         chunked response such that a client can be assured of buffering
         the entire response.</t>
    <t>If the transfer-coding being tested is one of the transfer-codings
         listed in the TE field, then it is acceptable unless it
         is accompanied by a qvalue of 0. (As defined in <xref target="quality.values"/>, a
         qvalue of 0 means "not acceptable".)</t>
    <t>If multiple transfer-codings are acceptable, then the
         acceptable transfer-coding with the highest non-zero qvalue is
         preferred.  The "chunked" transfer-coding always has a qvalue
         of 1.</t>
  </list>
</t>
<t>
   If the TE field-value is empty or if no TE field is present, the only
   transfer-coding is "chunked". A message with no transfer-coding is
   always acceptable.
</t>
</section>

<section title="Trailer" anchor="header.trailer">
  <iref primary="true" item="Trailer header field"/>
  <iref primary="true" item="Header Fields" subitem="Trailer"/>
  
<t>
   The "Trailer" header field indicates that the given set of
   header fields is present in the trailer of a message encoded with
   chunked transfer-coding.
</t>
<figure><iref primary="true" item="Grammar" subitem="Trailer"/><artwork type="abnf2616"><![CDATA[
  Trailer = 1#field-name
]]></artwork></figure>
<t>
   An HTTP/1.1 message SHOULD include a Trailer header field in a
   message using chunked transfer-coding with a non-empty trailer. Doing
   so allows the recipient to know which header fields to expect in the
   trailer.
</t>
<t>
   If no Trailer header field is present, the trailer SHOULD NOT  include
   any header fields. See <xref target="chunked.encoding"/> for restrictions on the use of
   trailer fields in a "chunked" transfer-coding.
</t>
<t>
   Message header fields listed in the Trailer header field MUST NOT
   include the following header fields:
  <list style="symbols">
    <t>Transfer-Encoding</t>
    <t>Content-Length</t>
    <t>Trailer</t>
  </list>
</t>
</section>

<section title="Transfer-Encoding" anchor="header.transfer-encoding">
  <iref primary="true" item="Transfer-Encoding header field"/>
  <iref primary="true" item="Header Fields" subitem="Transfer-Encoding"/>
  
<t>
   The "Transfer-Encoding" header field indicates what transfer-codings
   (if any) have been applied to the message body. It differs from 
   Content-Encoding (Section 2.2 of <xref target="Part3"/>) in that transfer-codings are a property
   of the message (and therefore are removed by intermediaries), whereas
   content-codings are not.
</t>
<figure><iref primary="true" item="Grammar" subitem="Transfer-Encoding"/><artwork type="abnf2616"><![CDATA[
  Transfer-Encoding = 1#transfer-coding
]]></artwork></figure>
<t>
   Transfer-codings are defined in <xref target="transfer.codings"/>. An example is:
</t>
<figure><artwork type="example"><![CDATA[
  Transfer-Encoding: chunked
]]></artwork></figure>
<t>
   If multiple encodings have been applied to a representation, the transfer-codings
   MUST be listed in the order in which they were applied.
   Additional information about the encoding parameters MAY be provided
   by other header fields not defined by this specification.
</t>
<t>
   Many older HTTP/1.0 applications do not understand the Transfer-Encoding
   header field.
</t>
</section>

<section title="Upgrade" anchor="header.upgrade">
  <iref primary="true" item="Upgrade header field"/>
  <iref primary="true" item="Header Fields" subitem="Upgrade"/>
  
<t>
   The "Upgrade" header field allows the client to specify what
   additional communication protocols it would like to use, if the server 
   chooses to switch protocols. Servers can use it to indicate what protocols
   they are willing to switch to.
</t>
<figure><iref primary="true" item="Grammar" subitem="Upgrade"/><artwork type="abnf2616"><![CDATA[
  Upgrade = 1#product
]]></artwork></figure>
<t>
   For example,
</t>
<figure><artwork type="example"><![CDATA[
  Upgrade: HTTP/2.0, SHTTP/1.3, IRC/6.9, RTA/x11
]]></artwork></figure>
<t>
   The Upgrade header field is intended to provide a simple mechanism
   for transition from HTTP/1.1 to some other, incompatible protocol. It
   does so by allowing the client to advertise its desire to use another
   protocol, such as a later version of HTTP with a higher major version
   number, even though the current request has been made using HTTP/1.1.
   This eases the difficult transition between incompatible protocols by
   allowing the client to initiate a request in the more commonly
   supported protocol while indicating to the server that it would like
   to use a "better" protocol if available (where "better" is determined
   by the server, possibly according to the nature of the request method
   or target resource).
</t>
<t>
   The Upgrade header field only applies to switching application-layer
   protocols upon the existing transport-layer connection. Upgrade
   cannot be used to insist on a protocol change; its acceptance and use
   by the server is optional. The capabilities and nature of the
   application-layer communication after the protocol change is entirely
   dependent upon the new protocol chosen, although the first action
   after changing the protocol MUST be a response to the initial HTTP
   request containing the Upgrade header field.
</t>
<t>
   The Upgrade header field only applies to the immediate connection.
   Therefore, the upgrade keyword MUST be supplied within a Connection
   header field (<xref target="header.connection"/>) whenever Upgrade is present in an
   HTTP/1.1 message.
</t>
<t>
   The Upgrade header field cannot be used to indicate a switch to a
   protocol on a different connection. For that purpose, it is more
   appropriate to use a 3xx redirection response (Section 8.3 of <xref target="Part2"/>).
</t>
<t>
   Servers MUST include the "Upgrade" header field in 101 (Switching
   Protocols) responses to indicate which protocol(s) are being switched to,
   and MUST include it in 426 (Upgrade Required) responses to indicate
   acceptable protocols to upgrade to. Servers MAY include it in any other
   response to indicate that they are willing to upgrade to one of the
   specified protocols.
</t>
<t>
   This specification only defines the protocol name "HTTP" for use by
   the family of Hypertext Transfer Protocols, as defined by the HTTP
   version rules of <xref target="http.version"/> and future updates to this
   specification. Additional tokens can be registered with IANA using the
   registration procedure defined below.  
</t>

<section title="Upgrade Token Registry" anchor="upgrade.token.registry">
<t>
   The HTTP Upgrade Token Registry defines the name space for product
   tokens used to identify protocols in the Upgrade header field.
   Each registered token is associated with contact information and
   an optional set of specifications that details how the connection
   will be processed after it has been upgraded.
</t>
<t>
   Registrations are allowed on a First Come First Served basis as
   described in Section 4.1 of <xref target="RFC5226"/>. The
   specifications need not be IETF documents or be subject to IESG review.
   Registrations are subject to the following rules:
  <list style="numbers">
    <t>A token, once registered, stays registered forever.</t>
    <t>The registration MUST name a responsible party for the
       registration.</t>
    <t>The registration MUST name a point of contact.</t>
    <t>The registration MAY name a set of specifications associated with that
       token. Such specifications need not be publicly available.</t>
    <t>The responsible party MAY change the registration at any time.
       The IANA will keep a record of all such changes, and make them
       available upon request.</t>
    <t>The responsible party for the first registration of a "product"
       token MUST approve later registrations of a "version" token
       together with that "product" token before they can be registered.</t>
    <t>If absolutely required, the IESG MAY reassign the responsibility
       for a token. This will normally only be used in the case when a
       responsible party cannot be contacted.</t>
  </list>
</t>
</section>


</section>

<section title="Via" anchor="header.via">
  <iref primary="true" item="Via header field"/>
  <iref primary="true" item="Header Fields" subitem="Via"/>
  
  
  
  
  
  
<t>
   The "Via" header field MUST be sent by a proxy or gateway to
   indicate the intermediate protocols and recipients between the user
   agent and the server on requests, and between the origin server and
   the client on responses. It is analogous to the "Received" field
   used by email systems (Section 3.6.7 of <xref target="RFC5322"/>)
   and is intended to be used for tracking message forwards,
   avoiding request loops, and identifying the protocol capabilities of
   all senders along the request/response chain.
</t>
<figure><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"/><artwork type="abnf2616"><![CDATA[
  Via               = 1#( received-protocol RWS received-by
                          [ RWS comment ] )
  received-protocol = [ protocol-name "/" ] protocol-version
  protocol-name     = token
  protocol-version  = token
  received-by       = ( uri-host [ ":" port ] ) / pseudonym
  pseudonym         = token
]]></artwork></figure>
<t>
   The received-protocol indicates the protocol version of the message
   received by the server or client along each segment of the
   request/response chain. The received-protocol version is appended to
   the Via field value when the message is forwarded so that information
   about the protocol capabilities of upstream applications remains
   visible to all recipients.
</t>
<t>
   The protocol-name is excluded if and only if it would be "HTTP". The
   received-by field is normally the host and optional port number of a
   recipient server or client that subsequently forwarded the message.
   However, if the real host is considered to be sensitive information,
   it MAY be replaced by a pseudonym. If the port is not given, it MAY
   be assumed to be the default port of the received-protocol.
</t>
<t>
   Multiple Via field values represent each proxy or gateway that has
   forwarded the message. Each recipient MUST append its information
   such that the end result is ordered according to the sequence of
   forwarding applications.
</t>
<t>
   Comments MAY be used in the Via header field to identify the software
   of each recipient, analogous to the User-Agent and Server header fields.
   However, all comments in the Via field are optional and MAY be removed
   by any recipient prior to forwarding the message.
</t>
<t>
   For example, a request message could be sent from an HTTP/1.0 user
   agent to an internal proxy code-named "fred", which uses HTTP/1.1 to
   forward the request to a public proxy at p.example.net, which completes
   the request by forwarding it to the origin server at www.example.com.
   The request received by www.example.com would then have the following
   Via header field:
</t>
<figure><artwork type="example"><![CDATA[
  Via: 1.0 fred, 1.1 p.example.net (Apache/1.1)
]]></artwork></figure>
<t>
   A proxy or gateway used as a portal through a network firewall
   SHOULD NOT forward the names and ports of hosts within the firewall
   region unless it is explicitly enabled to do so. If not enabled, the
   received-by host of any host behind the firewall SHOULD be replaced
   by an appropriate pseudonym for that host.
</t>
<t>
   For organizations that have strong privacy requirements for hiding
   internal structures, a proxy or gateway MAY combine an ordered
   subsequence of Via header field entries with identical received-protocol
   values into a single such entry. For example,
</t>
<figure><artwork type="example"><![CDATA[
  Via: 1.0 ricky, 1.1 ethel, 1.1 fred, 1.0 lucy
]]></artwork></figure>
<t>
  could be collapsed to
</t>
<figure><artwork type="example"><![CDATA[
  Via: 1.0 ricky, 1.1 mertz, 1.0 lucy
]]></artwork></figure>
<t>
   Senders SHOULD NOT combine multiple entries unless they are all
   under the same organizational control and the hosts have already been
   replaced by pseudonyms. Senders MUST NOT combine entries which
   have different received-protocol values.
</t>
</section>

