HTTP Working Group                                      R. Fielding, Ed.
Internet-Draft                                                     Adobe
Obsoletes: 7230 (if approved)                         M. Nottingham, Ed.
Intended status: Standards Track                                  Fastly
Expires: October 5, December 2, 2018                                J. Reschke, Ed.
                                                              greenbytes
                                                           April 3,
                                                            May 31, 2018

   Hypertext Transfer Protocol (HTTP/1.1): Message Syntax and Routing
                    draft-ietf-httpbis-messaging-00

                           HTTP/1.1 Messaging
                    draft-ietf-httpbis-messaging-01

Abstract

   The Hypertext Transfer Protocol (HTTP) is a stateless application-
   level protocol for distributed, collaborative, hypertext information
   systems.  This document provides an overview of HTTP architecture and
   its associated terminology, defines the "http" and "https" Uniform
   Resource Identifier (URI) schemes, defines specifies the HTTP/1.1 message
   syntax and parsing requirements, syntax,
   message parsing, connection management, and describes related security
   concerns for implementations.
   concerns.

   This document obsoletes portions of RFC 7230.

Editorial Note

   This note is to be removed before publishing as an RFC.

   Discussion of this draft takes place on the HTTP working group
   mailing list (ietf-http-wg@w3.org), which is archived at
   <http://lists.w3.org/Archives/Public/ietf-http-wg/>.
   <https://lists.w3.org/Archives/Public/ietf-http-wg/>.

   Working Group information can be found at <http://httpwg.github.io/>; <https://httpwg.org/>;
   source code and issues list for this draft can be found at
   <https://github.com/httpwg/http-core>.

   The changes in this draft are summarized in Appendix C.1. D.2.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
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   Drafts is at https://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
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   This Internet-Draft will expire on October 5, December 2, 2018.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   5   4
     1.1.  Requirements Notation . . . . . . . . . . . . . . . . . .   6   5
     1.2.  Syntax Notation . . . . . . . . . . . . . . . . . . . . .   6   5
   2.  Architecture  Message . . . . . . . . . . . . . . . . . . . . . . . . . . .   6
     2.1.  Client/Server Messaging  Message Format  . . . . . . . . . . . . . . . . .   7
     2.2.  Implementation Diversity . . . .   6
     2.2.  HTTP Version  . . . . . . . . . . . .   8
     2.3.  Intermediaries . . . . . . . . . .   6
     2.3.  Message Parsing . . . . . . . . . . .   9
     2.4.  Caches . . . . . . . . . .   7
   3.  Request Line  . . . . . . . . . . . . . . .  11
     2.5.  Conformance and Error Handling . . . . . . . . .   8
     3.1.  Method  . . . .  12
     2.6.  Protocol Versioning . . . . . . . . . . . . . . . . . . .  13
     2.7.  Uniform Resource Identifiers . .   9
     3.2.  Request Target  . . . . . . . . . . . .  16
       2.7.1.  http URI Scheme . . . . . . . . .   9
       3.2.1.  origin-form . . . . . . . . . .  16
       2.7.2.  https URI Scheme . . . . . . . . . . .  10
       3.2.2.  absolute-form . . . . . . .  18
       2.7.3.  http and https URI Normalization and Comparison . . .  19
   3.  Message Format . . . . . . . . . .  10
       3.2.3.  authority-form  . . . . . . . . . . . . .  19
     3.1.  Start Line . . . . . .  11
       3.2.4.  asterisk-form . . . . . . . . . . . . . . . . . .  20
       3.1.1.  Request Line . .  11

     3.3.  Effective Request URI . . . . . . . . . . . . . . . . . .  21
       3.1.2.  12
   4.  Status Line . . . . . . . . . . . . . . . . . . . . .  22
     3.2.  Header Fields . . . .  13
   5.  Header Fields . . . . . . . . . . . . . . . . . .  22
       3.2.1.  Field Extensibility . . . . . .  14
     5.1.  Field Parsing . . . . . . . . . . . . .  23
       3.2.2.  Field Order . . . . . . . . .  15
     5.2.  Obsolete Line Folding . . . . . . . . . . . .  23
       3.2.3.  Whitespace . . . . . .  15
   6.  Message Body  . . . . . . . . . . . . . . .  24
       3.2.4.  Field Parsing . . . . . . . . .  16
     6.1.  Transfer-Encoding . . . . . . . . . . .  24
       3.2.5.  Field Limits . . . . . . . . .  17
     6.2.  Content-Length  . . . . . . . . . . .  26
       3.2.6.  Field Value Components . . . . . . . . . .  18
     6.3.  Message Body Length . . . . .  26
     3.3.  Message Body . . . . . . . . . . . . . .  19
   7.  Transfer Codings  . . . . . . . .  27
       3.3.1.  Transfer-Encoding . . . . . . . . . . . . . .  21
     7.1.  Chunked Transfer Coding . . . .  28
       3.3.2.  Content-Length . . . . . . . . . . . . .  22
       7.1.1.  Chunk Extensions  . . . . . .  29
       3.3.3.  Message Body Length . . . . . . . . . . . .  22
       7.1.2.  Chunked Trailer Part  . . . . .  31
     3.4.  Handling Incomplete Messages . . . . . . . . . . .  23
       7.1.3.  Decoding Chunked  . . .  33
     3.5.  Message Parsing Robustness . . . . . . . . . . . . . . .  34
   4.  24
     7.2.  Transfer Codings for Compression  . . . . . . . . . . . .  24
     7.3.  Transfer Coding Registry  . . . . . . . . . .  34
     4.1.  Chunked Transfer Coding . . . . . .  25
     7.4.  TE  . . . . . . . . . . .  35
       4.1.1.  Chunk Extensions . . . . . . . . . . . . . . . .  25
   8.  Handling Incomplete Messages  . .  36
       4.1.2.  Chunked Trailer Part . . . . . . . . . . . . . .  26
   9.  Connection Management . .  36
       4.1.3.  Decoding Chunked . . . . . . . . . . . . . . . . . .  37
     4.2.  Compression Codings  27
     9.1.  Connection  . . . . . . . . . . . . . . . . . . .  37
       4.2.1.  Compress Coding . . . .  27
     9.2.  Establishment . . . . . . . . . . . . . . .  38
       4.2.2.  Deflate Coding  . . . . . . . .  29
     9.3.  Persistence . . . . . . . . . . .  38
       4.2.3.  Gzip Coding . . . . . . . . . . . .  29
       9.3.1.  Retrying Requests . . . . . . . . .  38
     4.3.  TE . . . . . . . . .  30
       9.3.2.  Pipelining  . . . . . . . . . . . . . . . . . .  38
     4.4.  Trailer . . .  31
     9.4.  Concurrency . . . . . . . . . . . . . . . . . . . . . .  39
   5.  Message Routing .  31
     9.5.  Failures and Timeouts . . . . . . . . . . . . . . . . . .  32
     9.6.  Tear-down . . . .  39
     5.1.  Identifying a Target Resource . . . . . . . . . . . . . .  40
     5.2.  Connecting Inbound . . . . . .  33
     9.7.  Upgrade . . . . . . . . . . . . .  40
     5.3.  Request Target . . . . . . . . . . . .  34
       9.7.1.  Upgrade Protocol Names  . . . . . . . . .  41
       5.3.1.  origin-form . . . . . .  36
       9.7.2.  Upgrade Token Registry  . . . . . . . . . . . . . . .  41
       5.3.2.  absolute-form  36
   10. Enclosing Messages as Data  . . . . . . . . . . . . . . . . .  37
     10.1.  Media Type message/http  . . .  41
       5.3.3.  authority-form . . . . . . . . . . . . .  37
     10.2.  Media Type application/http  . . . . . .  42
       5.3.4.  asterisk-form . . . . . . . .  38
   11. Security Considerations . . . . . . . . . . . .  42
     5.4.  Host . . . . . . .  39
     11.1.  Response Splitting . . . . . . . . . . . . . . . . . . .  43
     5.5.  Effective  39
     11.2.  Request URI Smuggling  . . . . . . . . . . . . . . . . . .  44
     5.6.  Associating a Response to a Request .  40
     11.3.  Message Integrity  . . . . . . . . . .  46
     5.7.  Message Forwarding . . . . . . . . .  40
     11.4.  Message Confidentiality  . . . . . . . . . .  46
       5.7.1.  Via . . . . . .  41
   12. IANA Considerations . . . . . . . . . . . . . . . . . . .  46
       5.7.2.  Transformations . .  41
     12.1.  Header Field Registration  . . . . . . . . . . . . . . .  41
     12.2.  Media Type Registration  . .  48
   6.  Connection Management . . . . . . . . . . . . . .  42
     12.3.  Transfer Coding Registration . . . . . .  49
     6.1.  Connection . . . . . . . .  42
     12.4.  Upgrade Token Registration . . . . . . . . . . . . . . .  50
     6.2.  Establishment  42
   13. References  . . . . . . . . . . . . . . . . . . . . . .  51
     6.3.  Persistence . . .  42
     13.1.  Normative References . . . . . . . . . . . . . . . . . .  42
     13.2.  Informative References . .  51
       6.3.1.  Retrying Requests . . . . . . . . . . . . . . .  43
   Appendix A.  Collected ABNF . . .  52
       6.3.2.  Pipelining . . . . . . . . . . . . . . . .  45
   Appendix B.  Differences between HTTP and MIME  . . . . .  53
     6.4.  Concurrency . . . .  46
     B.1.  MIME-Version  . . . . . . . . . . . . . . . . . . .  54
     6.5.  Failures and Timeouts . . .  47
     B.2.  Conversion to Canonical Form  . . . . . . . . . . . . . .  47
     B.3.  Conversion of Date Formats  .  54
     6.6.  Tear-down . . . . . . . . . . . . . .  47
     B.4.  Conversion of Content-Encoding  . . . . . . . . . .  55
     6.7.  Upgrade . . .  48
     B.5.  Conversion of Content-Transfer-Encoding . . . . . . . . .  48
     B.6.  MHTML and Line Length Limitations . . . . . . . . . . . .  48
   Appendix C.  HTTP Version History .  56
   7.  ABNF List Extension: #rule . . . . . . . . . . . . . . .  48
     C.1.  Changes from HTTP/1.0 . .  58
   8.  IANA Considerations . . . . . . . . . . . . . . . .  49
       C.1.1.  Multihomed Web Servers  . . . . .  59
     8.1.  Header Field Registration . . . . . . . . . .  49
       C.1.2.  Keep-Alive Connections  . . . . . .  59
     8.2.  URI Scheme Registration . . . . . . . . .  50
       C.1.3.  Introduction of Transfer-Encoding . . . . . . . .  60
     8.3.  Internet Media Type Registration . .  50
     C.2.  Changes from RFC 7230 . . . . . . . . . .  60
       8.3.1.  Internet Media Type message/http . . . . . . . .  50
   Appendix D.  Change Log . .  61
       8.3.2.  Internet Media Type application/http . . . . . . . .  62
     8.4.  Transfer Coding Registry . . . . . . . . . . .  51
     D.1.  Between RFC7230 and draft 00  . . . . .  63
       8.4.1.  Procedure . . . . . . . . .  51
     D.2.  Since draft-ietf-httpbis-messaging-00 . . . . . . . . . .  51
   Index . . .  63
       8.4.2.  Registration . . . . . . . . . . . . . . . . . . . .  64
     8.5.  Content Coding Registration . . . . . . .  51
   Acknowledgments . . . . . . . .  64
     8.6.  Upgrade Token Registry . . . . . . . . . . . . . . . . .  65
       8.6.1.  Procedure  54
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . .  65
       8.6.2.  Upgrade Token Registration  . . . . . . . . . . . . .  65
   9.  Security Considerations . . . . . . . . . . . . . . . . . . .  66
     9.1.  Establishing Authority  . . . . . . . . . . . . . . . . .  66
     9.2.  Risks of Intermediaries . . . . . . . . . . . . . . . . .  67
     9.3.  Attacks via  54

1.  Introduction

   The Hypertext Transfer Protocol Element Length . . . . . . . . . . .  67
     9.4.  Response Splitting  . . . . . . . . . . . . . . . . . . .  68
     9.5.  Request Smuggling . . . . . . . . . . . . . . . . . . . .  69
     9.6.  Message Integrity . . . . . . . . . . . . . . . . . . . .  69
     9.7.  Message Confidentiality . . . . . . . . . . . . . . . . .  70
     9.8.  Privacy (HTTP) is a stateless application-
   level request/response protocol that uses extensible semantics and
   self-descriptive messages for flexible interaction with network-based
   hypertext information systems.  HTTP is defined by a series of Server Log Information . . . . . . . . . . . .  70
   10. References  . . . . . . . . . . . . . . . . . . . . . . . . .  70
     10.1.  Normative References . . . . . . . . . . . . . . . . . .  70
     10.2.  Informative References . . . . . . . . . . . . . . . . .  72
   Appendix A.  HTTP Version History . . . . . . . . . . . . . . . .  75
     A.1.  Changes from HTTP/1.0 . . . . . . . . . . . . . . . . . .  75
       A.1.1.  Multihomed Web Servers  . . . . . . . . . . . . . . .  75
       A.1.2.  Keep-Alive Connections  . . . . . . . . . . . . . . .  76
       A.1.3.  Introduction
   documents that collectively form the HTTP/1.1 specification:

   o  "HTTP Semantics" [Semantics]

   o  "HTTP Caching" [Caching]

   o  "HTTP/1.1 Messaging" (this document)

   This document defines HTTP/1.1 message syntax and framing
   requirements and their associated connection management.  Our goal is
   to define all of the mechanisms necessary for HTTP/1.1 message
   handling that are independent of message semantics, thereby defining
   the complete set of requirements for message parsers and message-
   forwarding intermediaries.