</section>

<section title="IANA Considerations" anchor="IANA.considerations">

<section title="Header Field Registration" anchor="header.field.registration">
<t>
   The Message Header Field Registry located at <eref target="http://www.iana.org/assignments/message-headers/message-header-index.html"/> shall be updated
   with the permanent registrations below (see <xref target="RFC3864"/>):
</t>

<!--AUTOGENERATED FROM extract-header-defs.xslt, do not edit manually-->
<texttable align="left" suppress-title="true" anchor="iana.header.registration.table">
   <ttcol>Header Field Name</ttcol>
   <ttcol>Protocol</ttcol>
   <ttcol>Status</ttcol>
   <ttcol>Reference</ttcol>

   <c>Connection</c>
   <c>http</c>
   <c>standard</c>
   <c>
      <xref target="header.connection"/>
   </c>
   <c>Content-Length</c>
   <c>http</c>
   <c>standard</c>
   <c>
      <xref target="header.content-length"/>
   </c>
   <c>Date</c>
   <c>http</c>
   <c>standard</c>
   <c>
      <xref target="header.date"/>
   </c>
   <c>Host</c>
   <c>http</c>
   <c>standard</c>
   <c>
      <xref target="header.host"/>
   </c>
   <c>TE</c>
   <c>http</c>
   <c>standard</c>
   <c>
      <xref target="header.te"/>
   </c>
   <c>Trailer</c>
   <c>http</c>
   <c>standard</c>
   <c>
      <xref target="header.trailer"/>
   </c>
   <c>Transfer-Encoding</c>
   <c>http</c>
   <c>standard</c>
   <c>
      <xref target="header.transfer-encoding"/>
   </c>
   <c>Upgrade</c>
   <c>http</c>
   <c>standard</c>
   <c>
      <xref target="header.upgrade"/>
   </c>
   <c>Via</c>
   <c>http</c>
   <c>standard</c>
   <c>
      <xref target="header.via"/>
   </c>
</texttable>
<!--(END)-->

<t>
   Furthermore, the header field name "Close" shall be registered as "reserved", as its use as
   HTTP header field would be in conflict with the use of the "close" connection
   option for the "Connection" header field (<xref target="header.connection"/>).
</t>
<texttable align="left" suppress-title="true">
   <ttcol>Header Field Name</ttcol>
   <ttcol>Protocol</ttcol>
   <ttcol>Status</ttcol>
   <ttcol>Reference</ttcol>

   <c>Close</c>
   <c>http</c>
   <c>reserved</c>
   <c>
      <xref target="header.field.registration"/>
   </c>
</texttable>
<t>
   The change controller is: "IETF (iesg@ietf.org) - Internet Engineering Task Force".
</t>
</section>

<section title="URI Scheme Registration" anchor="uri.scheme.registration">
<t>
   The entries for the "http" and "https" URI Schemes in the registry located at
   <eref target="http://www.iana.org/assignments/uri-schemes.html"/>
   shall be updated to point to Sections <xref target="http.uri" format="counter"/>
   and <xref target="https.uri" format="counter"/> of this document
   (see <xref target="RFC4395"/>).
</t>
</section>

<section title="Internet Media Type Registrations" anchor="internet.media.type.http">
<t>
   This document serves as the specification for the Internet media types
   "message/http" and "application/http". The following is to be registered with
   IANA (see <xref target="RFC4288"/>).
</t>
<section title="Internet Media Type message/http" anchor="internet.media.type.message.http">
<iref item="Media Type" subitem="message/http" primary="true"/>
<iref item="message/http Media Type" primary="true"/>
<t>
   The message/http type can be used to enclose a single HTTP request or
   response message, provided that it obeys the MIME restrictions for all
   "message" types regarding line length and encodings.
</t>
<t>
  <list style="hanging">
    <t hangText="Type name:">
      message
    </t>
    <t hangText="Subtype name:">
      http
    </t>
    <t hangText="Required parameters:">
      none
    </t>
    <t hangText="Optional parameters:">
      version, msgtype
      <list style="hanging">
        <t hangText="version:">
          The HTTP-Version number of the enclosed message
          (e.g., "1.1"). If not present, the version can be
          determined from the first line of the body.
        </t>
        <t hangText="msgtype:">
          The message type — "request" or "response". If not
          present, the type can be determined from the first
          line of the body.
        </t>
      </list>
    </t>
    <t hangText="Encoding considerations:">
      only "7bit", "8bit", or "binary" are permitted
    </t>
    <t hangText="Security considerations:">
      none
    </t>
    <t hangText="Interoperability considerations:">
      none
    </t>
    <t hangText="Published specification:">
      This specification (see <xref target="internet.media.type.message.http"/>).
    </t>
    <t hangText="Applications that use this media type:">
    </t>
    <t hangText="Additional information:">
      <list style="hanging">
        <t hangText="Magic number(s):">none</t>
        <t hangText="File extension(s):">none</t>
        <t hangText="Macintosh file type code(s):">none</t>
      </list>
    </t>
    <t hangText="Person and email address to contact for further information:">
      See Authors Section.
    </t>
    <t hangText="Intended usage:">
      COMMON
    </t>
    <t hangText="Restrictions on usage:">
      none
    </t>
    <t hangText="Author/Change controller:">
      IESG
    </t>
  </list>
</t>
</section>
<section title="Internet Media Type application/http" anchor="internet.media.type.application.http">
<iref item="Media Type" subitem="application/http" primary="true"/>
<iref item="application/http Media Type" primary="true"/>
<t>
   The application/http type can be used to enclose a pipeline of one or more
   HTTP request or response messages (not intermixed).
</t>
<t>
  <list style="hanging">
    <t hangText="Type name:">
      application
    </t>
    <t hangText="Subtype name:">
      http
    </t>
    <t hangText="Required parameters:">
      none
    </t>
    <t hangText="Optional parameters:">
      version, msgtype
      <list style="hanging">
        <t hangText="version:">
          The HTTP-Version number of the enclosed messages
          (e.g., "1.1"). If not present, the version can be
          determined from the first line of the body.
        </t>
        <t hangText="msgtype:">
          The message type — "request" or "response". If not
          present, the type can be determined from the first
          line of the body.
        </t>
      </list>
    </t>
    <t hangText="Encoding considerations:">
      HTTP messages enclosed by this type
      are in "binary" format; use of an appropriate
      Content-Transfer-Encoding is required when
      transmitted via E-mail.
    </t>
    <t hangText="Security considerations:">
      none
    </t>
    <t hangText="Interoperability considerations:">
      none
    </t>
    <t hangText="Published specification:">
      This specification (see <xref target="internet.media.type.application.http"/>).
    </t>
    <t hangText="Applications that use this media type:">
    </t>
    <t hangText="Additional information:">
      <list style="hanging">
        <t hangText="Magic number(s):">none</t>
        <t hangText="File extension(s):">none</t>
        <t hangText="Macintosh file type code(s):">none</t>
      </list>
    </t>
    <t hangText="Person and email address to contact for further information:">
      See Authors Section.
    </t>
    <t hangText="Intended usage:">
      COMMON
    </t>
    <t hangText="Restrictions on usage:">
      none
    </t>
    <t hangText="Author/Change controller:">
      IESG
    </t>
  </list>
</t>
</section>
</section>

<section title="Transfer Coding Registry" anchor="transfer.coding.registration">
<t>
   The registration procedure for HTTP Transfer Codings is now defined by
   <xref target="transfer.coding.registry"/> of this document.
</t>
<t>
   The HTTP Transfer Codings Registry located at <eref target="http://www.iana.org/assignments/http-parameters"/>
   shall be updated with the registrations below:
</t>
<texttable align="left" suppress-title="true" anchor="iana.transfer.coding.registration.table">
   <ttcol>Name</ttcol>
   <ttcol>Description</ttcol>
   <ttcol>Reference</ttcol>
   <c>chunked</c>
   <c>Transfer in a series of chunks</c>
   <c>
      <xref target="chunked.encoding"/>
   </c>
   <c>compress</c>
   <c>UNIX "compress" program method</c>
   <c>
      <xref target="compress.coding"/>
   </c>
   <c>deflate</c>
   <c>"deflate" compression mechanism (<xref target="RFC1951"/>) used inside
   the "zlib" data format (<xref target="RFC1950"/>)
   </c>
   <c>
      <xref target="deflate.coding"/>
   </c>
   <c>gzip</c>
   <c>Same as GNU zip <xref target="RFC1952"/></c>
   <c>
      <xref target="gzip.coding"/>
   </c>
</texttable>
</section>

<section title="Upgrade Token Registration" anchor="upgrade.token.registration">
<t>
   The registration procedure for HTTP Upgrade Tokens — previously defined
   in Section 7.2 of <xref target="RFC2817"/> — is now defined
   by <xref target="upgrade.token.registry"/> of this document.
</t>
<t>
   The HTTP Status Code Registry located at <eref target="http://www.iana.org/assignments/http-upgrade-tokens/"/>
   shall be updated with the registration below:
</t>
<texttable align="left" suppress-title="true">
   <ttcol>Value</ttcol>
   <ttcol>Description</ttcol>
   <ttcol>Reference</ttcol>

   <c>HTTP</c>
   <c>Hypertext Transfer Protocol</c> 
   <c><xref target="http.version"/> of this specification</c>

</texttable>
</section>

</section>

<section title="Security Considerations" anchor="security.considerations">
<t>
   This section is meant to inform application developers, information
   providers, and users of the security limitations in HTTP/1.1 as
   described by this document. The discussion does not include
   definitive solutions to the problems revealed, though it does make
   some suggestions for reducing security risks.
</t>

<section title="Personal Information" anchor="personal.information">
<t>
   HTTP clients are often privy to large amounts of personal information
   (e.g., the user's name, location, mail address, passwords, encryption
   keys, etc.), and SHOULD be very careful to prevent unintentional
   leakage of this information.
   We very strongly recommend that a convenient interface be provided
   for the user to control dissemination of such information, and that
   designers and implementors be particularly careful in this area.
   History shows that errors in this area often create serious security
   and/or privacy problems and generate highly adverse publicity for the
   implementor's company.
</t>
</section>

<section title="Abuse of Server Log Information" anchor="abuse.of.server.log.information">
<t>
   A server is in the position to save personal data about a user's
   requests which might identify their reading patterns or subjects of
   interest. This information is clearly confidential in nature and its
   handling can be constrained by law in certain countries. People using
   HTTP to provide data are responsible for ensuring that
   such material is not distributed without the permission of any
   individuals that are identifiable by the published results.
</t>
</section>