   This document obsoletes the portions of Transfer-Encoding . . . . . . . . . .  76
     A.2.  Changes from RFC 7230 . . . . . . . . . . . . . . . . . .  77
   Appendix B.  Collected ABNF . . . . . . . . . . . . . . . . . . .  77 related to HTTP/1.1
   messaging and connection management, with the changes being
   summarized in Appendix C.  Change Log . . . . . . . . . . . . . . . . . . . . .  79
     C.1.  Since C.2.  The other parts of RFC 7230  . . . . . . . . . . . . . . . . . . . . .  79
   Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  80
   Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . .  84
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  84

1.  Introduction are
   obsoleted by "HTTP Semantics" [Semantics].

1.1.  Requirements Notation

   The Hypertext Transfer Protocol (HTTP) is a stateless application-
   level request/response protocol that uses extensible semantics key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and
   self-descriptive message payloads for flexible interaction with
   network-based hypertext information systems.  This "OPTIONAL" in this
   document is the
   first are to be interpreted as described in a series [RFC2119].

   Conformance criteria and considerations regarding error handling are
   defined in Section 3 of documents that collectively form the HTTP/1.1
   specification:

   1.  "Message [Semantics].

1.2.  Syntax and Routing" (this document)

   2.  "Semantics and Content" [SEMNTCS]

   3.  "Conditional Requests" [CONDTNL]

   4.  "Range Requests" [RANGERQ]

   5.  "Caching" [CACHING]

   6.  "Authentication" [AUTHFRM] Notation

   This specification obsoletes RFC 7230, with the changes being
   summarized in Appendix A.2.

   HTTP is a generic interface protocol for information systems.  It is
   designed to hide uses the details Augmented Backus-Naur Form (ABNF)
   notation of how [RFC5234] with 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 list extension, defined in
   isolation rather than being associated with a specific type of client
   or a predetermined sequence Section 11 of application steps.  The result is a
   protocol
   [Semantics], that can be used effectively in many different contexts and
   for which implementations can evolve independently over time.

   HTTP is also designed for use as an intermediation protocol allows for
   translating communication compact definition of comma-separated
   lists using a '#' operator (similar to and from non-HTTP information systems.
   HTTP proxies and gateways can provide access how the '*' operator indicates
   repetition).  Appendix A shows the collected grammar with all list
   operators expanded to alternative
   information services by translating their diverse protocols into standard ABNF notation.

   As a
   hypertext format convention, ABNF rule names prefixed with "obs-" denote
   "obsolete" grammar rules that can be viewed and manipulated appear for historical reasons.

   The following core rules are included by clients in the
   same way reference, as HTTP services.

   One consequence of this 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
   [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), HTAB (horizontal tab), LF (line
   feed), OCTET (any 8-bit sequence of
   received communication, data), SP (space), 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 VCHAR (any
   visible [USASCII] character).

   The rules below are defined in parallel and perhaps at cross-purposes, we cannot require that
   such changes be observable beyond the scope [Semantics]:

     BWS           = <BWS, see [Semantics], Section 4.3>
     OWS           = <OWS, see [Semantics], Section 4.3>
     RWS           = <RWS, see [Semantics], Section 4.3>
     absolute-URI  = <absolute-URI, see [RFC3986], Section 4.3>
     absolute-path = <absolute-path, see [Semantics], Section 2.4>
     authority     = <authority, see [RFC3986], Section 3.2>
     comment       = <comment, see [Semantics], Section 4.2.3>
     field-name    = <field-name, see [Semantics], Section 4.2>
     field-value   = <field-value, see [Semantics], Section 4.2>
     obs-text      = <obs-text, see [Semantics], Section 4.2.3>
     port          = <port, see [RFC3986], Section 3.2.3>
     query         = <query, see [RFC3986], Section 3.4>
     quoted-string = <quoted-string, see [Semantics], Section 4.2.3>
     token         = <token, see [Semantics], Section 4.2.3>
     uri-host      = <host, see [RFC3986], Section 3.2.2>

2.  Message

2.1.  Message Format

   All HTTP/1.1 messages consist of a single response.

   This document describes start-line followed by a sequence
   of octets in a format similar to the architectural elements that are used Internet Message Format
   [RFC5322]: zero or more header fields (collectively referred to in HTTP, defines as
   the "http" and "https" URI schemes,
   describes overall network operation and connection management, "headers" or the "header section"), an empty line indicating the
   end of the header section, and
   defines an optional message body.

     HTTP-message   = start-line
                      *( header-field CRLF )
                      CRLF
                      [ message-body ]

   An HTTP message framing and forwarding requirements.  Our goal
   is can be either a request from client to define all of server or a
   response from server to client.  Syntactically, the mechanisms necessary for HTTP message
   handling that are independent two types of
   message semantics, thereby defining differ only in the complete set of requirements 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 parsers and message-
   forwarding intermediaries.

1.1.  Requirements Notation

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", body (Section 6).

     start-line     = request-line / status-line

   In theory, a client could receive requests and "OPTIONAL" in this
   document a server could receive
   responses, distinguishing them by their different start-line formats.
   In practice, servers are implemented to be only expect a request (a
   response is interpreted as described in [RFC2119].

   Conformance criteria an unknown or invalid request method) and considerations regarding error handling
   clients are
   defined in Section 2.5.

1.2.  Syntax Notation

   This specification uses the Augmented Backus-Naur Form (ABNF)
   notation of [RFC5234] with implemented to only expect a list extension, defined in Section 7,
   that allows for compact definition response.

   [[CREF1: Although HTTP makes use of comma-separated lists using a
   '#' operator (similar some protocol elements similar to how
   the '*' operator indicates repetition). Multipurpose Internet Mail Extensions (MIME) [RFC2045], see
   Appendix B shows the collected grammar with all list operators
   expanded to standard ABNF notation.

   The following core rules are included by reference, as defined in
   [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), HTAB (horizontal tab), LF (line
   feed), OCTET (any 8-bit sequence of data), SP (space), and VCHAR (any
   visible [USASCII] character).

   As a convention, ABNF rule names prefixed with "obs-" denote
   "obsolete" grammar rules that appear for historical reasons.

2.  Architecture

   HTTP was created for the World Wide Web (WWW) architecture differences between HTTP and has
   evolved over time MIME messages.]]

2.2.  HTTP Version

   HTTP uses a "<major>.<minor>" numbering scheme to support indicate versions
   of the scalability needs protocol.  This specification defines version "1.1".
   Section 3.5 of a worldwide
   hypertext system.  Much [Semantics] specifies the semantics of that architecture HTTP version
   numbers.

   The version of an HTTP/1.x message is reflected indicated by an HTTP-version
   field in the
   terminology and syntax productions used to define HTTP.

2.1.  Client/Server Messaging

   HTTP start-line.  HTTP-version is a stateless request/response protocol that operates by
   exchanging messages (Section 3) across a reliable transport- or
   session-layer "connection" (Section 6).  An HTTP "client" case-sensitive.

     HTTP-version  = HTTP-name "/" DIGIT "." DIGIT
     HTTP-name     = %x48.54.54.50 ; "HTTP", case-sensitive

   When an HTTP/1.1 message is a
   program that establishes a connection sent to a server for the purpose of
   sending one an HTTP/1.0 recipient [RFC1945]
   or more HTTP requests.  An HTTP "server" is a program
   that accepts connections in order to service HTTP requests by sending
   HTTP responses.

   The terms "client" and "server" refer only to recipient whose version is unknown, the roles HTTP/1.1 message is
   constructed such that these
   programs perform for a particular connection.  The same program might
   act it can be interpreted as a client valid HTTP/1.0
   message if all of the newer features are ignored.  This specification
   places recipient-version requirements on some connections and new features so that a server on others.  The term
   "user agent" refers to any
   conformant sender will only use compatible features until it has
   determined, through configuration or the receipt of a message, that
   the various client programs recipient supports HTTP/1.1.

   Intermediaries that
   initiate a request, including (but process HTTP messages (i.e., all intermediaries
   other than those acting as tunnels) MUST send their own HTTP-version
   in forwarded messages.  In other words, they are not limited to) browsers, spiders
   (web-based robots), command-line tools, custom applications, and
   mobile apps.  The term "origin server" refers allowed to
   blindly forward the program that can
   originate authoritative responses for a given target resource.  The
   terms "sender" and "recipient" refer to any implementation start-line without ensuring that sends
   or receives a given message, respectively.

   HTTP relies upon the Uniform Resource Identifier (URI) standard
   [RFC3986] to indicate the target resource (Section 5.1) and
   relationships between resources.  Messages are passed protocol
   version in that message matches a format
   similar version to which that used by Internet mail [RFC5322] and the Multipurpose
   Internet Mail Extensions (MIME) [RFC2045] (see Appendix A of
   [SEMNTCS] intermediary
   is conformant for both the differences between HTTP receiving and MIME messages).

   Most HTTP communication consists of a retrieval request (GET) for a
   representation sending of some resource identified by a URI.  In messages.
   Forwarding an HTTP message without rewriting the simplest
   case, this HTTP-version might be accomplished via a single bidirectional
   connection (===) between the user agent (UA) and
   result in communication errors when downstream recipients use the origin server
   (O).

            request   >
       UA ======================================= O
                                   <   response
   message sender's version to determine what features are safe to use
   for later communication with that sender.

   A client sends server MAY send an HTTP request HTTP/1.0 response to a server in the form of a an HTTP/1.1 request
   message, beginning with a request-line if it
   is known or suspected that includes a method, URI, the client incorrectly implements the HTTP
   specification and protocol is incapable of correctly processing later version (Section 3.1.1), followed by header fields
   containing request modifiers,
   responses, such as when a client information, and representation
   metadata (Section 3.2), fails to parse the version number
   correctly or when an empty line intermediary is known to indicate blindly forward the end of
   HTTP-version even when it doesn't conform to the
   header section, and finally a message body containing given minor version
   of the payload
   body (if any, Section 3.3).

   A server responds to a client's request protocol.  Such protocol downgrades SHOULD NOT be performed
   unless triggered by sending specific client attributes, such as when one or
   more HTTP
   response messages, each beginning with a status line that includes of the protocol version, a success or error code, and textual reason
   phrase (Section 3.1.2), possibly followed by request header fields containing
   server information, resource metadata, and representation metadata
   (Section 3.2), (e.g., User-Agent) uniquely match
   the values sent by a client known to be in error.

2.3.  Message Parsing

   The normal procedure for parsing an empty line HTTP message is to indicate the end of read the
   start-line into a structure, read each header
   section, field into a hash table
   by field name until the empty line, and finally then use the parsed data to
   determine if a message body containing the payload body (if
   any, Section 3.3).

   A connection might be used for multiple request/response exchanges,
   as defined in Section 6.3.

   The following example illustrates is expected.  If a typical message exchange for a
   GET request (Section 4.3.1 of [SEMNTCS]) on the URI
   "http://www.example.com/hello.txt":

   Client request:

     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-Language: en, mi

   Server response:

     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: 51
     Vary: Accept-Encoding
     Content-Type: text/plain

     Hello World! My payload includes a trailing CRLF.

2.2.  Implementation Diversity

   When considering the design of HTTP, body has been
   indicated, then it is easy to fall into read as a trap stream until an amount of thinking that all user agents are general-purpose browsers and all
   origin servers are large public websites.  That octets
   equal to the message body length is not read or the case in
   practice.  Common connection is closed.

   A recipient MUST parse an HTTP user agents include household appliances,
   stereos, scales, firmware update scripts, command-line programs,
   mobile apps, and communication devices in message as a multitude sequence of shapes and
   sizes.  Likewise, common HTTP origin servers include home automation
   units, configurable networking components, office machines,
   autonomous robots, news feeds, traffic cameras, ad selectors, and
   video-delivery platforms.

   The term "user agent" does not imply octets in an
   encoding that there is a human user
   directly interacting with the software agent at the time superset of US-ASCII [USASCII].  Parsing an HTTP
   message as a
   request.  In many cases, a user agent is installed or configured to
   run in the background and save its results for later inspection (or
   save only a subset stream of those results that might be interesting or
   erroneous).  Spiders, Unicode characters, without regard for example, are typically given a start URI
   and configured the
   specific encoding, creates security vulnerabilities due to follow certain behavior while crawling the Web as a
   hypertext graph.

   The implementation diversity of HTTP means
   varying ways that not all user agents string processing libraries handle invalid
   multibyte character sequences that contain the octet LF (%x0A).
   String-based parsers can make interactive suggestions to their user or provide adequate
   warning for security or privacy concerns.  In only be safely used within protocol elements
   after the few cases where
   this specification requires reporting of errors to element has been extracted from the user, it is
   acceptable for message, such reporting to only be observable in an error
   console or log file.  Likewise, requirements that an automated action
   be confirmed by as within
   a header field-value after message parsing has delineated the user before proceeding might be met via advance
   configuration choices, run-time options, or simple avoidance of
   individual fields.

   Although the
   unsafe action; confirmation does not imply any specific user
   interface or interruption of normal processing if line terminator for the user has
   already made that choice.

2.3.  Intermediaries

   HTTP enables start-line and header fields is
   the use of intermediaries to satisfy requests through sequence CRLF, a
   chain of connections.  There are three common forms of HTTP
   intermediary: proxy, gateway, and tunnel.  In some cases, recipient MAY recognize a single
   intermediary might act LF as an origin server, proxy, gateway, or
   tunnel, switching behavior based on the nature of each request.

            >             >             >             >
       UA =========== A =========== B =========== C =========== O
                  <             <             <             <

   The figure above shows three intermediaries (A, B, a line
   terminator and C) between the ignore any preceding CR.

   Older HTTP/1.0 user agent and origin server.  A implementations might send an extra CRLF
   after a POST request or response as a workaround for some early server
   applications that failed to read message body content that
   travels was not
   terminated by a line-ending.  An HTTP/1.1 user agent MUST NOT preface
   or follow a request with an extra CRLF.  If terminating the whole chain will pass through four separate connections.
   Some HTTP communication options might apply only to the connection request
   message body with a line-ending is desired, then the nearest, non-tunnel neighbor, only to user agent MUST
   count the endpoints terminating CRLF octets as part 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 message body length.

   In the same time interest of robustness, a server that it is handling A's
   request.  Likewise, later requests might be sent through a different
   path of connections, often based on dynamic configuration for load
   balancing.

   The terms "upstream" and "downstream" are used to describe
   directional requirements in relation to the message flow: all
   messages flow from upstream expecting to downstream.  The terms "inbound" receive
   and
   "outbound" are used to describe directional requirements in relation parse a request-line SHOULD ignore at least one empty line (CRLF)
   received prior to the request route: "inbound" means toward request-line.

   A sender MUST NOT send whitespace between the origin server start-line and
   "outbound" means toward the user agent.
   first header field.  A "proxy" is a message-forwarding agent recipient that is selected receives whitespace between the
   start-line and the first header field MUST either reject the message
   as invalid or consume each whitespace-preceded line without further
   processing of it (i.e., ignore the entire line, along with any
   subsequent lines preceded by whitespace, until a properly formed
   header field is received or the
   client, usually via local configuration rules, to receive requests
   for some type(s) header section is terminated).

   The presence of absolute URI and such whitespace in a request might be an attempt to satisfy those
   requests via translation through
   trick a server into ignoring that field or processing the HTTP interface.  Some
   translations are minimal, such line after
   it as for proxy requests for "http" URIs,
   whereas other requests a new request, either of which might require translation to and from entirely
   different application-level protocols.  Proxies are often used to
   group an organization's HTTP requests through result in a common intermediary
   for security
   vulnerability if other implementations within the sake request chain
   interpret the same message differently.  Likewise, the presence of security, annotation services, or shared caching.
   Some proxies are designed to apply transformations to selected
   messages or payloads while they are being forwarded, as described
   such whitespace in
   Section 5.7.2.

   A "gateway" (a.k.a. "reverse proxy") is an intermediary that acts as
   an origin server for the outbound connection but translates received
   requests and forwards them inbound to another server or servers.
   Gateways are often used to encapsulate legacy a response might be ignored by some clients or untrusted
   information services,
   cause others to improve cease parsing.

   When a server performance through
   "accelerator" caching, and to enable partitioning or load balancing
   of HTTP services across multiple machines.

   All listening only for HTTP requirements applicable to an origin server also apply to request messages, or processing
   what appears from the outbound communication of a gateway.  A gateway communicates with
   inbound servers using any protocol that it desires, including private
   extensions start-line to be an HTTP that are outside the scope request message,
   receives a sequence of this specification.
   However, an HTTP-to-HTTP gateway octets that wishes to interoperate with
   third-party HTTP servers ought to conform to user agent requirements
   on does not match the gateway's inbound connection.

   A "tunnel" acts as a blind relay between two connections without
   changing HTTP-message
   grammar aside from the messages.  Once active, robustness exceptions listed above, the server
   SHOULD respond with a tunnel is not considered 400 (Bad Request) response.

3.  Request Line

   A request-line begins with a
   party to method token, followed by a single space
   (SP), the HTTP communication, though request-target, another single space (SP), the tunnel might have been
   initiated by an HTTP request.  A tunnel ceases to exist when both protocol
   version, and ends with CRLF.

     request-line   = method SP request-target SP HTTP-version CRLF

   Although the request-line grammar rule requires that each of the relayed connection are closed.  Tunnels are used to
   extend a virtual connection through an intermediary, such as when
   Transport Layer Security (TLS, [RFC5246]) is used to establish
   confidential communication through
   component elements be separated by a shared firewall proxy.

   The above categories for intermediary only consider those acting as
   participants in the HTTP communication.  There are also
   intermediaries that can act single SP octet, recipients MAY
   instead parse on lower layers whitespace-delimited word boundaries and, aside from
   the CRLF terminator, treat any form of whitespace as the network protocol
   stack, filtering SP separator
   while ignoring preceding or redirecting HTTP traffic without the knowledge trailing whitespace; such whitespace
   includes one or
   permission more of message senders.  Network intermediaries are
   indistinguishable (at a protocol level) from a man-in-the-middle
   attack, often introducing security flaws or interoperability problems
   due to mistakenly violating HTTP semantics.

   For example, an "interception proxy" [RFC3040] (also commonly known
   as a "transparent proxy" [RFC1919] or "captive portal") differs from
   an HTTP proxy because it is not selected by the client.  Instead, an
   interception proxy filters following octets: SP, HTAB, VT (%x0B), FF
   (%x0C), or redirects outgoing TCP port 80 packets
   (and occasionally other common port traffic).  Interception proxies bare CR.  However, lenient parsing can result in request
   smuggling security vulnerabilities if there are commonly found on public network access points, as a means of
   enforcing account subscription prior to allowing use multiple recipients
   of non-local
   Internet services, the message and within corporate firewalls to enforce network
   usage policies. each has its own unique interpretation of
   robustness (see Section 11.2).

   HTTP is defined as does not place a stateless protocol, meaning that each request
   message can be understood in isolation.  Many implementations depend predefined limit on HTTP's stateless design in order to reuse proxied connections or
   dynamically load balance requests across multiple servers.  Hence, the length of a request-
   line, as described in Section 3 of [Semantics].  A server MUST NOT assume that two requests
   receives a method longer than any that it implements SHOULD respond
   with a 501 (Not Implemented) status code.  A server that receives a
   request-target longer than any URI it wishes to parse MUST respond
   with a 414 (URI Too Long) status code (see Section 9.5.15 of
   [Semantics]).

   Various ad hoc limitations on the same connection request-line length are
   from the same user agent unless the connection found in
   practice.  It is secured and
   specific to RECOMMENDED that agent.  Some non-standard all HTTP extensions (e.g.,
   [RFC4559]) have been known to violate this requirement, resulting in
   security senders and interoperability problems.

2.4.  Caches

   A "cache" is recipients
   support, at a local store minimum, request-line lengths of previous response messages and 8000 octets.

3.1.  Method

   The method token indicates the
   subsystem that controls its message storage, retrieval, and deletion.
   A cache stores cacheable responses in order request method 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.

   The effect of a cache is that performed on the request/response chain
   target resource.  The request method is shortened
   if one case-sensitive.

     method         = token

   The request methods defined by this specification can be found in
   Section 7 of the participants [Semantics], along with information regarding the chain has a cached response
   applicable HTTP
   method registry and considerations for defining new methods.

3.2.  Request Target

   The request-target identifies the target resource upon which to that apply
   the request.  The following illustrates the resulting
   chain if B has client derives a cached copy of an earlier response request-target from O (via C)
   for a request that has not been cached by UA or A.

               >             >
          UA =========== A =========== B - - - - - - C - - - - - - O
                     <             <

   A response is "cacheable" if a cache is allowed to store a copy of
   the response message its desired
   target URI.  There are four distinct formats 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 request-target,
   depending on when that cached
   response can be used for a particular request.  HTTP requirements for
   cache behavior both the method being requested and cacheable responses are defined in Section 2 of
   [CACHING].

   There whether the request
   is to a wide variety of architectures and configurations of caches
   deployed across proxy.

     request-target = origin-form
                    / absolute-form
                    / authority-form
                    / asterisk-form

   No whitespace is allowed in the World Wide Web and inside large organizations.
   These include national hierarchies of proxy caches request-target.  Unfortunately, some
   user agents fail to save
   transoceanic bandwidth, collaborative systems that broadcast properly encode or
   multicast cache entries, archives of pre-fetched cache entries for
   use exclude whitespace found in off-line or high-latency environments, and so on.

2.5.  Conformance and Error Handling

   This specification targets conformance criteria according to the role
   of a participant
   hypertext references, resulting in HTTP communication.  Hence, HTTP requirements are
   placed on senders, recipients, clients, servers, user agents,
   intermediaries, origin servers, proxies, gateways, or caches,
   depending on what behavior is those disallowed characters being constrained by the requirement.
   Additional (social) requirements are placed on implementations,
   resource owners, and protocol element registrations when they apply
   beyond
   sent as the scope of request-target in a single communication.

   The verb "generate" is used instead malformed request-line.

   Recipients of "send" where a requirement
   differentiates between creating an invalid request-line SHOULD respond with either a protocol element and merely
   forwarding
   400 (Bad Request) error or a received element downstream.

   An implementation is considered conformant if it complies with all of
   the requirements associated 301 (Moved Permanently) redirect with
   the roles it partakes in HTTP.

   Conformance includes both the syntax and semantics of protocol
   elements. request-target properly encoded.  A sender MUST recipient SHOULD NOT generate protocol elements that convey a
   meaning that is known by that sender attempt
   to autocorrect and then process the request without a redirect, since
   the invalid request-line might be false.  A sender MUST NOT
   generate protocol elements that do not match deliberately crafted to bypass
   security filters along the grammar defined by request chain.

3.2.1.  origin-form

   The most common form of request-target is the corresponding ABNF rules.  Within a given message, origin-form.

     origin-form    = absolute-path [ "?" query ]

   When making a sender MUST
   NOT generate protocol elements or syntax alternatives that are only
   allowed request directly to be generated by participants in an origin server, other roles (i.e., than a role
   that the sender does not have for that message).

   When
   CONNECT or server-wide OPTIONS request (as detailed below), a received protocol element is parsed, the recipient client
   MUST be
   able to parse any value send only the absolute path and query components of reasonable length that the target
   URI as the request-target.  If the target URI's path component is applicable to
   empty, the recipient's role and that matches client MUST send "/" as the grammar defined by path within the
   corresponding ABNF rules.  Note, however, that some received protocol
   elements might not be parsed. origin-form of
   request-target.  A Host header field is also sent, as defined in
   Section 5.4 of [Semantics].

   For example, an intermediary
   forwarding a message might parse client wishing to retrieve a header-field into generic field-
   name and field-value components, but then forward representation of the header field
   without further parsing inside
   resource identified as

     http://www.example.org/where?q=now

   directly from the field-value.

   HTTP does not have specific length limitations for many origin server would open (or reuse) a TCP
   connection to port 80 of its
   protocol elements because the lengths that might be appropriate will
   vary widely, depending on the deployment context host "www.example.org" and purpose of send the
   implementation.  Hence, interoperability between senders and
   recipients depends on shared expectations regarding what is a
   reasonable length for each protocol element.  Furthermore, what is
   commonly understood to be a reasonable length for some protocol
   elements has changed over
   lines:

     GET /where?q=now HTTP/1.1
     Host: www.example.org

   followed by the course remainder of the past two decades of HTTP
   use and is expected request message.

3.2.2.  absolute-form

   When making a request to continue changing in the future.

   At a minimum, proxy, other than a recipient CONNECT or server-wide
   OPTIONS request (as detailed below), a client MUST be able to parse and process protocol
   element lengths that are at least as long as send the values that it
   generates for those same protocol elements in other messages.  For
   example, an origin server that publishes very long target
   URI references to
   its own resources needs to be able to parse and process those same
   references when received in absolute-form as a the request-target.

     absolute-form  = absolute-URI

   The proxy is requested to either service that request target.

   A recipient MUST interpret from a received protocol element according to valid
   cache, if possible, or make the semantics defined for it by this specification, including
   extensions same request on the client's behalf
   to this specification, unless either the recipient has determined
   (through experience next inbound proxy server or configuration) that directly to the sender incorrectly
   implements what is implied by those semantics.  For example, an origin
   server might disregard indicated by the contents of a received Accept-
   Encoding header field if inspection request-target.  Requirements on such
   "forwarding" of the User-Agent header field
   indicates a specific implementation messages are defined in Section 5.6 of [Semantics].

   An example absolute-form of request-line would be:

     GET http://www.example.org/pub/WWW/TheProject.html HTTP/1.1

   To allow for transition to the absolute-form for all requests in some
   future version that is known of HTTP, a server MUST accept the absolute-form in
   requests, even though HTTP/1.1 clients will only send them in
   requests to fail on
   receipt proxies.

3.2.3.  authority-form

   The authority-form of certain content codings.

   Unless noted otherwise, request-target is only used for CONNECT
   requests (Section 7.3.6 of [Semantics]).

     authority-form = authority

   When making a recipient MAY attempt CONNECT request to recover establish a usable
   protocol element from an invalid construct.  HTTP does not define
   specific error handling mechanisms except when they have tunnel through one or
   more proxies, a direct
   impact on security, since different applications of client MUST send only the protocol
   require different error handling strategies. target URI's authority
   component (excluding any userinfo and its "@" delimiter) as the
   request-target.  For example,

     CONNECT www.example.com:80 HTTP/1.1

3.2.4.  asterisk-form

   The asterisk-form of request-target is only used for a Web
   browser might wish to transparently recover from server-wide
   OPTIONS request (Section 7.3.7 of [Semantics]).

     asterisk-form  = "*"

   When a response where the
   Location header field doesn't parse according client wishes to request OPTIONS for the ABNF, whereas server as a
   systems control client might consider any form of error recovery whole, as
   opposed to
   be dangerous.

2.6.  Protocol Versioning

   HTTP uses a "<major>.<minor>" numbering scheme to indicate versions specific named resource of that server, the protocol.  This specification defines version "1.1".  The
   protocol version client MUST
   send only "*" (%x2A) as a whole indicates the sender's conformance request-target.  For example,

     OPTIONS * HTTP/1.1

   If a proxy receives an OPTIONS request with
   the set an absolute-form of requirements laid out
   request-target in that version's corresponding
   specification of HTTP.

   The version of an HTTP message is indicated by which the URI has an HTTP-version field
   in empty path and no query
   component, then the first line of last proxy on the message.  HTTP-version is case-sensitive.

     HTTP-version  = HTTP-name "/" DIGIT "." DIGIT
     HTTP-name     = %x48.54.54.50 ; "HTTP", case-sensitive

   The HTTP version number consists of two decimal digits separated by request chain MUST send a
   "." (period or decimal point).  The first digit ("major version")
   indicates
   request-target of "*" when it forwards the HTTP messaging syntax, whereas request to the second digit ("minor
   version") indicates indicated
   origin server.