<section title="Attacks Based On File and Path Names" anchor="attack.pathname">
<t>
   Implementations of HTTP origin servers SHOULD be careful to restrict
   the documents returned by HTTP requests to be only those that were
   intended by the server administrators. If an HTTP server translates
   HTTP URIs directly into file system calls, the server MUST take
   special care not to serve files that were not intended to be
   delivered to HTTP clients. For example, UNIX, Microsoft Windows, and
   other operating systems use ".." as a path component to indicate a
   directory level above the current one. On such a system, an HTTP
   server MUST disallow any such construct in the request-target if it
   would otherwise allow access to a resource outside those intended to
   be accessible via the HTTP server. Similarly, files intended for
   reference only internally to the server (such as access control
   files, configuration files, and script code) MUST be protected from
   inappropriate retrieval, since they might contain sensitive
   information. Experience has shown that minor bugs in such HTTP server
   implementations have turned into security risks.
</t>
</section>

<section title="DNS-related Attacks" anchor="dns.related.attacks">
<t>
   HTTP clients rely heavily on the Domain Name Service (DNS), and are thus
   generally prone to security attacks based on the deliberate misassociation
   of IP addresses and DNS names not protected by DNSSec. Clients need to be
   cautious in assuming the validity of an IP number/DNS name association unless
   the response is protected by DNSSec (<xref target="RFC4033"/>).
</t>
</section>

<section title="Proxies and Caching" anchor="attack.proxies">
<t>
   By their very nature, HTTP proxies are men-in-the-middle, and
   represent an opportunity for man-in-the-middle attacks. Compromise of
   the systems on which the proxies run can result in serious security
   and privacy problems. Proxies have access to security-related
   information, personal information about individual users and
   organizations, and proprietary information belonging to users and
   content providers. A compromised proxy, or a proxy implemented or
   configured without regard to security and privacy considerations,
   might be used in the commission of a wide range of potential attacks.
</t>
<t>
   Proxy operators need to protect the systems on which proxies run as
   they would protect any system that contains or transports sensitive
   information. In particular, log information gathered at proxies often
   contains highly sensitive personal information, and/or information
   about organizations. Log information needs to be carefully guarded, and
   appropriate guidelines for use need to be developed and followed.
   (<xref target="abuse.of.server.log.information"/>).
</t>
<t>
   Proxy implementors need to consider the privacy and security
   implications of their design and coding decisions, and of the
   configuration options they provide to proxy operators (especially the
   default configuration).
</t>
<t>
   Users of a proxy need to be aware that proxies are no trustworthier than
   the people who run them; HTTP itself cannot solve this problem.
</t>
<t>
   The judicious use of cryptography, when appropriate, might suffice to
   protect against a broad range of security and privacy attacks. Such
   cryptography is beyond the scope of the HTTP/1.1 specification.
</t>
</section>

<section title="Protocol Element Size Overflows" anchor="attack.protocol.element.size.overflows">
<t>
   Because HTTP uses mostly textual, character-delimited fields, attackers can
   overflow buffers in implementations, and/or perform a Denial of Service
   against implementations that accept fields with unlimited lengths.
</t>
<t>
   To promote interoperability, this specification makes specific
   recommendations for size limits on request-targets (<xref target="request-target"/>)
   and blocks of header fields (<xref target="header.fields"/>). These are
   minimum recommendations, chosen to be supportable even by implementations
   with limited resources; it is expected that most implementations will choose
   substantially higher limits.
</t>
<t>
   This specification also provides a way for servers to reject messages that
   have request-targets that are too long (Section 8.4.15 of <xref target="Part2"/>) or request entities
   that are too large (Section 8.4 of <xref target="Part2"/>).
</t>
<t>
   Other fields (including but not limited to request methods, response status
   phrases, header field-names, and body chunks) SHOULD be limited by
   implementations carefully, so as to not impede interoperability.
</t>
</section>

<section title="Denial of Service Attacks on Proxies" anchor="attack.DoS">
<t>
   They exist. They are hard to defend against. Research continues.
   Beware.
</t>
</section>
</section>

<section title="Acknowledgments" anchor="acks">
<t>
   This document revision builds on the work that went into
   <xref target="RFC2616" format="none">RFC 2616</xref> and its predecessors.
   See Section 16 of <xref target="RFC2616"/> for detailed
   acknowledgements.
</t>
<t>
  <cref anchor="todoacks">Insert HTTPbis-specific acknowledgements here.</cref>
</t>
<!--

Acknowledgements TODO list

- Jeff Hodges ("effective request URI")

-->
</section>

</middle>
<back>

<references title="Normative References">

<reference anchor="ISO-8859-1">
  <front>
    <title>
     Information technology -- 8-bit single-byte coded graphic character sets -- Part 1: Latin alphabet No. 1
    </title>
    <author>
      <organization>International Organization for Standardization</organization>
    </author>
    <date year="1998"/>
  </front>
  <seriesInfo name="ISO/IEC" value="8859-1:1998"/>
</reference>

<reference anchor="Part2">
  <front>
    <title abbrev="HTTP/1.1">HTTP/1.1, part 2: Message Semantics</title>
    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
      <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
      <address><email>fielding@gbiv.com</email></address>
    </author>
    <author initials="J." surname="Gettys" fullname="Jim Gettys">
      <organization abbrev="Alcatel-Lucent">Alcatel-Lucent Bell Labs</organization>
      <address><email>jg@freedesktop.org</email></address>
    </author>
    <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
      <organization abbrev="HP">Hewlett-Packard Company</organization>
      <address><email>JeffMogul@acm.org</email></address>
    </author>
    <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
      <organization abbrev="Microsoft">Microsoft Corporation</organization>
      <address><email>henrikn@microsoft.com</email></address>
    </author>
    <author initials="L." surname="Masinter" fullname="Larry Masinter">
      <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
      <address><email>LMM@acm.org</email></address>
    </author>
    <author initials="P." surname="Leach" fullname="Paul J. Leach">
      <organization abbrev="Microsoft">Microsoft Corporation</organization>
      <address><email>paulle@microsoft.com</email></address>
    </author>
    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
      <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
      <address><email>timbl@w3.org</email></address>
    </author>
    <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
      <organization abbrev="W3C">World Wide Web Consortium</organization>
      <address><email>ylafon@w3.org</email></address>
    </author>
    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
      <organization abbrev="greenbytes">greenbytes GmbH</organization>
      <address><email>julian.reschke@greenbytes.de</email></address>
    </author>
    <date month="August" year="2011"/>
  </front>
  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p2-semantics-16"/>
  
</reference>

<reference anchor="Part3">
  <front>
    <title abbrev="HTTP/1.1">HTTP/1.1, part 3: Message Payload and Content Negotiation</title>
    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
      <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
      <address><email>fielding@gbiv.com</email></address>
    </author>
    <author initials="J." surname="Gettys" fullname="Jim Gettys">
      <organization abbrev="Alcatel-Lucent">Alcatel-Lucent Bell Labs</organization>
      <address><email>jg@freedesktop.org</email></address>
    </author>
    <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
      <organization abbrev="HP">Hewlett-Packard Company</organization>
      <address><email>JeffMogul@acm.org</email></address>
    </author>
    <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
      <organization abbrev="Microsoft">Microsoft Corporation</organization>
      <address><email>henrikn@microsoft.com</email></address>
    </author>
    <author initials="L." surname="Masinter" fullname="Larry Masinter">
      <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
      <address><email>LMM@acm.org</email></address>
    </author>
    <author initials="P." surname="Leach" fullname="Paul J. Leach">
      <organization abbrev="Microsoft">Microsoft Corporation</organization>
      <address><email>paulle@microsoft.com</email></address>
    </author>
    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
      <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
      <address><email>timbl@w3.org</email></address>
    </author>
    <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
      <organization abbrev="W3C">World Wide Web Consortium</organization>
      <address><email>ylafon@w3.org</email></address>
    </author>
    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
      <organization abbrev="greenbytes">greenbytes GmbH</organization>
      <address><email>julian.reschke@greenbytes.de</email></address>
    </author>
    <date month="August" year="2011"/>
  </front>
  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p3-payload-16"/>
  
</reference>

<reference anchor="Part6">
  <front>
    <title abbrev="HTTP/1.1">HTTP/1.1, part 6: Caching</title>
    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
      <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
      <address><email>fielding@gbiv.com</email></address>
    </author>
    <author initials="J." surname="Gettys" fullname="Jim Gettys">
      <organization abbrev="Alcatel-Lucent">Alcatel-Lucent Bell Labs</organization>
      <address><email>jg@freedesktop.org</email></address>
    </author>
    <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
      <organization abbrev="HP">Hewlett-Packard Company</organization>
      <address><email>JeffMogul@acm.org</email></address>
    </author>
    <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
      <organization abbrev="Microsoft">Microsoft Corporation</organization>
      <address><email>henrikn@microsoft.com</email></address>
    </author>
    <author initials="L." surname="Masinter" fullname="Larry Masinter">
      <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
      <address><email>LMM@acm.org</email></address>
    </author>
    <author initials="P." surname="Leach" fullname="Paul J. Leach">
      <organization abbrev="Microsoft">Microsoft Corporation</organization>
      <address><email>paulle@microsoft.com</email></address>
    </author>
    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
      <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
      <address><email>timbl@w3.org</email></address>
    </author>
    <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
      <organization abbrev="W3C">World Wide Web Consortium</organization>
      <address><email>ylafon@w3.org</email></address>
    </author>
    <author initials="M." surname="Nottingham" fullname="Mark Nottingham" role="editor">
      <address><email>mnot@mnot.net</email></address>
    </author>
    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
      <organization abbrev="greenbytes">greenbytes GmbH</organization>
      <address><email>julian.reschke@greenbytes.de</email></address>
    </author>
    <date month="August" year="2011"/>
  </front>
  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p6-cache-16"/>
  
</reference>

<reference anchor="RFC5234">
  <front>
    <title abbrev="ABNF for Syntax Specifications">Augmented BNF for Syntax Specifications: ABNF</title>
    <author initials="D." surname="Crocker" fullname="Dave Crocker" role="editor">
      <organization>Brandenburg InternetWorking</organization>
      <address>
        <email>dcrocker@bbiw.net</email>
      </address>  
    </author>
    <author initials="P." surname="Overell" fullname="Paul Overell">
      <organization>THUS plc.</organization>
      <address>
        <email>paul.overell@thus.net</email>
      </address>
    </author>
    <date month="January" year="2008"/>
  </front>
  <seriesInfo name="STD" value="68"/>
  <seriesInfo name="RFC" value="5234"/>
</reference>