   For example, the highest minor version within that major
   version to which request

     OPTIONS http://www.example.org:8001 HTTP/1.1

   would be forwarded by the sender is conformant and able final proxy as

     OPTIONS * HTTP/1.1
     Host: www.example.org:8001

   after connecting to understand for
   future communication.  The minor version advertises the sender's
   communication capabilities even when port 8001 of host "www.example.org".

3.3.  Effective Request URI

   Since the sender is request-target often contains only using a
   backwards-compatible subset part of the protocol, thereby letting user agent's
   target URI, a server reconstructs the
   recipient know that more advanced features can be used in response
   (by servers) or in future requests (by clients).

   When intended target as an HTTP/1.1 message is sent effective
   request URI to an HTTP/1.0 recipient [RFC1945]
   or a recipient whose version properly service the request (Section 5.3 of
   [Semantics]).

   If the request-target is unknown, in absolute-form, the HTTP/1.1 message effective request URI
   is the same as the request-target.  Otherwise, the effective request
   URI is constructed such that it can be interpreted as follows:

      If the server's configuration (or outbound gateway) provides a valid HTTP/1.0
   message
      fixed URI scheme, that scheme is used for the effective request
      URI.  Otherwise, if all of the newer features are ignored.  This specification
   places recipient-version requirements on some new features so that request is received over a
   conformant sender will only use compatible features until it has
   determined, through configuration or TLS-secured TCP
      connection, the receipt of a message, that effective request URI's scheme is "https"; if not,
      the recipient supports HTTP/1.1.

   The interpretation of scheme is "http".

      If the server's configuration (or outbound gateway) provides a header field does not change between minor
   versions of
      fixed URI authority component, that authority is used for the same major HTTP version, though
      effective request URI.  If not, then if the default behavior
   of a recipient request-target is in
      authority-form, 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, effective request URI's authority component is
      the same as the request-target.  If not, then if a Host and Connection header fields ought to be implemented by all HTTP/1.x implementations
   whether or not they advertise conformance
      field is supplied with HTTP/1.1.

   New header fields can be introduced without changing a non-empty field-value, the protocol
   version if their defined semantics allow them to be safely ignored by
   recipients that do not recognize them.  Header field extensibility authority
      component is
   discussed in Section 3.2.1.

   Intermediaries that process HTTP messages (i.e., all intermediaries
   other than those acting the same as tunnels) MUST send their own HTTP-version
   in forwarded messages.  In other words, they are not allowed to
   blindly forward the first line of an HTTP message without ensuring
   that Host field-value.  Otherwise, the protocol version in that message matches a version to which
   that intermediary
      authority component is conformant assigned the default name configured for both
      the receiving and sending of
   messages.  Forwarding an HTTP message without rewriting server and, if the HTTP-
   version might result in communication errors when downstream
   recipients use connection's incoming TCP port number
      differs from the message sender's version to determine what
   features are safe to use default port for later communication with that sender.

   A client SHOULD send a the effective request version equal to URI's
      scheme, then a colon (":") and the highest version incoming port number (in
      decimal form) are appended to which the client authority component.

      If the request-target is conformant in authority-form or asterisk-form, the
      effective request URI's combined path and whose major version query component is no
   higher than the highest version supported by
      empty.  Otherwise, the server, if this is
   known.  A client MUST NOT send a version to which it is not
   conformant.

   A client MAY send a lower request version if it combined path and query component is known that the
   server incorrectly implements
      same as the HTTP specification, but only after request-target.

      The components of the client has attempted at least one normal effective request and URI, once determined
   from the response status code or header fields (e.g., Server) that
   the server improperly handles higher request versions.

   A server SHOULD send a response version equal to as
      above, can be combined into absolute-URI form by concatenating the highest version
   to which
      scheme, "://", authority, and combined path and query component.

   Example 1: the server is conformant that following message received over an insecure TCP
   connection

     GET /pub/WWW/TheProject.html HTTP/1.1
     Host: www.example.org:8080

   has a major version less than
   or equal to an effective request URI of

     http://www.example.org:8080/pub/WWW/TheProject.html

   Example 2: the one following message received in the request.  A server MUST NOT send over a version to which it is not conformant.  A server can send TLS-secured TCP
   connection

     OPTIONS * HTTP/1.1
     Host: www.example.org

   has an effective request URI of

     https://www.example.org

   Recipients of an HTTP/1.0 request that lacks a 505
   (HTTP Version Not Supported) response if it wishes, for any reason, Host header field
   might need to refuse service use heuristics (e.g., examination of the client's major protocol version.

   A server MAY send an HTTP/1.0 response URI path for
   something unique to a particular host) in order to guess the
   effective request if it URI's authority component.

4.  Status Line

   The first line of a response message is known or
   suspected that the client incorrectly implements the HTTP
   specification and is incapable status-line, consisting
   of correctly processing later version
   responses, such as when the protocol version, a client fails to parse space (SP), the version number
   correctly or when an intermediary is known to blindly forward status code, another
   space, a possibly empty textual phrase describing the status code,
   and ending with CRLF.

     status-line = HTTP-version even when it doesn't conform to SP status-code SP reason-phrase CRLF

   Although the given minor version status-line grammar rule requires that each of the protocol.  Such protocol downgrades SHOULD NOT
   component elements be performed
   unless triggered separated by specific client attributes, such a single SP octet, recipients MAY
   instead parse on whitespace-delimited word boundaries and, aside from
   the line terminator, treat any form of whitespace as when the SP separator
   while ignoring preceding or trailing whitespace; such whitespace
   includes one or more of the request header fields (e.g., User-Agent) uniquely match
   the values sent by a client known to be following octets: SP, HTAB, VT (%x0B), FF
   (%x0C), or bare CR.  However, lenient parsing can result in error.

   The intention of HTTP's versioning design is that the major number
   will only be incremented response
   splitting security vulnerabilities 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 there are multiple recipients
   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 [RFC2068] and [RFC2616], and this revision each has
   specifically avoided any such changes to the protocol.

   When an HTTP message its own unique interpretation of
   robustness (see Section 11.1).

   The status-code element is received with a major version number that the
   recipient implements, but a higher minor version number than what the
   recipient implements, the recipient SHOULD process 3-digit integer code describing the message as if
   it were in
   result of the highest minor version within that major version server's attempt to
   which understand and satisfy the recipient is conformant.  A recipient can assume that a client's
   corresponding request.  The rest of the response message with a higher minor version, when sent to a recipient that
   has not yet indicated support for that higher version, is
   sufficiently backwards-compatible to be safely processed by any
   implementation
   interpreted in light of the same major version.

2.7.  Uniform Resource Identifiers

   Uniform Resource Identifiers (URIs) [RFC3986] are used throughout
   HTTP as the means semantics defined for identifying resources (Section 2 that status code.
   See Section 9 of [SEMNTCS]).
   URI references are used to target requests, indicate redirects, and
   define relationships.

   The definitions [Semantics] for information about the semantics of "URI-reference", "absolute-URI", "relative-part",
   "scheme", "authority", "port", "host", "path-abempty", "segment",
   "query", and "fragment" are adopted from
   status codes, including the URI generic syntax.  An
   "absolute-path" rule is classes of status code (indicated by the
   first digit), the status codes defined by this specification,
   considerations for protocol elements that can
   contain a non-empty path component.  (This rule differs slightly from the path-abempty rule definition of RFC 3986, which allows new status codes, and the IANA
   registry.

     status-code    = 3DIGIT

   The reason-phrase element exists for an empty path the sole purpose of providing a
   textual description associated with the numeric status code, mostly
   out of deference to
   be used in references, and path-absolute rule, which does not allow
   paths earlier Internet application protocols that begin were
   more frequently used with "//".) interactive text clients.  A "partial-URI" rule is defined for
   protocol elements that can contain a relative URI but not a fragment
   component.

     URI-reference = <URI-reference, see [RFC3986], Section 4.1>
     absolute-URI  = <absolute-URI, see [RFC3986], Section 4.3>
     relative-part = <relative-part, see [RFC3986], Section 4.2>
     scheme        = <scheme, see [RFC3986], Section 3.1>
     authority     = <authority, see [RFC3986], Section 3.2>
     uri-host      = <host, see [RFC3986], Section 3.2.2>
     port          = <port, see [RFC3986], Section 3.2.3>
     path-abempty  = <path-abempty, see [RFC3986], Section 3.3>
     segment       = <segment, see [RFC3986], Section 3.3>
     query         = <query, see [RFC3986], Section 3.4>
     fragment      = <fragment, see [RFC3986], Section 3.5>

     absolute-path client SHOULD
   ignore the reason-phrase content.

     reason-phrase  = 1*( "/" segment *( HTAB / SP / VCHAR / obs-text )
     partial-URI   = relative-part [ "?" query ]

5.  Header Fields

   Each protocol element in HTTP that allows a URI reference will
   indicate in its ABNF production whether the element allows any form header field consists of reference (URI-reference), only a URI in absolute form (absolute-
   URI), only case-insensitive field name followed
   by a colon (":"), optional leading whitespace, the path field value, and
   optional query components, or some
   combination of trailing whitespace.

     header-field   = field-name ":" OWS field-value OWS

   [[CREF2: Most HTTP field names and the above.  Unless otherwise indicated, URI references rules for parsing within field
   values are parsed relative to the effective request URI (Section 5.5).

2.7.1.  http URI Scheme

   The "http" URI scheme is hereby defined for the purpose in Section 4 of minting
   identifiers according to their association with [Semantics].  This section covers
   the hierarchical
   namespace governed by a potential HTTP origin server listening for
   TCP ([RFC0793]) connections on a given port.

     http-URI = "http:" "//" authority path-abempty [ "?" query ]
                [ "#" fragment ]

   The origin server generic syntax for an "http" URI is identified by the authority
   component, which includes a host identifier and optional TCP port
   ([RFC3986], Section 3.2.2).  The hierarchical path component header field inclusion within, and
   optional query component serve as an identifier for a potential
   target resource within that origin server's name space.  The optional
   fragment component allows for indirect identification of a secondary
   resource, independent of extraction
   from, HTTP/1.1 messages.  In addition, the URI scheme, as following header fields
   are defined in by this document because they are specific to HTTP/1.1
   message processing: ]]

   +-------------------+----------+----------+---------------+
   | Header Field Name | Protocol | Status   | Reference     |
   +-------------------+----------+----------+---------------+
   | Connection        | http     | standard | Section 3.5 of
   [RFC3986].

   A sender MUST NOT generate an "http" URI with an empty host
   identifier.  A recipient that processes such a URI reference MUST
   reject it as invalid.

   If the host identifier is provided as an IP address, the origin
   server is the listener (if any) on the indicated TCP port at that IP
   address.  If host is a registered name, 9.1   |
   | MIME-Version      | http     | standard | Appendix B.1  |
   | TE                | http     | standard | Section 7.4   |
   | Transfer-Encoding | http     | standard | Section 6.1   |
   | Upgrade           | http     | standard | Section 9.7   |
   +-------------------+----------+----------+---------------+

   Furthermore, the registered field name "Close" is an
   indirect identifier for use with a reserved, since using that
   name resolution service, such as
   DNS, to find an address for that origin server.  If HTTP header field might conflict with the port
   subcomponent is empty or not given, TCP port 80 (the reserved port
   for WWW services) is "close"
   connection option of the default.

   Note that the presence of Connection header field (Section 9.1).

   +-------------------+----------+----------+------------+
   | Header Field Name | Protocol | Status   | Reference  |
   +-------------------+----------+----------+------------+
   | Close             | http     | reserved | Section 5  |
   +-------------------+----------+----------+------------+

5.1.  Field Parsing

   Messages are parsed using a URI with generic algorithm, independent of the
   individual header field names.  The contents within a given authority component does field
   value are not imply that there is always an HTTP server listening for
   connections on that host and port.  Anyone can mint parsed until a URI.  What later stage of message interpretation
   (usually after the
   authority component determines is who message's entire header section has been
   processed).

   No whitespace is allowed between the right to respond
   authoritatively to requests that target the identified resource.  The
   delegated nature of registered names and IP addresses creates a
   federated namespace, based on control over the indicated host header field-name and
   port, whether or not an HTTP server is present.  See Section 9.1 for
   security considerations related to establishing authority.

   When an "http" URI is used within a context that calls for access to colon.  In
   the indicated resource, a client MAY attempt access by resolving past, differences in the
   host to an IP address, establishing a TCP connection handling of such whitespace have led to that address
   on the indicated port,
   security vulnerabilities in request routing and sending an HTTP response handling.  A
   server MUST reject any received request message
   (Section 3) containing the URI's identifying data (Section 5) to the
   server.  If the server responds to that request contains
   whitespace between a header field-name and colon with a non-interim
   HTTP response message, as described in Section 6 code
   of [SEMNTCS], then
   that 400 (Bad Request).  A proxy MUST remove any such whitespace from a
   response is considered an authoritative answer to message before forwarding the client's
   request.

   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 message downstream.

   A field value might be identified using preceded and/or followed by optional
   whitespace (OWS); a different URI scheme, just as single SP preceding the "https"
   scheme (below) field-value is used preferred
   for resources that require an end-to-end
   secured connection.  Other protocols might also be used to provide
   access to "http" identified resources -- it is only the authoritative
   interface that is specific to TCP. consistent readability by humans.  The URI generic syntax for authority also includes a deprecated
   userinfo subcomponent ([RFC3986], Section 3.2.1) for including user
   authentication information in field value does not
   include any leading or trailing whitespace: OWS occurring before the URI.  Some implementations make use
   first non-whitespace octet of the userinfo component for internal configuration of
   authentication information, such as within command invocation
   options, configuration files, field value or bookmark lists, even though such
   usage might expose after the last non-
   whitespace octet of the field value ought to be excluded by parsers
   when extracting the field value from a user identifier header field.