<reference anchor="RFC2119">
  <front>
    <title>Key words for use in RFCs to Indicate Requirement Levels</title>
    <author initials="S." surname="Bradner" fullname="Scott Bradner">
      <organization>Harvard University</organization>
      <address><email>sob@harvard.edu</email></address>
    </author>
    <date month="March" year="1997"/>
  </front>
  <seriesInfo name="BCP" value="14"/>
  <seriesInfo name="RFC" value="2119"/>
</reference>

<reference anchor="RFC3986">
 <front>
  <title abbrev="URI Generic Syntax">Uniform Resource Identifier (URI): Generic Syntax</title>
  <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
    <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
    <address>
       <email>timbl@w3.org</email>
       <uri>http://www.w3.org/People/Berners-Lee/</uri>
    </address>
  </author>
  <author initials="R." surname="Fielding" fullname="Roy T. Fielding">
    <organization abbrev="Day Software">Day Software</organization>
    <address>
      <email>fielding@gbiv.com</email>
      <uri>http://roy.gbiv.com/</uri>
    </address>
  </author>
  <author initials="L." surname="Masinter" fullname="Larry Masinter">
    <organization abbrev="Adobe Systems">Adobe Systems Incorporated</organization>
    <address>
      <email>LMM@acm.org</email>
      <uri>http://larry.masinter.net/</uri>
    </address>
  </author>
  <date month="January" year="2005"/>
 </front>
 <seriesInfo name="STD" value="66"/>
 <seriesInfo name="RFC" value="3986"/>
</reference>

<reference anchor="USASCII">
  <front>
    <title>Coded Character Set -- 7-bit American Standard Code for Information Interchange</title>
    <author>
      <organization>American National Standards Institute</organization>
    </author>
    <date year="1986"/>
  </front>
  <seriesInfo name="ANSI" value="X3.4"/>
</reference>

<reference anchor="RFC1950">
  <front>
    <title>ZLIB Compressed Data Format Specification version 3.3</title>
    <author initials="L.P." surname="Deutsch" fullname="L. Peter Deutsch">
      <organization>Aladdin Enterprises</organization>
      <address><email>ghost@aladdin.com</email></address>
    </author>
    <author initials="J-L." surname="Gailly" fullname="Jean-Loup Gailly"/>
    <date month="May" year="1996"/>
  </front>
  <seriesInfo name="RFC" value="1950"/>
  <annotation>
    RFC 1950 is an Informational RFC, thus it might be less stable than
    this specification. On the other hand, this downward reference was 
    present since the publication of RFC 2068 in 1997 (<xref target="RFC2068"/>),
    therefore it is unlikely to cause problems in practice. See also
    <xref target="BCP97"/>.
  </annotation>
</reference>

<reference anchor="RFC1951">
  <front>
    <title>DEFLATE Compressed Data Format Specification version 1.3</title>
    <author initials="P." surname="Deutsch" fullname="L. Peter Deutsch">
      <organization>Aladdin Enterprises</organization>
      <address><email>ghost@aladdin.com</email></address>
    </author>
    <date month="May" year="1996"/>
  </front>
  <seriesInfo name="RFC" value="1951"/>
  <annotation>
    RFC 1951 is an Informational RFC, thus it might be less stable than
    this specification. On the other hand, this downward reference was 
    present since the publication of RFC 2068 in 1997 (<xref target="RFC2068"/>),
    therefore it is unlikely to cause problems in practice. See also
    <xref target="BCP97"/>.
  </annotation>
</reference>

<reference anchor="RFC1952">
  <front>
    <title>GZIP file format specification version 4.3</title>
    <author initials="P." surname="Deutsch" fullname="L. Peter Deutsch">
      <organization>Aladdin Enterprises</organization>
      <address><email>ghost@aladdin.com</email></address>
    </author>
    <author initials="J-L." surname="Gailly" fullname="Jean-Loup Gailly">
      <address><email>gzip@prep.ai.mit.edu</email></address>
    </author>
    <author initials="M." surname="Adler" fullname="Mark Adler">
      <address><email>madler@alumni.caltech.edu</email></address>
    </author>
    <author initials="L.P." surname="Deutsch" fullname="L. Peter Deutsch">
      <address><email>ghost@aladdin.com</email></address>
    </author>
    <author initials="G." surname="Randers-Pehrson" fullname="Glenn Randers-Pehrson">
      <address><email>randeg@alumni.rpi.edu</email></address>
    </author>
    <date month="May" year="1996"/>
  </front>
  <seriesInfo name="RFC" value="1952"/>
  <annotation>
    RFC 1952 is an Informational RFC, thus it might be less stable than
    this specification. On the other hand, this downward reference was 
    present since the publication of RFC 2068 in 1997 (<xref target="RFC2068"/>),
    therefore it is unlikely to cause problems in practice. See also
    <xref target="BCP97"/>.
  </annotation>
</reference>

</references>

<references title="Informative References">

<reference anchor="Nie1997" target="http://doi.acm.org/10.1145/263105.263157">
  <front>
    <title>Network Performance Effects of HTTP/1.1, CSS1, and PNG</title>
    <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen"/>
    <author initials="J." surname="Gettys" fullname="J. Gettys"/>
    <author initials="E." surname="Prud'hommeaux" fullname="E. Prud'hommeaux"/>
    <author initials="H." surname="Lie" fullname="H. Lie"/>
    <author initials="C." surname="Lilley" fullname="C. Lilley"/>
    <date year="1997" month="September"/>
  </front>
  <seriesInfo name="ACM" value="Proceedings of the ACM SIGCOMM '97 conference on Applications, technologies, architectures, and protocols for computer communication SIGCOMM '97"/>
</reference>

<reference anchor="Pad1995" target="http://portal.acm.org/citation.cfm?id=219094">
  <front>
    <title>Improving HTTP Latency</title>
    <author initials="V.N." surname="Padmanabhan" fullname="Venkata N. Padmanabhan"/>
    <author initials="J.C." surname="Mogul" fullname="Jeffrey C. Mogul"/>
    <date year="1995" month="December"/>
  </front>
  <seriesInfo name="Computer Networks and ISDN Systems" value="v. 28, pp. 25-35"/>
</reference>

<reference anchor="RFC1123">
  <front>
    <title>Requirements for Internet Hosts - Application and Support</title>
    <author initials="R." surname="Braden" fullname="Robert Braden">
      <organization>University of Southern California (USC), Information Sciences Institute</organization>
      <address><email>Braden@ISI.EDU</email></address>
    </author>
    <date month="October" year="1989"/>
  </front>
  <seriesInfo name="STD" value="3"/>
  <seriesInfo name="RFC" value="1123"/>
</reference>

<reference anchor="RFC1919">
  <front>
    <title>Classical versus Transparent IP Proxies</title>
    <author initials="M." surname="Chatel" fullname="Marc Chatel">
      <address><email>mchatel@pax.eunet.ch</email></address>
    </author>
    <date year="1996" month="March"/>
  </front>
  <seriesInfo name="RFC" value="1919"/>
</reference>

<reference anchor="RFC1945">
  <front>
    <title abbrev="HTTP/1.0">Hypertext Transfer Protocol -- HTTP/1.0</title>
    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
      <organization>MIT, Laboratory for Computer Science</organization>
      <address><email>timbl@w3.org</email></address>
    </author>
    <author initials="R.T." surname="Fielding" fullname="Roy T. Fielding">
      <organization>University of California, Irvine, Department of Information and Computer Science</organization>
      <address><email>fielding@ics.uci.edu</email></address>
    </author>
    <author initials="H.F." surname="Nielsen" fullname="Henrik Frystyk Nielsen">
      <organization>W3 Consortium, MIT Laboratory for Computer Science</organization>
      <address><email>frystyk@w3.org</email></address>
    </author>
    <date month="May" year="1996"/>
  </front>
  <seriesInfo name="RFC" value="1945"/>
</reference>

<reference anchor="RFC2045">
  <front>
    <title abbrev="Internet Message Bodies">Multipurpose Internet Mail Extensions (MIME) Part One: Format of Internet Message Bodies</title>
    <author initials="N." surname="Freed" fullname="Ned Freed">
      <organization>Innosoft International, Inc.</organization>
      <address><email>ned@innosoft.com</email></address>
    </author>
    <author initials="N.S." surname="Borenstein" fullname="Nathaniel S. Borenstein">
      <organization>First Virtual Holdings</organization>
      <address><email>nsb@nsb.fv.com</email></address>
    </author>
    <date month="November" year="1996"/>
  </front>
  <seriesInfo name="RFC" value="2045"/>
</reference>

<reference anchor="RFC2047">
  <front>
    <title abbrev="Message Header Extensions">MIME (Multipurpose Internet Mail Extensions) Part Three: Message Header Extensions for Non-ASCII Text</title>
    <author initials="K." surname="Moore" fullname="Keith Moore">
      <organization>University of Tennessee</organization>
      <address><email>moore@cs.utk.edu</email></address>
    </author>
    <date month="November" year="1996"/>
  </front>
  <seriesInfo name="RFC" value="2047"/>
</reference>

<reference anchor="RFC2068">
  <front>
    <title abbrev="HTTP/1.1">Hypertext Transfer Protocol -- HTTP/1.1</title>
    <author initials="R." surname="Fielding" fullname="Roy T. Fielding">
      <organization>University of California, Irvine, Department of Information and Computer Science</organization>
      <address><email>fielding@ics.uci.edu</email></address>
    </author>
    <author initials="J." surname="Gettys" fullname="Jim Gettys">
      <organization>MIT Laboratory for Computer Science</organization>
      <address><email>jg@w3.org</email></address>
    </author>
    <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
      <organization>Digital Equipment Corporation, Western Research Laboratory</organization>
      <address><email>mogul@wrl.dec.com</email></address>
    </author>
    <author initials="H." surname="Nielsen" fullname="Henrik Frystyk Nielsen">
      <organization>MIT Laboratory for Computer Science</organization>
      <address><email>frystyk@w3.org</email></address>
    </author>
    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
      <organization>MIT Laboratory for Computer Science</organization>
      <address><email>timbl@w3.org</email></address>
    </author>
    <date month="January" year="1997"/>
  </front>
  <seriesInfo name="RFC" value="2068"/>
</reference>

<reference anchor="RFC2145">
  <front>
    <title abbrev="HTTP Version Numbers">Use and Interpretation of HTTP Version Numbers</title>
    <author initials="J.C." surname="Mogul" fullname="Jeffrey C. Mogul">
      <organization>Western Research Laboratory</organization>
      <address><email>mogul@wrl.dec.com</email></address>
    </author>
    <author initials="R.T." surname="Fielding" fullname="Roy T. Fielding">
      <organization>Department of Information and Computer Science</organization>
      <address><email>fielding@ics.uci.edu</email></address>
    </author>
    <author initials="J." surname="Gettys" fullname="Jim Gettys">
      <organization>MIT Laboratory for Computer Science</organization>
      <address><email>jg@w3.org</email></address>
    </author>
    <author initials="H.F." surname="Nielsen" fullname="Henrik Frystyk Nielsen">
      <organization>W3 Consortium</organization>
      <address><email>frystyk@w3.org</email></address>
    </author>
    <date month="May" year="1997"/>
  </front>
  <seriesInfo name="RFC" value="2145"/>
</reference>

<reference anchor="RFC2616">
  <front>
    <title>Hypertext Transfer Protocol -- HTTP/1.1</title>
    <author initials="R." surname="Fielding" fullname="R. Fielding">
      <organization>University of California, Irvine</organization>
      <address><email>fielding@ics.uci.edu</email></address>
    </author>
    <author initials="J." surname="Gettys" fullname="J. Gettys">
      <organization>W3C</organization>
      <address><email>jg@w3.org</email></address>
    </author>
    <author initials="J." surname="Mogul" fullname="J. Mogul">
      <organization>Compaq Computer Corporation</organization>
      <address><email>mogul@wrl.dec.com</email></address>
    </author>
    <author initials="H." surname="Frystyk" fullname="H. Frystyk">
      <organization>MIT Laboratory for Computer Science</organization>
      <address><email>frystyk@w3.org</email></address>
    </author>
    <author initials="L." surname="Masinter" fullname="L. Masinter">
      <organization>Xerox Corporation</organization>
      <address><email>masinter@parc.xerox.com</email></address>
    </author>
    <author initials="P." surname="Leach" fullname="P. Leach">
      <organization>Microsoft Corporation</organization>
      <address><email>paulle@microsoft.com</email></address>
    </author>
    <author initials="T." surname="Berners-Lee" fullname="T. Berners-Lee">
      <organization>W3C</organization>
      <address><email>timbl@w3.org</email></address>
    </author>
    <date month="June" year="1999"/>
  </front>
  <seriesInfo name="RFC" value="2616"/>
</reference>

<reference anchor="RFC2817">
  <front>
    <title>Upgrading to TLS Within HTTP/1.1</title>
    <author initials="R." surname="Khare" fullname="R. Khare">
      <organization>4K Associates / UC Irvine</organization>
      <address><email>rohit@4K-associates.com</email></address>
    </author>
    <author initials="S." surname="Lawrence" fullname="S. Lawrence">
      <organization>Agranat Systems, Inc.</organization>
      <address><email>lawrence@agranat.com</email></address>
    </author>
    <date year="2000" month="May"/>
  </front>
  <seriesInfo name="RFC" value="2817"/>
</reference>

<reference anchor="RFC2818">
  <front>
    <title>HTTP Over TLS</title>
    <author initials="E." surname="Rescorla" fullname="Eric Rescorla">
      <organization>RTFM, Inc.</organization>
      <address><email>ekr@rtfm.com</email></address>
    </author>
    <date year="2000" month="May"/>
  </front>
  <seriesInfo name="RFC" value="2818"/>
</reference>

<reference anchor="RFC2965">
  <front>
    <title>HTTP State Management Mechanism</title>
    <author initials="D. M." surname="Kristol" fullname="David M. Kristol">
      <organization>Bell Laboratories, Lucent Technologies</organization>
      <address><email>dmk@bell-labs.com</email></address>
    </author>
    <author initials="L." surname="Montulli" fullname="Lou Montulli">
      <organization>Epinions.com, Inc.</organization>
      <address><email>lou@montulli.org</email></address>
    </author>
    <date year="2000" month="October"/>
  </front>
  <seriesInfo name="RFC" value="2965"/>
</reference>

<reference anchor="RFC3040">
  <front>
    <title>Internet Web Replication and Caching Taxonomy</title>
    <author initials="I." surname="Cooper" fullname="I. Cooper">
      <organization>Equinix, Inc.</organization>
    </author>
    <author initials="I." surname="Melve" fullname="I. Melve">
      <organization>UNINETT</organization>
    </author>
    <author initials="G." surname="Tomlinson" fullname="G. Tomlinson">
      <organization>CacheFlow Inc.</organization>
    </author>
    <date year="2001" month="January"/>
  </front>
  <seriesInfo name="RFC" value="3040"/>
</reference>

<reference anchor="RFC3864">
  <front>
    <title>Registration Procedures for Message Header Fields</title>
    <author initials="G." surname="Klyne" fullname="G. Klyne">
      <organization>Nine by Nine</organization>
      <address><email>GK-IETF@ninebynine.org</email></address>
    </author>
    <author initials="M." surname="Nottingham" fullname="M. Nottingham">
      <organization>BEA Systems</organization>
      <address><email>mnot@pobox.com</email></address>
    </author>
    <author initials="J." surname="Mogul" fullname="J. Mogul">
      <organization>HP Labs</organization>
      <address><email>JeffMogul@acm.org</email></address>
    </author>
    <date year="2004" month="September"/>
  </front>
  <seriesInfo name="BCP" value="90"/>
  <seriesInfo name="RFC" value="3864"/>
</reference>

<reference anchor="RFC4033">
  <front>
    <title>DNS Security Introduction and Requirements</title>
    <author initials="R." surname="Arends" fullname="R. Arends"/>
    <author initials="R." surname="Austein" fullname="R. Austein"/>
    <author initials="M." surname="Larson" fullname="M. Larson"/>
    <author initials="D." surname="Massey" fullname="D. Massey"/>
    <author initials="S." surname="Rose" fullname="S. Rose"/>
    <date year="2005" month="March"/>
  </front>
  <seriesInfo name="RFC" value="4033"/>
</reference>

<reference anchor="RFC4288">
  <front>
    <title>Media Type Specifications and Registration Procedures</title>
    <author initials="N." surname="Freed" fullname="N. Freed">
      <organization>Sun Microsystems</organization>
      <address>
        <email>ned.freed@mrochek.com</email>
      </address>
    </author>
    <author initials="J." surname="Klensin" fullname="J. Klensin">
      <address>
        <email>klensin+ietf@jck.com</email>
      </address>
    </author>
    <date year="2005" month="December"/>
  </front>
  <seriesInfo name="BCP" value="13"/>
  <seriesInfo name="RFC" value="4288"/>
</reference>

<reference anchor="RFC4395">
  <front>
    <title>Guidelines and Registration Procedures for New URI Schemes</title>
    <author initials="T." surname="Hansen" fullname="T. Hansen">
      <organization>AT&amp;T Laboratories</organization>
      <address>
        <email>tony+urireg@maillennium.att.com</email>
      </address>
    </author>
    <author initials="T." surname="Hardie" fullname="T. Hardie">
      <organization>Qualcomm, Inc.</organization>
      <address>
        <email>hardie@qualcomm.com</email>
      </address>
    </author>
    <author initials="L." surname="Masinter" fullname="L. Masinter">
      <organization>Adobe Systems</organization>
      <address>
        <email>LMM@acm.org</email>
      </address>
    </author>
    <date year="2006" month="February"/>
  </front>
  <seriesInfo name="BCP" value="115"/>
  <seriesInfo name="RFC" value="4395"/>
</reference>

<reference anchor="RFC4559">
  <front>
    <title>SPNEGO-based Kerberos and NTLM HTTP Authentication in Microsoft Windows</title>
    <author initials="K." surname="Jaganathan" fullname="K. Jaganathan"/>
    <author initials="L." surname="Zhu" fullname="L. Zhu"/>
    <author initials="J." surname="Brezak" fullname="J. Brezak"/>
    <date year="2006" month="June"/>
  </front>
  <seriesInfo name="RFC" value="4559"/>
</reference>

<reference anchor="RFC5226">
  <front>
    <title>Guidelines for Writing an IANA Considerations Section in RFCs</title>
    <author initials="T." surname="Narten" fullname="T. Narten">
      <organization>IBM</organization>
      <address><email>narten@us.ibm.com</email></address>
    </author>
    <author initials="H." surname="Alvestrand" fullname="H. Alvestrand">
      <organization>Google</organization>
      <address><email>Harald@Alvestrand.no</email></address>
    </author>
    <date year="2008" month="May"/>
  </front>
  <seriesInfo name="BCP" value="26"/>
  <seriesInfo name="RFC" value="5226"/>
</reference>

<reference anchor="RFC5322">
  <front>
    <title>Internet Message Format</title>
    <author initials="P." surname="Resnick" fullname="P. Resnick">
      <organization>Qualcomm Incorporated</organization>
    </author>
    <date year="2008" month="October"/>
  </front> 
  <seriesInfo name="RFC" value="5322"/>
</reference>

<reference anchor="RFC6265">
  <front>
    <title>HTTP State Management Mechanism</title>
    <author initials="A." surname="Barth" fullname="Adam Barth">
      <organization abbrev="U.C. Berkeley">
        University of California, Berkeley
      </organization>
      <address><email>abarth@eecs.berkeley.edu</email></address>
    </author>
    <date year="2011" month="April"/>
  </front>
  <seriesInfo name="RFC" value="6265"/>
</reference>

<reference anchor="BCP97">
  <front>
    <title>Handling Normative References to Standards-Track Documents</title>
    <author initials="J." surname="Klensin" fullname="J. Klensin">
      <address>
        <email>klensin+ietf@jck.com</email>
      </address>
    </author>
    <author initials="S." surname="Hartman" fullname="S. Hartman">
      <organization>MIT</organization>
      <address>
        <email>hartmans-ietf@mit.edu</email>
      </address>
    </author>
    <date year="2007" month="June"/>
  </front>
  <seriesInfo name="BCP" value="97"/>
  <seriesInfo name="RFC" value="4897"/>
</reference>

<reference anchor="Kri2001" target="http://arxiv.org/abs/cs.SE/0105018">
  <front>
    <title>HTTP Cookies: Standards, Privacy, and Politics</title>
    <author initials="D." surname="Kristol" fullname="David M. Kristol"/>
    <date year="2001" month="November"/>
  </front>
  <seriesInfo name="ACM Transactions on Internet Technology" value="Vol. 1, #2"/>
</reference>

<reference anchor="Spe" target="http://sunsite.unc.edu/mdma-release/http-prob.html">
  <front>
    <title>Analysis of HTTP Performance Problems</title>
    <author initials="S." surname="Spero" fullname="Simon E. Spero"/>
    <date/>
  </front>
</reference>