5.2.  Obsolete Line Folding

   Historically, HTTP header field values could be extended over
   multiple lines by preceding each extra line with at least one space
   or password. horizontal tab (obs-fold).  This specification deprecates such
   line folding except within the message/http media type
   (Section 10.1).

     obs-fold     = CRLF 1*( SP / HTAB )
                 ; obsolete line folding

   A sender MUST NOT generate the userinfo subcomponent (and its "@" delimiter) when an
   "http" URI reference is generated within a message as that includes line folding
   (i.e., that has any field-value that contains a request
   target or header field value.  Before making use of an "http" URI
   reference received from an untrusted source, a recipient SHOULD parse
   for userinfo and treat its presence as an error; it is likely being
   used match to obscure the authority for obs-fold
   rule) unless the sake of phishing attacks.

2.7.2.  https URI Scheme

   The "https" URI scheme message is hereby defined intended for packaging within the purpose of minting
   identifiers according to their association with
   message/http media type.

   A server that receives an obs-fold in a request message that is not
   within a message/http container MUST either reject the hierarchical
   namespace governed message by
   sending a potential HTTP origin server listening to 400 (Bad Request), preferably with a
   given TCP port for TLS-secured connections ([RFC5246]).

   All of the requirements listed above for the "http" scheme are also
   requirements for the "https" scheme, except representation
   explaining that TCP port 443 obsolete line folding is unacceptable, or replace
   each received obs-fold with one or more SP octets prior to
   interpreting the
   default if field value or forwarding the port subcomponent is empty message downstream.

   A proxy or gateway that receives an obs-fold in a response message
   that is not given, within a message/http container MUST either discard the
   message and replace it with a 502 (Bad Gateway) response, preferably
   with a representation explaining that unacceptable line folding was
   received, or replace each received obs-fold with one or more SP
   octets prior to interpreting the field value or forwarding the
   message downstream.

   A user agent MUST ensure that its connection to the origin server receives an obs-fold in a response message that is secured
   through the use of strong encryption, end-to-end,
   not within a message/http container MUST replace each received obs-
   fold with one or more SP octets prior to sending
   the first HTTP request.

     https-URI = "https:" "//" authority path-abempty [ "?" query ]
                 [ "#" fragment ]

   Note that the "https" URI scheme depends on both TLS and TCP for
   establishing authority.  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 interpreting the same TCP port).  They are distinct namespaces and
   are considered to be distinct origin servers.  However, field
   value.

6.  Message Body

   The message body (if any) of an extension
   to HTTP that message is defined to apply used to entire host domains, such as carry the
   Cookie protocol [RFC6265], can allow information set by one service
   to impact communication with other services within a matching group
   payload body of host domains. that request or response.  The process for authoritative access to an "https" identified
   resource message body is defined in [RFC2818].

2.7.3.  http and https URI Normalization and Comparison

   Since the "http" and "https" schemes conform to the URI generic
   syntax, such URIs are normalized and compared according
   identical to the
   algorithm defined payload body unless a transfer coding has been
   applied, as described in Section 6 of [RFC3986], using the defaults
   described above 6.1.

     message-body = *OCTET

   The rules for each scheme.

   If the port when a message body is equal to the default port allowed in a message differ for
   requests and responses.

   The presence of a scheme, the normal
   form message body in a request is to omit signaled by a Content-
   Length or Transfer-Encoding header field.  Request message framing is
   independent of method semantics, even if the port subcomponent.  When method does not being used define
   any use for a message body.

   The presence of a message body in
   absolute form as a response depends on both the
   request target of an OPTIONS request, an empty
   path component is equivalent method to an absolute path of "/", so the
   normal form which it is responding and the response status code
   (Section 4).  Responses to provide a path the HEAD request method (Section 7.3.2 of "/" instead.  The scheme and host
   are case-insensitive and normally provided in lowercase; all other
   components are compared in
   [Semantics]) never include a case-sensitive manner.  Characters other
   than those in message body because the "reserved" set are equivalent to associated
   response header fields (e.g., Transfer-Encoding, Content-Length,
   etc.), if present, indicate only what their percent-
   encoded octets: values would have been if
   the normal form is request method had been GET (Section 7.3.1 of [Semantics]).  2xx
   (Successful) responses to not encode them (see Sections
   2.1 and 2.2 a CONNECT request method (Section 7.3.6 of [RFC3986]).

   For example, the following three URIs are equivalent:

      http://example.com:80/~smith/home.html
      http://EXAMPLE.com/%7Esmith/home.html
      http://EXAMPLE.com:/%7esmith/home.html

3.  Message Format

   All HTTP/1.1 messages consist
   [Semantics]) switch to tunnel mode instead of having a start-line followed by message body.
   All 1xx (Informational), 204 (No Content), and 304 (Not Modified)
   responses do not include a sequence
   of octets in message body.  All other responses do
   include a format similar to message body, although the Internet Message Format
   [RFC5322]: body might be of zero or more length.

6.1.  Transfer-Encoding

   The Transfer-Encoding header fields (collectively referred to as
   the "headers" or field lists the "header section"), an empty line indicating transfer coding names
   corresponding to the
   end sequence of transfer codings that have been (or
   will be) applied to the payload body in order to form the header section, and an optional message
   body.

     HTTP-message  Transfer codings are defined in Section 7.

     Transfer-Encoding = start-line
                      *( header-field CRLF )
                      CRLF
                      [ message-body ]

   The normal procedure for parsing an HTTP message 1#transfer-coding

   Transfer-Encoding is analogous to read the
   start-line into a structure, read each header field into a hash table
   by Content-Transfer-Encoding field name until the empty line, and then use the parsed data
   of MIME, which was designed to
   determine if a message body is expected.  If enable safe transport of binary data
   over a message body 7-bit transport service ([RFC2045], Section 6).  However, safe
   transport has been
   indicated, then it is read as a stream until different focus for an amount of octets
   equal to the message body length 8bit-clean transfer protocol.
   In HTTP's case, Transfer-Encoding is read primarily intended to accurately
   delimit a dynamically generated payload and to distinguish payload
   encodings that are only applied for transport efficiency or security
   from those that are characteristics of the connection is closed. selected resource.

   A recipient MUST be able to parse an HTTP message as the chunked transfer coding
   (Section 7.1) because it plays a sequence of octets crucial role in an
   encoding that framing messages
   when the payload body size is not known in advance.  A sender MUST
   NOT apply chunked more than once to a superset of US-ASCII [USASCII].  Parsing message body (i.e., chunking an HTTP
   already chunked message as is not allowed).  If any transfer coding
   other than chunked is applied to a stream of Unicode characters, without regard for request payload body, the
   specific encoding, creates security vulnerabilities due to sender
   MUST apply chunked as the
   varying ways that string processing libraries handle invalid
   multibyte character sequences final transfer coding to ensure that contain the octet LF (%x0A).
   String-based parsers can only be safely used within protocol elements
   after the element has been extracted from the message, such as within
   a header field-value after
   message parsing has delineated the
   individual fields.

   An HTTP message can be parsed as is properly framed.  If any transfer coding other than
   chunked is applied to a stream for incremental processing
   or forwarding downstream.  However, recipients cannot rely on
   incremental delivery of partial messages, since some implementations
   will buffer or delay message forwarding for the sake of network
   efficiency, security checks, or response payload transformations.

   A body, the sender MUST NOT send whitespace between either
   apply chunked as the start-line and final transfer coding or terminate the
   first header field.  A recipient message
   by closing the connection.

   For example,

     Transfer-Encoding: gzip, chunked

   indicates that receives whitespace between the
   start-line payload body has been compressed using the gzip
   coding and then chunked using the first header field MUST either reject chunked coding while forming the
   message
   as invalid or consume each whitespace-preceded line without further
   processing body.

   Unlike Content-Encoding (Section 6.1.2 of [Semantics]), Transfer-
   Encoding is a property of it (i.e., ignore the entire line, along with message, not of the representation, and
   any
   subsequent lines preceded by whitespace, until a properly formed
   header field is recipient along the request/response chain MAY decode the
   received transfer coding(s) or apply additional transfer coding(s) to
   the message body, assuming that corresponding changes are made to the
   Transfer-Encoding field-value.  Additional information about the
   encoding parameters can be provided by other header section is terminated).

   The presence of such whitespace fields not
   defined by this specification.

   Transfer-Encoding MAY be sent in a request might be an attempt response to
   trick a server into ignoring that field HEAD request or processing the line after
   it as in a new
   304 (Not Modified) response (Section 9.4.5 of [Semantics]) to a GET
   request, either neither of which might result in includes a security
   vulnerability if other implementations within the request chain
   interpret the same message differently.  Likewise, body, to indicate that
   the presence of
   such whitespace in origin server would have applied a response might be ignored by some clients or
   cause others transfer coding to cease parsing.

3.1.  Start Line

   An HTTP the message can be either a
   body if the request from client to had been an unconditional GET.  This indication
   is not required, however, because any recipient on the response chain
   (including the origin server) can remove transfer codings when they
   are not needed.

   A server or MUST NOT send a Transfer-Encoding header field in any
   response from with a status code of 1xx (Informational) or 204 (No
   Content).  A server MUST NOT send a Transfer-Encoding header field in
   any 2xx (Successful) response to client.  Syntactically, the two types a CONNECT request (Section 7.3.6 of
   message differ only
   [Semantics]).

   Transfer-Encoding was added in the start-line, which HTTP/1.1.  It is either generally assumed
   that implementations advertising only HTTP/1.0 support will not
   understand how to process a request-line
   (for requests) or transfer-encoded payload.  A client MUST
   NOT send a status-line (for responses), and in request containing Transfer-Encoding unless it knows the algorithm
   for determining
   server will handle HTTP/1.1 (or later) requests; such knowledge might
   be in the length form of specific user configuration or by remembering the message body (Section 3.3).

   In theory, a client could receive requests and
   version of a prior received response.  A server could receive
   responses, distinguishing them by their different start-line formats,
   but, in practice, servers are implemented to only expect MUST NOT send a request (a
   response is interpreted as an unknown or invalid containing Transfer-Encoding unless the corresponding
   request method) and
   clients are implemented to only expect a response.

     start-line     = request-line / status-line

3.1.1.  Request Line indicates HTTP/1.1 (or later).

   A request-line begins server that receives a request message with a method token, followed by transfer coding it
   does not understand SHOULD respond with 501 (Not Implemented).

6.2.  Content-Length

   When a single space
   (SP), message does not have a Transfer-Encoding header field, a
   Content-Length header field can provide the request-target, another single space (SP), anticipated size, as a
   decimal number of octets, for a potential payload body.  For messages
   that do include a payload body, the protocol
   version, and ends with CRLF.

     request-line   = method SP request-target SP HTTP-version CRLF

   The method token Content-Length field-value
   provides the framing information necessary for determining where the
   body (and message) ends.  For messages that do not include a payload
   body, the Content-Length indicates the request method to be performed on size of the
   target resource.  The request method selected
   representation (Section 6.2.4 of [Semantics]).

      Note: HTTP's use of Content-Length for message framing differs
      significantly from the same field's use in MIME, where it is case-sensitive.

     method         = token an
      optional field used only within the "message/external-body" media-
      type.

6.3.  Message Body Length

   The request methods defined length of a message body is determined by this specification can be found in
   Section 4 one of [SEMNTCS], along with information regarding the HTTP
   method registry following
   (in order of precedence):

   1.  Any response to a HEAD request and considerations for defining new methods.

   The request-target identifies any response with a 1xx
       (Informational), 204 (No Content), or 304 (Not Modified) status
       code is always terminated by the target resource upon which to apply first empty line after the request, as defined in Section 5.3.

   Recipients typically parse
       header fields, regardless of the request-line into its component parts
   by splitting on whitespace (see Section 3.5), since no whitespace is
   allowed header fields present in the three components.  Unfortunately, some user agents
   fail
       message, and thus cannot contain a message body.

   2.  Any 2xx (Successful) response to properly encode or exclude whitespace found in hypertext
   references, resulting in those disallowed characters being sent in a
   request-target.

   Recipients of an invalid request-line SHOULD respond with either CONNECT request implies that
       the connection will become a
   400 (Bad Request) error tunnel immediately after the empty
       line that concludes the header fields.  A client MUST ignore any
       Content-Length or Transfer-Encoding header fields received in
       such a 301 (Moved Permanently) redirect with message.

   3.  If a Transfer-Encoding header field is present and the request-target properly encoded.  A recipient SHOULD NOT attempt
   to autocorrect chunked
       transfer coding (Section 7.1) is the final encoding, the message
       body length is determined by reading and then process decoding the request without a redirect, since chunked
       data until the invalid request-line might be deliberately crafted to bypass
   security filters along transfer coding indicates the request chain.

   HTTP does not place data is complete.

       If a predefined limit on Transfer-Encoding header field is present in a response and
       the chunked transfer coding is not the final encoding, the
       message body length of is determined by reading the connection until
       it is closed by the server.  If a request-
   line, as described Transfer-Encoding header field
       is present in Section 2.5.  A server that receives a method
   longer than any that it implements SHOULD respond with a 501 (Not
   Implemented) status code.  A request and the chunked transfer coding is not
       the final encoding, the message body length cannot be determined
       reliably; the server that receives a request-target
   longer than any URI it wishes to parse MUST respond with a 414 (URI
   Too Long) the 400 (Bad Request)
       status code (see Section 6.5.12 of [SEMNTCS]).

   Various ad hoc limitations on request-line length are found in
   practice.  It and then close the connection.

       If a message is RECOMMENDED that all HTTP senders received with both a Transfer-Encoding and recipients
   support, at a minimum, request-line lengths of 8000 octets.

3.1.2.  Status Line

   The first line of
       Content-Length header field, the Transfer-Encoding overrides the
       Content-Length.  Such a message might indicate an attempt to
       perform request smuggling (Section 11.2) or response splitting
       (Section 11.1) and ought to be handled as an error.  A sender
       MUST remove the received Content-Length field prior to forwarding
       such a message downstream.