<reference anchor="Tou1998" target="http://www.isi.edu/touch/pubs/http-perf96/">
  <front>
  <title>Analysis of HTTP Performance</title>
  <author initials="J." surname="Touch" fullname="Joe Touch">
    <organization>USC/Information Sciences Institute</organization>
    <address><email>touch@isi.edu</email></address>
  </author>
  <author initials="J." surname="Heidemann" fullname="John Heidemann">
    <organization>USC/Information Sciences Institute</organization>
    <address><email>johnh@isi.edu</email></address>
  </author>
  <author initials="K." surname="Obraczka" fullname="Katia Obraczka">
    <organization>USC/Information Sciences Institute</organization>
    <address><email>katia@isi.edu</email></address>
  </author>
  <date year="1998" month="Aug"/>
  </front>
  <seriesInfo name="ISI Research Report" value="ISI/RR-98-463"/>
  <annotation>(original report dated Aug. 1996)</annotation>
</reference>

</references>


<section title="HTTP Version History" anchor="compatibility">
<t>
   HTTP has been in use by the World-Wide Web global information initiative
   since 1990. The first version of HTTP, later referred to as HTTP/0.9,
   was a simple protocol for hypertext data transfer across the Internet
   with only a single request method (GET) and no metadata.
   HTTP/1.0, as defined by <xref target="RFC1945"/>, added a range of request
   methods and MIME-like messaging that could include metadata about the data
   transferred and modifiers on the request/response semantics. However,
   HTTP/1.0 did not sufficiently take into consideration the effects of
   hierarchical proxies, caching, the need for persistent connections, or
   name-based virtual hosts. The proliferation of incompletely-implemented
   applications calling themselves "HTTP/1.0" further necessitated a
   protocol version change in order for two communicating applications
   to determine each other's true capabilities.
</t>
<t>
   HTTP/1.1 remains compatible with HTTP/1.0 by including more stringent
   requirements that enable reliable implementations, adding only
   those new features that will either be safely ignored by an HTTP/1.0
   recipient or only sent when communicating with a party advertising
   compliance with HTTP/1.1.
</t>
<t>
   It is beyond the scope of a protocol specification to mandate
   compliance with previous versions. HTTP/1.1 was deliberately
   designed, however, to make supporting previous versions easy.
   We would expect a general-purpose HTTP/1.1 server to understand
   any valid request in the format of HTTP/1.0 and respond appropriately
   with an HTTP/1.1 message that only uses features understood (or
   safely ignored) by HTTP/1.0 clients.  Likewise, would expect
   an HTTP/1.1 client to understand any valid HTTP/1.0 response.
</t>
<t>
   Since HTTP/0.9 did not support header fields in a request,
   there is no mechanism for it to support name-based virtual
   hosts (selection of resource by inspection of the Host header
   field).  Any server that implements name-based virtual hosts
   ought to disable support for HTTP/0.9.  Most requests that
   appear to be HTTP/0.9 are, in fact, badly constructed HTTP/1.x
   requests wherein a buggy client failed to properly encode
   linear whitespace found in a URI reference and placed in
   the request-target.
</t>

<section title="Changes from HTTP/1.0" anchor="changes.from.1.0">
<t>
   This section summarizes major differences between versions HTTP/1.0
   and HTTP/1.1.
</t>

<section title="Multi-homed Web Servers" anchor="changes.to.simplify.multi-homed.web.servers.and.conserve.ip.addresses">
<t>
   The requirements that clients and servers support the Host header
   field (<xref target="header.host"/>), report an error if it is
   missing from an HTTP/1.1 request, and accept absolute URIs (<xref target="request-target"/>)
   are among the most important changes defined by HTTP/1.1.
</t>
<t>
   Older HTTP/1.0 clients assumed a one-to-one relationship of IP
   addresses and servers; there was no other established mechanism for
   distinguishing the intended server of a request than the IP address
   to which that request was directed. The Host header field was
   introduced during the development of HTTP/1.1 and, though it was
   quickly implemented by most HTTP/1.0 browsers, additional requirements
   were placed on all HTTP/1.1 requests in order to ensure complete
   adoption.  At the time of this writing, most HTTP-based services
   are dependent upon the Host header field for targeting requests.
</t>
</section>

<section title="Keep-Alive Connections" anchor="compatibility.with.http.1.0.persistent.connections">
<t>
   For most implementations of HTTP/1.0, each connection is established
   by the client prior to the request and closed by the server after
   sending the response. However, some implementations implement the
   Keep-Alive version of persistent connections described in
   Section 19.7.1 of <xref target="RFC2068"/>.
</t>
<t>
   Some clients and servers might wish to be compatible with some
   previous implementations of persistent connections in HTTP/1.0
   clients and servers. Persistent connections in HTTP/1.0 are
   explicitly negotiated as they are not the default behavior. HTTP/1.0
   experimental implementations of persistent connections are faulty,
   and the new facilities in HTTP/1.1 are designed to rectify these
   problems. The problem was that some existing HTTP/1.0 clients might
   send Keep-Alive to a proxy server that doesn't understand
   Connection, which would then erroneously forward it to the next
   inbound server, which would establish the Keep-Alive connection and
   result in a hung HTTP/1.0 proxy waiting for the close on the
   response. The result is that HTTP/1.0 clients must be prevented from
   using Keep-Alive when talking to proxies.
</t>
<t>
   However, talking to proxies is the most important use of persistent
   connections, so that prohibition is clearly unacceptable. Therefore,
   we need some other mechanism for indicating a persistent connection
   is desired, which is safe to use even when talking to an old proxy
   that ignores Connection. Persistent connections are the default for
   HTTP/1.1 messages; we introduce a new keyword (Connection: close) for
   declaring non-persistence. See <xref target="header.connection"/>.
</t>
</section>
</section>

<section title="Changes from RFC 2616" anchor="changes.from.rfc.2616">
<t>
  Empty list elements in list productions have been deprecated.
  (<xref target="notation.abnf"/>)
</t>
<t>
  Rules about implicit linear whitespace between certain grammar productions
  have been removed; now it's only allowed when specifically pointed out
  in the ABNF.
  (<xref target="basic.rules"/>)
</t>
<t>
  Clarify that the string "HTTP" in the HTTP-Version ABFN production is case
  sensitive. Restrict the version numbers to be single digits due to the fact
  that implementations are known to handle multi-digit version numbers
  incorrectly.
  (<xref target="http.version"/>)
</t>
<t>
  Require that invalid whitespace around field-names be rejected.
  (<xref target="header.fields"/>)
</t>
<t>  
  The NUL octet is no longer allowed in comment and quoted-string
  text. The quoted-pair rule no longer allows escaping control characters other than HTAB.
  Non-ASCII content in header fields and reason phrase has been obsoleted and
  made opaque (the TEXT rule was removed).
  (<xref target="field.rules"/>)
</t>
<t>
  Require recipients to handle bogus Content-Length header fields as errors.
  (<xref target="message.body"/>)
</t>
<t>
  Remove reference to non-existent identity transfer-coding value tokens.
  (Sections <xref format="counter" target="message.body"/> and
  <xref format="counter" target="transfer.codings"/>)
</t>
<t>
  Update use of abs_path production from RFC 1808 to the path-absolute + query
  components of RFC 3986. State that the asterisk form is allowed for the OPTIONS
  request method only.
  (<xref target="request-target"/>)
</t>
<t>
  Clarification that the chunk length does not include the count of the octets
  in the chunk header and trailer. Furthermore disallowed line folding
  in chunk extensions.
  (<xref target="chunked.encoding"/>)
</t>
<t>
  Remove hard limit of two connections per server.
  (<xref target="persistent.practical"/>)
</t>
<t>
  Change ABNF productions for header fields to only define the field value.
  (<xref target="header.field.definitions"/>)
</t>
<t>
  Clarify exactly when close connection options must be sent.
  (<xref target="header.connection"/>)
</t>
<t>
  Define the semantics of the "Upgrade" header field in responses other than
  101 (this was incorporated from <xref target="RFC2817"/>).
  (<xref target="header.upgrade"/>)
</t>
</section>
</section>


<section title="Collected ABNF" anchor="collected.abnf">
<figure>
<artwork type="abnf" name="p1-messaging.parsed-abnf"><![CDATA[
BWS = OWS

Chunked-Body = *chunk last-chunk trailer-part CRLF
Connection = *( "," OWS ) connection-token *( OWS "," [ OWS
 connection-token ] )
Content-Length = 1*DIGIT

Date = HTTP-date

GMT = %x47.4D.54 ; GMT

HTTP-Prot-Name = %x48.54.54.50 ; HTTP
HTTP-Version = HTTP-Prot-Name "/" DIGIT "." DIGIT
HTTP-date = rfc1123-date / obs-date
HTTP-message = start-line *( header-field CRLF ) CRLF [ message-body
 ]
Host = uri-host [ ":" port ]

Method = token

OWS = *( [ obs-fold ] WSP )

RWS = 1*( [ obs-fold ] WSP )
Reason-Phrase = *( WSP / VCHAR / obs-text )
Request = Request-Line *( header-field CRLF ) CRLF [ message-body ]
Request-Line = Method SP request-target SP HTTP-Version CRLF
Response = Status-Line *( header-field CRLF ) CRLF [ message-body ]

Status-Code = 3DIGIT
Status-Line = HTTP-Version SP Status-Code SP Reason-Phrase CRLF

TE = [ ( "," / t-codings ) *( OWS "," [ OWS t-codings ] ) ]
Trailer = *( "," OWS ) field-name *( OWS "," [ OWS field-name ] )
Transfer-Encoding = *( "," OWS ) transfer-coding *( OWS "," [ OWS
 transfer-coding ] )

URI-reference = <URI-reference, defined in [RFC3986], Section 4.1>
Upgrade = *( "," OWS ) product *( OWS "," [ OWS product ] )

Via = *( "," OWS ) received-protocol RWS received-by [ RWS comment ]
 *( OWS "," [ OWS received-protocol RWS received-by [ RWS comment ] ]
 )

absolute-URI = <absolute-URI, defined in [RFC3986], Section 4.3>
asctime-date = day-name SP date3 SP time-of-day SP year
attribute = token
authority = <authority, defined in [RFC3986], Section 3.2>

chunk = chunk-size *WSP [ chunk-ext ] CRLF chunk-data CRLF
chunk-data = 1*OCTET
chunk-ext = *( ";" *WSP chunk-ext-name [ "=" chunk-ext-val ] *WSP )
chunk-ext-name = token
chunk-ext-val = token / quoted-str-nf
chunk-size = 1*HEXDIG
comment = "(" *( ctext / quoted-cpair / comment ) ")"
connection-token = token
ctext = OWS / %x21-27 ; '!'-'''
 / %x2A-5B ; '*'-'['
 / %x5D-7E ; ']'-'~'
 / obs-text

date1 = day SP month SP year
date2 = day "-" month "-" 2DIGIT
date3 = month SP ( 2DIGIT / ( SP DIGIT ) )
day = 2DIGIT
day-name = %x4D.6F.6E ; Mon
 / %x54.75.65 ; Tue
 / %x57.65.64 ; Wed
 / %x54.68.75 ; Thu
 / %x46.72.69 ; Fri
 / %x53.61.74 ; Sat
 / %x53.75.6E ; Sun
day-name-l = %x4D.6F.6E.64.61.79 ; Monday
 / %x54.75.65.73.64.61.79 ; Tuesday
 / %x57.65.64.6E.65.73.64.61.79 ; Wednesday
 / %x54.68.75.72.73.64.61.79 ; Thursday
 / %x46.72.69.64.61.79 ; Friday
 / %x53.61.74.75.72.64.61.79 ; Saturday
 / %x53.75.6E.64.61.79 ; Sunday

field-content = *( WSP / VCHAR / obs-text )
field-name = token
field-value = *( field-content / OWS )

header-field = field-name ":" OWS [ field-value ] OWS
hour = 2DIGIT
http-URI = "http://" authority path-abempty [ "?" query ]
https-URI = "https://" authority path-abempty [ "?" query ]

last-chunk = 1*"0" *WSP [ chunk-ext ] CRLF

message-body = *OCTET
minute = 2DIGIT
month = %x4A.61.6E ; Jan
 / %x46.65.62 ; Feb
 / %x4D.61.72 ; Mar
 / %x41.70.72 ; Apr
 / %x4D.61.79 ; May
 / %x4A.75.6E ; Jun
 / %x4A.75.6C ; Jul
 / %x41.75.67 ; Aug
 / %x53.65.70 ; Sep
 / %x4F.63.74 ; Oct
 / %x4E.6F.76 ; Nov
 / %x44.65.63 ; Dec

obs-date = rfc850-date / asctime-date
obs-fold = CRLF
obs-text = %x80-FF

partial-URI = relative-part [ "?" query ]
path-abempty = <path-abempty, defined in [RFC3986], Section 3.3>
path-absolute = <path-absolute, defined in [RFC3986], Section 3.3>
port = <port, defined in [RFC3986], Section 3.2.3>
product = token [ "/" product-version ]
product-version = token
protocol-name = token
protocol-version = token
pseudonym = token

qdtext = OWS / "!" / %x23-5B ; '#'-'['
 / %x5D-7E ; ']'-'~'
 / obs-text
qdtext-nf = WSP / "!" / %x23-5B ; '#'-'['
 / %x5D-7E ; ']'-'~'
 / obs-text
query = <query, defined in [RFC3986], Section 3.4>
quoted-cpair = "\" ( WSP / VCHAR / obs-text )
quoted-pair = "\" ( WSP / VCHAR / obs-text )
quoted-str-nf = DQUOTE *( qdtext-nf / quoted-pair ) DQUOTE
quoted-string = DQUOTE *( qdtext / quoted-pair ) DQUOTE
qvalue = ( "0" [ "." *3DIGIT ] ) / ( "1" [ "." *3"0" ] )

received-by = ( uri-host [ ":" port ] ) / pseudonym
received-protocol = [ protocol-name "/" ] protocol-version
relative-part = <relative-part, defined in [RFC3986], Section 4.2>
request-target = "*" / absolute-URI / ( path-absolute [ "?" query ] )
 / authority
rfc1123-date = day-name "," SP date1 SP time-of-day SP GMT
rfc850-date = day-name-l "," SP date2 SP time-of-day SP GMT

second = 2DIGIT
special = "(" / ")" / "<" / ">" / "@" / "," / ";" / ":" / "\" /
 DQUOTE / "/" / "[" / "]" / "?" / "=" / "{" / "}"
start-line = Request-Line / Status-Line

t-codings = "trailers" / ( transfer-extension [ te-params ] )
tchar = "!" / "#" / "$" / "%" / "&" / "'" / "*" / "+" / "-" / "." /
 "^" / "_" / "`" / "|" / "~" / DIGIT / ALPHA
te-ext = OWS ";" OWS token [ "=" word ]
te-params = OWS ";" OWS "q=" qvalue *te-ext
time-of-day = hour ":" minute ":" second
token = 1*tchar
trailer-part = *( header-field CRLF )
transfer-coding = "chunked" / "compress" / "deflate" / "gzip" /
 transfer-extension
transfer-extension = token *( OWS ";" OWS transfer-parameter )
transfer-parameter = attribute BWS "=" BWS value

uri-host = <host, defined in [RFC3986], Section 3.2.2>

value = word

word = token / quoted-string

year = 4DIGIT
]]></artwork>
</figure>
<figure><preamble>ABNF diagnostics:</preamble><artwork type="inline"><![CDATA[
; Chunked-Body defined but not used
; Connection defined but not used
; Content-Length defined but not used
; Date defined but not used
; HTTP-message defined but not used
; Host defined but not used
; Request defined but not used
; Response defined but not used
; TE defined but not used
; Trailer defined but not used
; Transfer-Encoding defined but not used
; URI-reference defined but not used
; Upgrade defined but not used
; Via defined but not used
; http-URI defined but not used
; https-URI defined but not used
; partial-URI defined but not used
; special defined but not used
]]></artwork></figure></section>


<section title="Change Log (to be removed by RFC Editor before publication)" anchor="change.log">

<section title="Since RFC 2616">
<t>
  Extracted relevant partitions from <xref target="RFC2616"/>.
</t>
</section>

<section title="Since draft-ietf-httpbis-p1-messaging-00">
<t>
  Closed issues:
  <list style="symbols"> 
    <t>
      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/1"/>:
      "HTTP Version should be case sensitive"
      (<eref target="http://purl.org/NET/http-errata#verscase"/>)
    </t>
    <t>
      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/2"/>:
      "'unsafe' characters"
      (<eref target="http://purl.org/NET/http-errata#unsafe-uri"/>)
    </t>
    <t>
      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/3"/>:
      "Chunk Size Definition"
      (<eref target="http://purl.org/NET/http-errata#chunk-size"/>)
    </t>
    <t>
      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/4"/>:
      "Message Length"
      (<eref target="http://purl.org/NET/http-errata#msg-len-chars"/>)
    </t>
    <t>
      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/8"/>:
      "Media Type Registrations"
      (<eref target="http://purl.org/NET/http-errata#media-reg"/>)
    </t>
    <t>
      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/11"/>:
      "URI includes query"
      (<eref target="http://purl.org/NET/http-errata#uriquery"/>)
    </t>
    <t>
      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/15"/>:
      "No close on 1xx responses"
      (<eref target="http://purl.org/NET/http-errata#noclose1xx"/>)
    </t>
    <t>
      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/16"/>:
      "Remove 'identity' token references"
      (<eref target="http://purl.org/NET/http-errata#identity"/>)
    </t>
    <t>
      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/26"/>:
      "Import query BNF"
    </t>
    <t>
      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/31"/>:
      "qdtext BNF"
    </t>
    <t>
      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/35"/>:
      "Normative and Informative references"
    </t>
    <t>
      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/42"/>:
      "RFC2606 Compliance"
    </t>
    <t>
      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/45"/>:
      "RFC977 reference"
    </t>
    <t>
      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/46"/>:
      "RFC1700 references"
    </t>
    <t>
      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/47"/>:
      "inconsistency in date format explanation"
    </t>
    <t>
      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/48"/>:
      "Date reference typo"
    </t>
    <t>
      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/65"/>:
      "Informative references"
    </t>
    <t>
      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/66"/>:
      "ISO-8859-1 Reference"
    </t>
    <t>
      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/86"/>:
      "Normative up-to-date references"
    </t>
  </list>
</t>
<t>
  Other changes:
  <list style="symbols"> 
    <t>
      Update media type registrations to use RFC4288 template.
    </t>
    <t>
      Use names of RFC4234 core rules DQUOTE and WSP,
      fix broken ABNF for chunk-data
      (work in progress on <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/36"/>)
    </t>
  </list>
</t>
</section>

<section title="Since draft-ietf-httpbis-p1-messaging-01">
<t>
  Closed issues:
  <list style="symbols"> 
    <t>
      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/19"/>:
      "Bodies on GET (and other) requests"
    </t>
    <t>
      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/55"/>:
      "Updating to RFC4288"
    </t>
    <t>
      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/57"/>:
      "Status Code and Reason Phrase"
    </t>
    <t>
      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/82"/>:
      "rel_path not used"
    </t>
  </list>
</t>
<t>
  Ongoing work on ABNF conversion (<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/36"/>):
  <list style="symbols"> 
    <t>
      Get rid of duplicate BNF rule names ("host" -&gt; "uri-host", "trailer" -&gt;
      "trailer-part").
    </t>
    <t>
      Avoid underscore character in rule names ("http_URL" -&gt;
      "http-URL", "abs_path" -&gt; "path-absolute").
    </t>
    <t>
      Add rules for terms imported from URI spec ("absoluteURI", "authority",
      "path-absolute", "port", "query", "relativeURI", "host) — these will
      have to be updated when switching over to RFC3986.
    </t>
    <t>
      Synchronize core rules with RFC5234.
    </t>
    <t>
      Get rid of prose rules that span multiple lines.
    </t>
    <t>
      Get rid of unused rules LOALPHA and UPALPHA.
    </t>
    <t>
      Move "Product Tokens" section (back) into Part 1, as "token" is used
      in the definition of the Upgrade header field.
    </t>
    <t>
      Add explicit references to BNF syntax and rules imported from other parts of the specification.
    </t>
    <t>
      Rewrite prose rule "token" in terms of "tchar", rewrite prose rule "TEXT".
    </t>
  </list>
</t>
</section>

<section title="Since draft-ietf-httpbis-p1-messaging-02" anchor="changes.since.02">
<t>
  Closed issues:
  <list style="symbols"> 
    <t>
      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/51"/>:
      "HTTP-date vs. rfc1123-date"
    </t>
    <t>
      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/64"/>:
      "WS in quoted-pair"
    </t>
  </list>
</t>
<t>
  Ongoing work on IANA Message Header Field Registration (<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/40"/>):
  <list style="symbols"> 
    <t>
      Reference RFC 3984, and update header field registrations for headers defined
      in this document.
    </t>
  </list>
</t>
<t>
  Ongoing work on ABNF conversion (<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/36"/>):
  <list style="symbols"> 
    <t>
      Replace string literals when the string really is case-sensitive (HTTP-Version).
    </t>
  </list>
</t>
</section>