   4.  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 status-line, consisting
   of message framing is invalid and the protocol version,
       recipient MUST treat it as an unrecoverable error.  If this is a space (SP),
       request message, the status code, another
   space, server MUST respond with a possibly empty textual phrase describing the 400 (Bad Request)
       status code, code and ending with CRLF.

     status-line = HTTP-version SP status-code SP reason-phrase CRLF

   The status-code element then close the connection.  If this is a 3-digit integer code describing response
       message received by a proxy, the
   result of proxy MUST close the server's attempt connection
       to understand and satisfy the client's
   corresponding request.  The rest of server, discard the response message received response, and send a 502 (Bad
       Gateway) response to the client.  If this is a response message
       received by a user agent, the user agent MUST close the
       connection to be
   interpreted the server and discard the received response.

   5.  If a valid Content-Length header field is present without
       Transfer-Encoding, its decimal value defines the expected message
       body length in light of octets.  If the semantics defined for that status code.
   See Section 6 of [SEMNTCS] for information about sender closes the semantics of
   status codes, including connection or
       the classes recipient times out before the indicated number of status code (indicated by octets are
       received, the
   first digit), recipient MUST consider the status codes defined by this specification,
   considerations for message to be
       incomplete and close the definition of new status codes, connection.

   6.  If this is a request message and none of the IANA
   registry.

     status-code    = 3DIGIT

   The reason-phrase element exists for above are true, then
       the sole purpose of providing message body length is zero (no message body is present).

   7.  Otherwise, this is a
   textual description associated with response message without a declared message
       body length, so the numeric status code, mostly
   out message body length is determined by the
       number of deference octets received prior to earlier Internet application protocols that were
   more frequently used with interactive text clients.  A client SHOULD
   ignore the reason-phrase content.

     reason-phrase  = *( HTAB / SP / VCHAR / obs-text )

3.2.  Header Fields

   Each header field consists of server closing the
       connection.

   Since there is no way to distinguish a case-insensitive field name followed
   by successfully completed, close-
   delimited message from a colon (":"), optional leading whitespace, the field value, and
   optional trailing whitespace.

     header-field   = field-name ":" OWS field-value OWS

     field-name     = token
     field-value    = *( field-content / obs-fold )
     field-content  = field-vchar [ 1*( SP / HTAB ) field-vchar ]
     field-vchar    = VCHAR / obs-text

     obs-fold       = CRLF 1*( SP / HTAB )
                    ; obsolete line folding
                    ; see Section 3.2.4 partially received message interrupted by
   network failure, a server SHOULD generate encoding or length-
   delimited messages whenever possible.  The field-name token labels the corresponding field-value as having
   the semantics defined close-delimiting feature
   exists primarily for backwards compatibility with HTTP/1.0.

   A server MAY reject a request that contains a message body but not a
   Content-Length by responding with 411 (Length Required).

   Unless a transfer coding other than chunked has been applied, a
   client that header field.  For example, the Date sends a request containing a message body SHOULD use a
   valid Content-Length header field is defined in Section 7.1.1.2 of [SEMNTCS] as containing
   the origination timestamp for if the message in which it appears.

3.2.1.  Field Extensibility

   Header fields are fully extensible: there body length is no limit on the
   introduction of new field names, each presumably defining new
   semantics, nor on the number of header fields used known
   in advance, rather than the chunked transfer coding, since some
   existing services respond to chunked with a given
   message.  Existing fields 411 (Length Required)
   status code even though they understand the chunked transfer coding.
   This is typically because such services are defined implemented via a gateway
   that requires a content-length in each part advance of this
   specification being called and in many other specifications outside this document
   set.

   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, define preconditions on request
   evaluation,
   server is unable or refine unwilling to buffer the meaning of responses. entire request before
   processing.

   A proxy user agent that sends a request containing a message body MUST forward unrecognized header fields unless the field-name
   is listed in the Connection send
   a valid Content-Length header field (Section 6.1) or if it does not know the proxy
   is specifically configured to block, or otherwise transform, server
   will handle HTTP/1.1 (or later) requests; such
   fields.  Other recipients SHOULD ignore unrecognized header fields.
   These requirements allow HTTP's functionality to be enhanced without
   requiring prior update of deployed intermediaries.

   All defined header fields ought to knowledge can be registered with IANA in
   the
   "Message Headers" registry, as described in Section 8.3 form of [SEMNTCS].

3.2.2.  Field Order

   The order in which header fields with differing field names are specific user configuration or by remembering the version
   of a prior received is not significant.  However, it is good practice response.

   If the final response to send
   header fields that contain control data first, such as Host the last request on
   requests a connection has been
   completely received and Date on responses, so that implementations can decide
   when not there remains additional data to handle a message as early as possible.  A server MUST NOT
   apply read, a request user
   agent MAY discard the remaining data or attempt to determine if that
   data belongs as part of the target resource until prior response body, which might be the entire request
   header section
   case if the prior message's Content-Length value is received, since later header fields might include
   conditionals, authentication credentials, or deliberately misleading
   duplicate header fields that would impact request processing. incorrect.  A sender
   client MUST NOT generate multiple header fields with the same field
   name in a message unless either the entire field value for that
   header field is defined process, cache, or forward such extra data as a comma-separated list [i.e., #(values)]
   separate response, since such behavior would be vulnerable to cache
   poisoning.

7.  Transfer Codings

   Transfer coding names are used to indicate an encoding transformation
   that has been, can be, or the header field is might need to be applied to a well-known exception (as noted below).

   A recipient MAY combine multiple header fields with the same field
   name into one "field-name: field-value" pair, without changing the
   semantics of the message, by appending each subsequent field value payload body
   in order to ensure "safe transport" through the combined field value in order, separated by network.  This
   differs from a comma.  The order content coding in which header fields with that the same field name are received transfer coding is
   therefore significant to the interpretation a
   property of the combined field
   value; message rather than a proxy MUST NOT change the order property of these field values when
   forwarding a message.

      Note: In practice, the "Set-Cookie" header field ([RFC6265]) often
      appears multiple times representation
   that is being transferred.

     transfer-coding    = "chunked" ; Section 7.1
                        / "compress" ; [Semantics], Section 6.1.2.1
                        / "deflate" ; [Semantics], Section 6.1.2.2
                        / "gzip" ; [Semantics], Section 6.1.2.3
                        / transfer-extension
     transfer-extension = token *( OWS ";" OWS transfer-parameter )

   Parameters are in a response message and does not use the
      list syntax, violating the above requirements on multiple header
      fields with the same name.  Since it cannot be combined into form of a
      single field-value, recipients name or name=value pair.

     transfer-parameter = token BWS "=" BWS ( token / quoted-string )

   All transfer-coding names are case-insensitive and ought to handle "Set-Cookie" be
   registered within the HTTP Transfer Coding registry, as a
      special case while processing defined in
   Section 7.3.  They are used in the TE (Section 7.4) and Transfer-
   Encoding (Section 6.1) header fields.  (See Appendix A.2.3

   +------------+------------------------------------------+-----------+
   | Name       | Description                              | Reference |
   +------------+------------------------------------------+-----------+
   | chunked    | Transfer in a series of [Kri2001] for details.)

3.2.3.  Whitespace

   This specification uses three rules to denote chunks           | Section 7 |
   |            |                                          | .1        |
   | compress   | UNIX "compress" data format [Welch]      | Section 7 |
   |            |                                          | .2        |
   | deflate    | "deflate" compressed data ([RFC1951])    | Section 7 |
   |            | inside the use of linear
   whitespace: OWS (optional whitespace), RWS (required whitespace), and
   BWS ("bad" whitespace). "zlib" data format            | .2        |
   |            | ([RFC1950])                              |           |
   | gzip       | GZIP file format [RFC1952]               | Section 7 |
   |            |                                          | .2        |
   | x-compress | Deprecated (alias for compress)          | Section 7 |
   |            |                                          | .2        |
   | x-gzip     | Deprecated (alias for gzip)              | Section 7 |
   |            |                                          | .2        |
   +------------+------------------------------------------+-----------+

7.1.  Chunked Transfer Coding

   The OWS rule is used where zero or more linear whitespace octets
   might appear.  For protocol elements where optional whitespace is
   preferred to improve readability, a sender SHOULD generate chunked transfer coding wraps the
   optional whitespace payload body in order to
   transfer it as a single SP; otherwise, series of chunks, each with its own size indicator,
   followed by an OPTIONAL trailer containing header fields.  Chunked
   enables content streams of unknown size to be transferred as a
   sequence of length-delimited buffers, which enables the sender SHOULD NOT
   generate optional whitespace except as needed to white out invalid or
   unwanted protocol elements during in-place message filtering.

   The RWS rule is used
   retain connection persistence and the recipient to know when at least one linear whitespace octet is
   required to separate field tokens.  A sender SHOULD generate RWS as a
   single SP.

   The BWS rule is used where the grammar allows optional whitespace
   only for historical reasons.  A sender MUST NOT generate BWS in
   messages.  A recipient MUST parse for such bad whitespace and remove it before interpreting has
   received the protocol element.

     OWS entire message.

     chunked-body   = *( SP / HTAB )
                    ; optional whitespace
     RWS *chunk
                      last-chunk
                      trailer-part
                      CRLF

     chunk          = 1*( SP / HTAB )
                    ; required whitespace
     BWS chunk-size [ chunk-ext ] CRLF
                      chunk-data CRLF
     chunk-size     = OWS 1*HEXDIG
     last-chunk     = 1*("0") [ chunk-ext ] CRLF

     chunk-data     = 1*OCTET ; "bad" whitespace

3.2.4.  Field Parsing

   Messages are parsed using a generic algorithm, independent sequence of the
   individual header field names. chunk-size octets

   The contents within a given chunk-size field
   value are not parsed until is a later stage string of message interpretation
   (usually after hex digits indicating the message's entire header section has been
   processed).  Consequently, this specification does not use ABNF rules
   to define each "Field-Name: Field Value" pair, as was done size of
   the chunk-data in
   previous editions.  Instead, this specification uses ABNF rules that
   are named according octets.  The chunked transfer coding is complete
   when a chunk with a chunk-size of zero is received, possibly followed
   by a trailer, and finally terminated by an empty line.

   A recipient MUST be able to each registered field name, wherein parse and decode the rule
   defines chunked transfer
   coding.

7.1.1.  Chunk Extensions

   The chunked encoding allows each chunk to include zero or more chunk
   extensions, immediately following the valid grammar chunk-size, for that field's corresponding field values
   (i.e., after the field-value has been extracted from the header
   section by a generic field parser).

   No whitespace sake of
   supplying per-chunk metadata (such as a signature or hash), mid-
   message control information, or randomization of message body size.

     chunk-ext      = *( ";" chunk-ext-name [ "=" chunk-ext-val ] )

     chunk-ext-name = token
     chunk-ext-val  = token / quoted-string

   The chunked encoding is allowed between the header field-name specific to each connection and colon.  In
   the past, differences in is likely to
   be removed or recoded by each recipient (including intermediaries)
   before any higher-level application would have a chance to inspect
   the handling extensions.  Hence, use of such whitespace have led chunk extensions is generally limited
   to
   security vulnerabilities in request routing specialized HTTP services such as "long polling" (where client and response handling.  A
   server can have shared expectations regarding the use of chunk
   extensions) or for padding within an end-to-end secured connection.

   A recipient MUST reject any ignore unrecognized chunk extensions.  A server
   ought to limit the total length of chunk extensions received in a
   request message to an amount reasonable for the services provided, in the
   same way that contains
   whitespace between a header field-name it applies length limitations and colon with a response code timeouts for other
   parts of 400 (Bad Request).  A proxy MUST remove any such whitespace from a message, and generate an appropriate 4xx (Client Error)
   response message before forwarding if that amount is exceeded.

7.1.2.  Chunked Trailer Part

   A trailer allows the sender to include additional fields at the end
   of a chunked message downstream.

   A field value in order to supply metadata that might be preceded and/or followed by optional
   whitespace (OWS); a single SP preceding
   dynamically generated while the field-value message body is preferred
   for consistent readability by humans.  The field value does not
   include any leading or trailing whitespace: OWS occurring before the
   first non-whitespace octet of the field value sent, such as a
   message integrity check, digital signature, or after the last non-
   whitespace octet of the field value ought post-processing
   status.  The trailer fields are identical to be excluded by parsers
   when extracting the field value from a header field.

   Historically, HTTP header field values could be extended over
   multiple lines by preceding each extra line with at least one space
   or horizontal tab (obs-fold).  This specification deprecates such
   line folding fields, except within
   they are sent in a chunked trailer instead of the message/http media type
   (Section 8.3.1). message's header
   section.

     trailer-part   = *( header-field CRLF )

   A sender MUST NOT generate a message that includes
   line folding (i.e., that has any field-value trailer that contains a match field necessary
   for message framing (e.g., Transfer-Encoding and Content-Length),
   routing (e.g., Host), request modifiers (e.g., controls and
   conditionals in Section 8 of [Semantics]), authentication (e.g., see
   Section 8.5 of [Semantics] and [RFC6265]), response control data
   (e.g., see Section 10.1 of [Semantics]), or determining how to
   process the obs-fold rule) unless the payload (e.g., Content-Encoding, Content-Type, Content-
   Range, and Trailer).

   When a chunked message containing a non-empty trailer is intended for packaging
   within received,
   the message/http media type. recipient MAY process the fields (aside from those forbidden
   above) as if they were appended to the message's header section.  A server that receives
   recipient MUST ignore (or consider as an obs-fold in a request message error) any fields that is not
   within a message/http container MUST either reject the message by
   sending are
   forbidden to be sent in a 400 (Bad Request), preferably with a representation
   explaining that obsolete line folding is unacceptable, or replace
   each received obs-fold with one or more SP octets prior to
   interpreting trailer, since processing them as if they
   were present in the field value or forwarding header section might bypass external security
   filters.

   Unless the message downstream.

   A proxy or gateway that receives an obs-fold in request includes a response message
   that TE header field indicating "trailers"
   is not within acceptable, as described in Section 7.4, a message/http container MUST either discard the
   message and replace server SHOULD NOT
   generate trailer fields that it with a 502 (Bad Gateway) response, preferably
   with believes are necessary for the user
   agent to receive.  Without a representation explaining that unacceptable line folding was
   received, or replace each received obs-fold with one or more SP
   octets prior TE containing "trailers", the server
   ought to interpreting assume that the field value or forwarding trailer fields might be silently discarded
   along the path to the
   message downstream.