<section title="Since draft-ietf-httpbis-p1-messaging-03" anchor="changes.since.03">
<t>
  Closed issues:
  <list style="symbols"> 
    <t>
      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/28"/>:
      "Connection closing"
    </t>
    <t>
      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/97"/>:
      "Move registrations and registry information to IANA Considerations"
    </t>
    <t>
      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/120"/>:
      "need new URL for PAD1995 reference"
    </t>
    <t>
      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/127"/>:
      "IANA Considerations: update HTTP URI scheme registration"
    </t>
    <t>
      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/128"/>:
      "Cite HTTPS URI scheme definition"
    </t>
    <t>
      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/129"/>:
      "List-type headers vs Set-Cookie"
    </t>
  </list>
</t>
<t>
  Ongoing work on ABNF conversion (<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/36"/>):
  <list style="symbols"> 
    <t>
      Replace string literals when the string really is case-sensitive (HTTP-Date).
    </t>
    <t>
      Replace HEX by HEXDIG for future consistence with RFC 5234's core rules. 
    </t>
  </list>
</t>
</section>

<section title="Since draft-ietf-httpbis-p1-messaging-04" anchor="changes.since.04">
<t>
  Closed issues:
  <list style="symbols"> 
    <t>
      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/34"/>:
      "Out-of-date reference for URIs"
    </t>
    <t>
      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/132"/>:
      "RFC 2822 is updated by RFC 5322"
    </t>
  </list>
</t>
<t>
  Ongoing work on ABNF conversion (<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/36"/>):
  <list style="symbols"> 
    <t>
      Use "/" instead of "|" for alternatives.
    </t>
    <t>
      Get rid of RFC822 dependency; use RFC5234 plus extensions instead.
    </t>
    <t>
      Only reference RFC 5234's core rules.
    </t>
    <t>
      Introduce new ABNF rules for "bad" whitespace ("BWS"), optional
      whitespace ("OWS") and required whitespace ("RWS").
    </t>
    <t>
      Rewrite ABNFs to spell out whitespace rules, factor out
      header field value format definitions.
    </t>
  </list>
</t>
</section>

<section title="Since draft-ietf-httpbis-p1-messaging-05" anchor="changes.since.05">
<t>
  Closed issues:
  <list style="symbols"> 
    <t>
      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/30"/>:
      "Header LWS"
    </t>
    <t>
      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/52"/>:
      "Sort 1.3 Terminology"
    </t>
    <t>
      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/63"/>:
      "RFC2047 encoded words"
    </t>
    <t>
      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/74"/>:
      "Character Encodings in TEXT"
    </t>
    <t>
      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/77"/>:
      "Line Folding"
    </t>
    <t>
      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/83"/>:
      "OPTIONS * and proxies"
    </t>
    <t>
      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/94"/>:
      "Reason-Phrase BNF"
    </t>
    <t>
      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/111"/>:
      "Use of TEXT"
    </t>
    <t>
      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/118"/>:
      "Join "Differences Between HTTP Entities and RFC 2045 Entities"?"
    </t>
    <t>
      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/134"/>:
      "RFC822 reference left in discussion of date formats"
    </t>
  </list>
</t>
<t>
  Final work on ABNF conversion (<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/36"/>):
  <list style="symbols"> 
    <t>
      Rewrite definition of list rules, deprecate empty list elements.
    </t>
    <t>
      Add appendix containing collected and expanded ABNF.
    </t>
  </list>
</t>
<t>
  Other changes:
  <list style="symbols"> 
    <t>
      Rewrite introduction; add mostly new Architecture Section.
    </t>
    <t>
      Move definition of quality values from Part 3 into Part 1;
      make TE request header field grammar independent of accept-params (defined in Part 3).
    </t>
  </list>
</t>
</section>

<section title="Since draft-ietf-httpbis-p1-messaging-06" anchor="changes.since.06">
<t>
  Closed issues:
  <list style="symbols"> 
    <t>
      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/161"/>:
      "base for numeric protocol elements"
    </t>
    <t>
      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/162"/>:
      "comment ABNF"
    </t>
  </list>
</t>
<t>
  Partly resolved issues:
  <list style="symbols"> 
    <t>
      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/88"/>:
      "205 Bodies" (took out language that implied that there might be
      methods for which a request body MUST NOT be included)
    </t>
    <t>
      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/163"/>:
      "editorial improvements around HTTP-date"
    </t>
  </list>
</t>
</section>

<section title="Since draft-ietf-httpbis-p1-messaging-07" anchor="changes.since.07">
<t>
  Closed issues:
  <list style="symbols"> 
    <t>
      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/93"/>:
      "Repeating single-value headers"
    </t>
    <t>
      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/131"/>:
      "increase connection limit"
    </t>
    <t>
      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/157"/>:
      "IP addresses in URLs"
    </t>
    <t>
      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/172"/>:
      "take over HTTP Upgrade Token Registry"
    </t>
    <t>
      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/173"/>:
      "CR and LF in chunk extension values"
    </t>
    <t>
      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/184"/>:
      "HTTP/0.9 support"
    </t>
    <t>
      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/188"/>:
      "pick IANA policy (RFC5226) for Transfer Coding / Content Coding"
    </t>
    <t>
      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/189"/>:
      "move definitions of gzip/deflate/compress to part 1"
    </t>
    <t>
      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/194"/>:
      "disallow control characters in quoted-pair"
    </t>
  </list>
</t>
<t>
  Partly resolved issues:
  <list style="symbols"> 
    <t>
      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/148"/>:
      "update IANA requirements wrt Transfer-Coding values" (add the
      IANA Considerations subsection)
    </t>
  </list>
</t>
</section>

<section title="Since draft-ietf-httpbis-p1-messaging-08" anchor="changes.since.08">
<t>
  Closed issues:
  <list style="symbols"> 
    <t>
      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/201"/>:
      "header parsing, treatment of leading and trailing OWS"
    </t>
  </list>
</t>
<t>
  Partly resolved issues:
  <list style="symbols"> 
    <t>
      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/60"/>:
      "Placement of 13.5.1 and 13.5.2"
    </t>
    <t>
      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/200"/>:
      "use of term "word" when talking about header structure"
    </t>
  </list>
</t>
</section>

<section title="Since draft-ietf-httpbis-p1-messaging-09" anchor="changes.since.09">
<t>
  Closed issues:
  <list style="symbols"> 
    <t>
      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/73"/>:
      "Clarification of the term 'deflate'"
    </t>
    <t>
      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/83"/>:
      "OPTIONS * and proxies"
    </t>
    <t>
      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/122"/>:
      "MIME-Version not listed in P1, general header fields"
    </t>
    <t>
      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/143"/>:
      "IANA registry for content/transfer encodings"
    </t>
    <t>
      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/165"/>:
      "Case-sensitivity of HTTP-date"
    </t>
    <t>
      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/200"/>:
      "use of term "word" when talking about header structure"
    </t>
  </list>
</t>
<t>
  Partly resolved issues:
  <list style="symbols"> 
    <t>
      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/196"/>:
      "Term for the requested resource's URI"
    </t>
  </list>
</t>
</section>

<section title="Since draft-ietf-httpbis-p1-messaging-10" anchor="changes.since.10">
<t>
  Closed issues:
  <list style="symbols">
    <t>
      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/28"/>:
      "Connection Closing"
    </t>
    <t>
      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/90"/>:
      "Delimiting messages with multipart/byteranges"
    </t>
    <t>
      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/95"/>:
      "Handling multiple Content-Length headers"
    </t>
    <t>
      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/109"/>:
      "Clarify entity / representation / variant terminology"
    </t>
    <t>
      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/220"/>:
      "consider removing the 'changes from 2068' sections"
    </t>
  </list>
</t>
<t>
  Partly resolved issues:
  <list style="symbols"> 
    <t>
      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/159"/>:
      "HTTP(s) URI scheme definitions"
    </t>
  </list>
</t>
</section>

<section title="Since draft-ietf-httpbis-p1-messaging-11" anchor="changes.since.11">
<t>
  Closed issues:
  <list style="symbols">
    <t>
      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/193"/>:
      "Trailer requirements"
    </t>
    <t>
      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/204"/>:
      "Text about clock requirement for caches belongs in p6"
    </t>
    <t>
      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/221"/>:
      "effective request URI: handling of missing host in HTTP/1.0"
    </t>
    <t>
      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/248"/>:
      "confusing Date requirements for clients"
    </t>
  </list>
</t>
<t>
  Partly resolved issues:
  <list style="symbols"> 
    <t>
      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/95"/>:
      "Handling multiple Content-Length headers"
    </t>
  </list>
</t>
</section>

<section title="Since draft-ietf-httpbis-p1-messaging-12" anchor="changes.since.12">
<t>
  Closed issues:
  <list style="symbols">
    <t>
      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/75"/>:
      "RFC2145 Normative"
    </t>
    <t>
      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/159"/>:
      "HTTP(s) URI scheme definitions" (tune the requirements on userinfo)
    </t>
    <t>
      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/210"/>:
      "define 'transparent' proxy"
    </t>
    <t>
      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/224"/>:
      "Header Classification"
    </t>
    <t>
      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/233"/>:
      "Is * usable as a request-uri for new methods?"
    </t>
    <t>
      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/240"/>:
      "Migrate Upgrade details from RFC2817"
    </t>
    <t>
      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/276"/>:
      "untangle ABNFs for header fields"
    </t>
    <t>
      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/279"/>:
      "update RFC 2109 reference"
    </t>
  </list>
</t>
</section>

<section title="Since draft-ietf-httpbis-p1-messaging-13" anchor="changes.since.13">
<t>
  Closed issues:
  <list style="symbols">
    <t>
      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/53"/>:
      "Allow is not in 13.5.2"
    </t>
    <t>
      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/95"/>:
      "Handling multiple Content-Length headers"
    </t>
    <t>
      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/276"/>:
      "untangle ABNFs for header fields"
    </t>
    <t>
      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/286"/>:
      "Content-Length ABNF broken"
    </t>
  </list>
</t>
</section>

<section title="Since draft-ietf-httpbis-p1-messaging-14" anchor="changes.since.14">
<t>
  Closed issues:
  <list style="symbols">
    <t>
      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/273"/>:
      "HTTP-Version should be redefined as fixed length pair of DIGIT . DIGIT"
    </t>
    <t>
      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/282"/>:
      "Recommend minimum sizes for protocol elements"
    </t>
    <t>
      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/283"/>:
      "Set expectations around buffering"
    </t>
    <t>
      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/288"/>:
      "Considering messages in isolation"
    </t>
  </list>
</t>
</section>

<section title="Since draft-ietf-httpbis-p1-messaging-15" anchor="changes.since.15">
<t>
  Closed issues:
  <list style="symbols">
    <t>
      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/100"/>:
      "DNS Spoofing / DNS Binding advice"
    </t>
    <t>
      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/254"/>:
      "move RFCs 2145, 2616, 2817 to Historic status"
    </t>
    <t>
      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/270"/>:
      "\-escaping in quoted strings"
    </t>
    <t>
      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/305"/>:
      "'Close' should be reserved in the HTTP header field registry"
    </t>
  </list>
</t>
</section>

</section>

</back>
</rfc>