   A user agent that receives agent.  This requirement allows
   intermediaries to forward a de-chunked message to an obs-fold in HTTP/1.0
   recipient without buffering the entire response.

   When a response message that is
   not within includes a message/http container MUST replace each received obs-
   fold message body encoded with one or more SP octets prior to interpreting the field
   value.

   Historically, HTTP has allowed field content with text chunked
   transfer coding and the sender desires to send metadata in the
   ISO-8859-1 charset [ISO-8859-1], supporting other charsets only
   through use form
   of [RFC2047] encoding.  In practice, most HTTP header
   field values use only a subset trailer fields at the end of the US-ASCII charset [USASCII].
   Newly defined header fields SHOULD limit their field values to
   US-ASCII octets.  A recipient message, the sender SHOULD treat other octets in field
   content (obs-text) as opaque data.

3.2.5.  Field Limits

   HTTP does not place
   generate a predefined limit on the length of each Trailer header field or on the length of before the header section as a whole, as described
   in Section 2.5.  Various ad hoc limitations on individual header
   field length are found message body to indicate
   which fields will be present in practice, often depending on the specific
   field semantics.

   A server that receives a request header field, or set trailers.  This allows the
   recipient to prepare for receipt of fields,
   larger than that metadata before it starts
   processing the body, which is useful if the message is being streamed
   and the recipient wishes to process MUST respond with confirm an appropriate 4xx
   (Client Error) status code.  Ignoring such header fields would
   increase integrity check on the server's vulnerability to request smuggling attacks
   (Section 9.5). fly.

7.1.3.  Decoding Chunked

   A client MAY discard or truncate received header fields that are
   larger than the client wishes to process if the field semantics are
   such that for decoding the dropped value(s) chunked transfer coding can be safely ignored without changing
   the message framing or response semantics.

3.2.6.  Field Value Components

   Most HTTP header field values are defined using common syntax
   components (token, quoted-string, represented
   in pseudo-code as:

     length := 0
     read chunk-size, chunk-ext (if any), and comment) separated by
   whitespace or specific delimiting characters.  Delimiters are chosen
   from the set of US-ASCII visual characters CRLF
     while (chunk-size > 0) {
        read chunk-data and CRLF
        append chunk-data to decoded-body
        length := length + chunk-size
        read chunk-size, chunk-ext (if any), and CRLF
     }
     read trailer field
     while (trailer field is not empty) {
        if (trailer field is allowed in a token
   (DQUOTE and "(),/:;<=>?@[\]{}").

     token          = 1*tchar

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

   A string of text is parsed as a single value if it is quoted using
   double-quote marks.

     quoted-string  = DQUOTE *( qdtext / quoted-pair ) DQUOTE
     qdtext         = HTAB / SP /%x21 / %x23-5B / %x5D-7E / obs-text
     obs-text       = %x80-FF

   Comments can to be included sent in some HTTP a trailer) {
            append trailer field to existing header fields
        }
        read trailer-field
     }
     Content-Length := length
     Remove "chunked" from Transfer-Encoding
     Remove Trailer from existing header fields

7.2.  Transfer Codings for Compression

   The following transfer coding names for compression are defined by surrounding
   the comment text with parentheses.  Comments are only allowed in
   fields containing "comment" same algorithm as part of their field value definition.

     comment        = "(" *( ctext / quoted-pair / comment ) ")"
     ctext          = HTAB / SP / %x21-27 / %x2A-5B / %x5D-7E / obs-text corresponding content coding:

   compress (and x-compress)
      See Section 6.1.2.1 of [Semantics].

   deflate
      See Section 6.1.2.2 of [Semantics].

   gzip (and x-gzip)
      See Section 6.1.2.3 of [Semantics].

7.3.  Transfer Coding Registry

   The backslash octet ("\") can be used as a single-octet quoting
   mechanism within quoted-string and comment constructs.  Recipients
   that process "HTTP Transfer Coding Registry" defines the value namespace for
   transfer coding names.  It is maintained at
   <https://www.iana.org/assignments/http-parameters>.

   Registrations MUST include the following fields:

   o  Name

   o  Description

   o  Pointer to specification text

   Names of a quoted-string transfer codings MUST handle a quoted-pair NOT overlap with names of content
   codings (Section 6.1.2 of [Semantics]) unless the encoding
   transformation is identical, as if it were replaced by is the octet following case for the backslash.

     quoted-pair    = "\" ( HTAB / SP / VCHAR / obs-text )

   A sender SHOULD NOT generate a quoted-pair compression
   codings defined in a quoted-string except
   where necessary Section 7.2.

   Values to quote DQUOTE and backslash octets occurring within
   that string.  A sender SHOULD NOT generate a quoted-pair in a comment
   except where necessary be added to quote parentheses ["(" and ")"] and
   backslash octets occurring within that comment.

3.3.  Message Body

   The message body (if any) this namespace require IETF Review (see
   Section 4.1 of an HTTP message is used [RFC5226]), and MUST conform to carry the
   payload body purpose of that request or response.  The message body is
   identical to the payload body unless a
   transfer coding has been
   applied, as described in Section 3.3.1.

     message-body = *OCTET

   The rules for when a message body is allowed defined in a message differ this specification.

   Use of program names for
   requests and responses.

   The presence the identification of a message body in a request is signaled by a Content-
   Length or Transfer-Encoding header field.  Request message framing encoding formats is
   independent of method semantics, even if the method does
   not define
   any use desirable and is discouraged for a message body. future encodings.

7.4.  TE

   The presence of a message body "TE" header field in a response depends on both the request method to which it indicates what transfer codings,
   besides chunked, the client is responding willing to accept in response, and
   whether or not the response status code
   (Section 3.1.2).  Responses client is willing to the HEAD request method (Section 4.3.2
   of [SEMNTCS]) never include a message body because the associated
   response header accept trailer fields (e.g., Transfer-Encoding, Content-Length,
   etc.), if present, indicate only what their values would have been if
   the request method had been GET (Section 4.3.1 of [SEMNTCS]).  2xx
   (Successful) responses to in a CONNECT request method (Section 4.3.6 of
   [SEMNTCS]) switch to tunnel mode instead
   chunked transfer coding.

   The TE field-value consists of having a message body.
   All 1xx (Informational), 204 (No Content), and 304 (Not Modified)
   responses do not include a message body.  All other responses do
   include a message body, although the body might be comma-separated list of zero length.

3.3.1.  Transfer-Encoding

   The Transfer-Encoding header field lists the transfer
   coding names
   corresponding to names, each allowing for optional parameters (as described in
   Section 7), and/or the sequence of keyword "trailers".  A client MUST NOT send
   the chunked transfer codings coding name in TE; chunked is always acceptable
   for HTTP/1.1 recipients.

     TE        = #t-codings
     t-codings = "trailers" / ( transfer-coding [ t-ranking ] )
     t-ranking = OWS ";" OWS "q=" rank
     rank      = ( "0" [ "." 0*3DIGIT ] )
                / ( "1" [ "." 0*3("0") ] )

   Three examples of TE use are below.

     TE: deflate
     TE:
     TE: trailers, deflate;q=0.5

   The presence of the keyword "trailers" indicates that have been (or
   will be) applied to the payload body in order client is
   willing to form the message
   body.  Transfer codings are accept trailer fields in a chunked transfer coding, as
   defined in Section 4.

     Transfer-Encoding = 1#transfer-coding

   Transfer-Encoding is analogous 7.1.2, on behalf of itself and any downstream
   clients.  For requests from an intermediary, this implies that
   either: (a) all downstream clients are willing to accept trailer
   fields in the Content-Transfer-Encoding field forwarded response; or, (b) the intermediary will
   attempt to buffer the response on behalf of MIME, which was designed downstream recipients.
   Note that HTTP/1.1 does not define any means to enable safe transport limit the size of binary data
   over a 7-bit transport service ([RFC2045], Section 6).  However, safe
   transport has a different focus for an 8bit-clean transfer protocol.
   In HTTP's case, Transfer-Encoding is primarily intended to accurately
   delimit a dynamically generated payload and to distinguish payload
   encodings that are only applied for transport efficiency or security
   from those
   chunked response such that are characteristics of the selected resource.

   A recipient MUST an intermediary can be able to parse assured of
   buffering the chunked entire response.

   When multiple transfer coding
   (Section 4.1) because it plays codings are acceptable, the client MAY rank
   the codings by preference using a crucial role in framing messages
   when case-insensitive "q" parameter
   (similar to the payload body size is not known qvalues used in advance.  A sender MUST
   NOT apply chunked more than once to content negotiation fields,
   Section 8.4.1 of [Semantics]).  The rank value is a message body (i.e., chunking an
   already chunked message real number in
   the range 0 through 1, where 0.001 is not allowed).  If any transfer coding
   other than chunked the least preferred and 1 is applied to
   the most preferred; a request payload body, value of 0 means "not acceptable".

   If the sender
   MUST apply chunked as TE field-value is empty or if no TE field is present, the final only
   acceptable transfer coding to ensure that the
   message is properly framed.  If any chunked.  A message with no transfer
   coding other than
   chunked is applied always acceptable.

   Since the TE header field only applies to a response payload body, the immediate connection, a
   sender of TE MUST either
   apply chunked as also send a "TE" connection option within the final transfer coding or terminate
   Connection header field (Section 9.1) in order to prevent the message TE
   field from being forwarded by closing the connection.

   For example,

     Transfer-Encoding: gzip, chunked

   indicates intermediaries that do not support its
   semantics.

8.  Handling Incomplete Messages

   A server that receives an incomplete request message, usually due to
   a canceled request or a triggered timeout exception, MAY send an
   error response prior to closing the payload body has been compressed using the gzip
   coding and then chunked using the connection.

   A client that receives an incomplete response message, which can
   occur when a connection is closed prematurely or when decoding a
   supposedly chunked transfer coding while forming fails, MUST record the message body.

   Unlike Content-Encoding (Section 3.1.2.1 as
   incomplete.  Cache requirements for incomplete responses are defined
   in Section 3 of [SEMNTCS]), Transfer-
   Encoding is [Caching].

   If a property of response terminates in the message, not middle of the representation, and
   any recipient along header section (before
   the request/response chain MAY decode empty line is received) and the
   received transfer coding(s) or apply additional transfer coding(s) status code might rely on header
   fields to convey the message body, assuming that corresponding changes are made to full meaning of the
   Transfer-Encoding field-value.  Additional information about response, then the
   encoding parameters can be provided by other header fields not
   defined by this specification.

   Transfer-Encoding MAY be sent in a response client
   cannot assume that meaning has been conveyed; the client might need
   to a HEAD repeat the request or in a
   304 (Not Modified) response (Section 4.1 of [CONDTNL]) order to a GET
   request, neither of which includes a message body, determine what action to indicate take next.

   A message body that uses the origin server would have applied a chunked transfer coding to is incomplete if
   the zero-sized chunk that terminates the encoding has not been
   received.  A message
   body that uses a valid Content-Length is incomplete
   if the request had been an unconditional GET.  This indication
   is not required, however, because any recipient on size of the response chain
   (including message body received (in octets) is less than the origin server) can remove transfer codings when they
   are not needed.
   value given by Content-Length.  A server MUST NOT send a Transfer-Encoding header field in any response with a status code that has neither chunked
   transfer coding nor Content-Length is terminated by closure of 1xx (Informational) or 204 (No
   Content).  A server MUST NOT send a Transfer-Encoding header field in
   any 2xx (Successful) response to a CONNECT request (Section 4.3.6 the
   connection and, thus, is considered complete regardless of
   [SEMNTCS]).

   Transfer-Encoding the number
   of message body octets received, provided that the header section was added in HTTP/1.1.  It
   received intact.

9.  Connection Management

   HTTP messaging is generally assumed
   that implementations advertising independent of the underlying transport- or
   session-layer connection protocol(s).  HTTP only HTTP/1.0 support will not
   understand how to process a transfer-encoded payload.  A client MUST
   NOT send presumes a reliable
   transport with in-order delivery of requests and the corresponding
   in-order delivery of responses.  The mapping of HTTP request containing Transfer-Encoding unless it knows and
   response structures onto the
   server will handle HTTP/1.1 (or later) requests; such knowledge might
   be in data units of an underlying transport
   protocol is outside the form scope of this specification.

   As described in Section 5.2 of [Semantics], the specific user configuration or connection
   protocols to be used for an HTTP interaction are determined by remembering client
   configuration and the
   version target URI.  For example, the "http" URI scheme
   (Section 2.5.1 of [Semantics]) indicates a prior received response.  A server MUST NOT send a
   response containing Transfer-Encoding unless the corresponding
   request indicates HTTP/1.1 (or later).

   A server that receives a request message default connection of TCP
   over IP, with a transfer coding it
   does not understand SHOULD respond with 501 (Not Implemented).

3.3.2.  Content-Length

   When default TCP port of 80, but the client might be
   configured to use a message does not have proxy via some other connection, port, or
   protocol.

   HTTP implementations are expected to engage in connection management,
   which includes maintaining the state of current connections,
   establishing a Transfer-Encoding header field, new connection or reusing an existing connection,
   processing messages received on a
   Content-Length connection, detecting connection
   failures, and closing each connection.  Most clients maintain
   multiple connections in parallel, including more than one connection
   per server endpoint.  Most servers are designed to maintain thousands
   of concurrent connections, while controlling request queues to enable
   fair use and detect denial-of-service attacks.

9.1.  Connection

   The "Connection" header field can provide allows the anticipated size, as a
   decimal number of octets, sender to indicate desired
   control options for a potential payload body.  For messages
   that do include a payload body, the Content-Length field-value
   provides current connection.  In order to avoid
   confusing downstream recipients, a proxy or gateway MUST remove or
   replace any received connection options before forwarding the framing
   message.

   When a header field aside from Connection is used to supply control
   information necessary for determining where or about the
   body (and message) ends.  For messages that do not include a payload
   body, current connection, the Content-Length indicates sender MUST list
   the size of corresponding field-name within the selected
   representation (Section 3 of [SEMNTCS]).

     Content-Length = 1*DIGIT

   An example is

     Content-Length: 3495 Connection header field.  A sender
   proxy or gateway MUST NOT send parse a Content-Length received Connection header field before
   a message is forwarded and, for each connection-option in this field,
   remove any message
   that contains a Transfer-Encoding header field.

   A user agent SHOULD send a Content-Length in a request field(s) from the message when
   no Transfer-Encoding is sent with the same name as the
   connection-option, and then remove the request method defines a meaning Connection header field itself
   (or replace it with the intermediary's own connection options for an enclosed payload body.  For example, a Content-Length the
   forwarded message).

   Hence, the Connection header field is normally sent in provides a POST request even when declarative way of
   distinguishing header fields that are only intended for the immediate
   recipient ("hop-by-hop") from those fields that are intended for all
   recipients on the chain ("end-to-end"), enabling the message to be
   self-descriptive and allowing future connection-specific extensions
   to be deployed without fear that they will be blindly forwarded by
   older intermediaries.

   The Connection header field's value is 0
   (indicating an empty payload body). has the following grammar:

     Connection        = 1#connection-option
     connection-option = token

   Connection options are case-insensitive.

   A user agent SHOULD sender MUST NOT send a
   Content-Length connection option corresponding to a header
   field when that is intended for all recipients of the request message does not contain payload.  For
   example, Cache-Control is never appropriate as a payload body and the method semantics connection option
   (Section 5.2 of [Caching]).

   The connection options do not anticipate such a
   body.

   A server MAY send always correspond to a Content-Length header field
   present in the message, since a response to a
   HEAD request (Section 4.3.2 of [SEMNTCS]); connection-specific header field
   might not be needed if there are no parameters associated with a server MUST NOT send
   Content-Length in such
   connection option.  In contrast, a response unless its field-value equals the
   decimal number of octets connection-specific header field
   that would have been sent in the payload
   body of is received without a response if corresponding connection option usually
   indicates that the same request had used field has been improperly forwarded by an
   intermediary and ought to be ignored by the GET method.

   A server MAY send a Content-Length recipient.

   When defining new connection options, specification authors ought to
   survey existing header field in names and ensure that the new connection
   option does not share the same name as an already deployed header
   field.  Defining a 304 (Not
   Modified) response new connection option 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.

   The "close" connection option is defined for a conditional GET request (Section 4.1 sender to signal that
   this connection will be closed after completion of
   [CONDTNL]); a server MUST NOT send Content-Length in such a response
   unless its field-value equals the decimal number of octets that would
   have been sent response.  For
   example,

     Connection: close

   in either the request or the payload body of a 200 (OK) response header fields indicates that
   the sender is going to close the same
   request. connection after the current
   request/response is complete (Section 9.6).

   A client that does not support persistent connections MUST send the
   "close" connection option in every request message.

   A server that does not support persistent connections MUST NOT send a Content-Length header field the
   "close" connection option in any every response
   with message that does not
   have a status code of 1xx (Informational) or 204 (No Content).  A
   server MUST NOT send status code.

9.2.  Establishment

   It is beyond the scope of this specification to describe how
   connections are established via various transport- or session-layer
   protocols.  Each connection applies to only one transport link.

9.3.  Persistence

   HTTP/1.1 defaults to the use of "persistent connections", allowing
   multiple requests and responses to be carried over a Content-Length header field in any 2xx
   (Successful) response single
   connection.  The "close" connection option is used to signal that a CONNECT request (Section 4.3.6 of
   [SEMNTCS]).

   Aside from the cases defined above, in
   connection will not persist after the absence of Transfer-
   Encoding, an origin server current request/response.  HTTP
   implementations SHOULD send support persistent connections.

   A recipient determines whether a Content-Length connection is persistent or not
   based on the most recently received message's protocol version and
   Connection header field
   when (if any):

   o  If the payload body size "close" connection option is known prior to sending present, the complete
   header section.  This connection will allow downstream recipients to measure
   transfer progress, know when a
      not persist after the current response; else,

   o  If the received message protocol is complete, and
   potentially reuse HTTP/1.1 (or later), the connection for additional requests.

   Any Content-Length field value greater than or equal to zero
      will persist after the current response; else,

   o  If the received protocol is
   valid.  Since there HTTP/1.0, the "keep-alive" connection
      option is no predefined limit to present, the length of a
   payload, a recipient MUST anticipate potentially large decimal
   numerals is not a proxy, and prevent parsing errors due the recipient
      wishes to integer conversion
   overflows (Section 9.3).

   If a message is received that has multiple Content-Length header
   fields with field-values consisting of honor 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 HTTP/1.0 "keep-alive" mechanism, the
   recipient MUST either reject
      connection will persist after the message as invalid or replace current response; otherwise,

   o  The connection will close after the
   duplicated field-values with current response.

   A client MAY send additional requests on a single valid Content-Length field
   containing that decimal value prior to determining the message body
   length or forwarding the message.

      Note: HTTP's use of Content-Length for message framing differs
      significantly from the same field's use in MIME, where persistent connection
   until it is sends or receives a "close" connection option or receives an
      optional field used only within the "message/external-body" media-
      type.

3.3.3.  Message Body Length

   The length of
   HTTP/1.0 response without a message body is determined by one of the following
   (in "keep-alive" connection option.

   In order of precedence):

   1.  Any response to remain persistent, all messages on a HEAD request and any response with connection need to
   have a 1xx
       (Informational), 204 (No Content), or 304 (Not Modified) status
       code is always terminated self-defined message length (i.e., one not defined by the first empty line after the
       header fields, regardless closure
   of the header fields present connection), as described in Section 6.  A server MUST read
   the
       message, and thus cannot contain a message body.

   2.  Any 2xx (Successful) response to a CONNECT entire request implies that message body or close the connection will become a tunnel immediately after sending
   its response, since otherwise the empty
       line that concludes remaining data on a persistent
   connection would be misinterpreted as the header fields.  A client MUST ignore any
       Content-Length or Transfer-Encoding header fields received in
       such a message.

   3.  If next request.  Likewise, a Transfer-Encoding header field is present and the chunked
       transfer coding (Section 4.1) is the final encoding,
   client MUST read the entire response message body length is determined by reading and decoding the chunked
       data until the transfer coding indicates if it intends to
   reuse the data is complete.

       If same connection for a Transfer-Encoding header field is present in subsequent request.

   A proxy server MUST NOT maintain a response persistent connection with an
   HTTP/1.0 client (see Section 19.7.1 of [RFC2068] for information and
   discussion of the chunked transfer coding is not the final encoding, problems with the
       message body length is determined Keep-Alive header field
   implemented by reading many HTTP/1.0 clients).

   See Appendix C.1.2 for more information on backwards compatibility
   with HTTP/1.0 clients.

9.3.1.  Retrying Requests

   Connections can be closed at any time, with or without intention.
   Implementations ought to anticipate the need to recover from
   asynchronous close events.

   When an inbound connection until
       it is closed by the server.  If prematurely, a Transfer-Encoding header field
       is present in client MAY open a request
   new connection and the chunked transfer coding is not
       the final encoding, the message body length cannot be determined
       reliably; the server automatically retransmit an aborted sequence of
   requests if all of those requests have idempotent methods
   (Section 7.2.2 of [Semantics]).  A proxy MUST respond with the 400 (Bad Request)
       status code and then close the connection.

       If NOT automatically retry
   non-idempotent requests.

   A user agent MUST NOT automatically retry a message is received request with both a Transfer-Encoding and a
       Content-Length header field, the Transfer-Encoding overrides the
       Content-Length.  Such a message might indicate an attempt non-
   idempotent method unless it has some means to
       perform know that the request smuggling (Section 9.5)
   semantics are actually idempotent, regardless of the method, or response splitting
       (Section 9.4) and ought some
   means to be handled as an error.  A sender MUST
       remove detect that the received Content-Length field prior to forwarding such
       a message downstream.

   4.  If original request was never applied.  For
   example, a message is received without Transfer-Encoding and with
       either multiple Content-Length header fields having differing
       field-values user agent that knows (through design or configuration)
   that a single Content-Length header field having an
       invalid value, then the message framing is invalid and the
       recipient MUST treat it as an unrecoverable error.  If this is POST request to a given resource is safe can repeat that
   request message, automatically.  Likewise, a user agent designed specifically
   to operate on a version control repository might be able to recover
   from partial failure conditions by checking the server MUST respond with target resource
   revision(s) after a 400 (Bad Request)
       status code failed connection, reverting or fixing any
   changes that were partially applied, and then close automatically retrying
   the connection.  If this is requests that failed.

   A client SHOULD NOT automatically retry a response
       message received by failed automatic retry.

9.3.2.  Pipelining

   A client that supports persistent connections MAY "pipeline" its
   requests (i.e., send multiple requests without waiting for each
   response).  A server MAY process a proxy, the proxy sequence of pipelined requests in
   parallel if they all have safe methods (Section 7.2.1 of
   [Semantics]), but it MUST close send the connection
       to corresponding responses in the server, discard
   same order that the received response, and send a 502 (Bad
       Gateway) response to the client.  If this is a response message
       received by a user agent, requests were received.

   A client that pipelines requests SHOULD retry unanswered requests if
   the user agent MUST close connection closes before it receives all of the corresponding
   responses.  When retrying pipelined requests after a failed
   connection to (a connection not explicitly closed by the server and discard the received response.

   5.  If a valid Content-Length header field is present without
       Transfer-Encoding, in its decimal value defines
   last complete response), a client MUST NOT pipeline immediately after
   connection establishment, since the expected message
       body length first remaining request in octets.  If the sender closes the
   prior pipeline might have caused an error response that can be lost
   again if multiple requests are sent on a prematurely closed
   connection or
       the recipient times out before (see the indicated number TCP reset problem described in Section 9.6).

   Idempotent methods (Section 7.2.2 of octets [Semantics]) are
       received, the recipient MUST consider the message significant to
   pipelining because they can be
       incomplete and close the connection.

   6.  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).

   7.  Otherwise, this is automatically retried after a response message without
   connection failure.  A user agent SHOULD NOT pipeline requests after
   a declared message
       body length, so non-idempotent method, until the message body length is determined by final response status code for
   that method has been received, unless the
       number of octets received prior user agent has a means to
   detect and recover from partial failure conditions involving the server closing the
       connection.

   Since there is no way
   pipelined sequence.

   An intermediary that receives pipelined requests MAY pipeline those
   requests when forwarding them inbound, since it can rely on the
   outbound user agent(s) to distinguish a successfully completed, close-
   delimited message from a partially received message interrupted by
   network failure, determine what requests can be safely
   pipelined.  If the inbound connection fails before receiving a server SHOULD generate encoding or length-
   delimited messages whenever possible.  The close-delimiting feature
   exists primarily for backwards compatibility with HTTP/1.0.

   A server
   response, the pipelining intermediary MAY reject a request that contains a message body but not a
   Content-Length by responding with 411 (Length Required).

   Unless a transfer coding other than chunked has been applied, attempt to retry a
   client sequence
   of requests that sends a request containing a message body SHOULD use have yet to receive a
   valid Content-Length header field response if the message body length is known
   in advance, rather than requests all
   have idempotent methods; otherwise, the chunked transfer coding, since some
   existing services respond to chunked with a 411 (Length Required)
   status code even though they understand pipelining intermediary
   SHOULD forward any received responses and then close the chunked transfer coding.
   This is typically because such services are implemented via a gateway
   corresponding outbound connection(s) so that requires a content-length in advance of being called and the
   server is unable or unwilling outbound user
   agent(s) can recover accordingly.

9.4.  Concurrency

   A client ought to buffer limit the entire request before
   processing.

   A user agent number of simultaneous open connections
   that sends it maintains to a request containing given server.

   Previous revisions of HTTP gave a message body MUST send specific number of connections as a valid Content-Length header field if it
   ceiling, but this was found to be impractical for many applications.
   As a result, this specification 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 mandate a prior received response.

   If the final response particular maximum
   number of connections but, instead, encourages clients to be
   conservative when opening multiple connections.

   Multiple connections are typically used to avoid the last request on "head-of-line
   blocking" problem, wherein a connection request that takes significant server-
   side processing and/or has been
   completely received and there remains additional data to read, a user
   agent MAY discard large payload blocks subsequent requests
   on the remaining data or attempt to determine if same connection.  However, each connection consumes server
   resources.  Furthermore, using multiple connections can cause
   undesirable side effects in congested networks.

   Note that
   data belongs a server might reject traffic that it deems abusive or
   characteristic of a denial-of-service attack, such as part an excessive
   number of the prior response body, open connections from a single client.

9.5.  Failures and Timeouts

   Servers will usually have some timeout 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
   case if same proxy server.  The use of
   persistent connections places no requirements on the length (or
   existence) of this timeout for either the prior message's Content-Length value is incorrect.  A client MUST NOT process, cache, or forward such extra data as a
   separate response, since such behavior would be vulnerable to cache
   poisoning.

3.4.  Handling Incomplete Messages the server.

   A client or server that receives an incomplete request message, usually due wishes to time out SHOULD issue a canceled request or a triggered timeout exception, MAY send an
   error response prior to closing graceful
   close on the connection.

   A client that receives an incomplete response message, which can
   occur when a connection is closed prematurely or when decoding a
   supposedly chunked transfer coding fails, MUST record the message as
   incomplete.  Cache requirements  Implementations SHOULD constantly monitor
   open connections for incomplete responses are defined
   in Section 3 of [CACHING].

   If a response terminates in the middle of the header section (before
   the empty line is received) received closure signal and the status code might rely on header
   fields respond to convey the full meaning it as
   appropriate, since prompt closure of both sides of a connection
   enables allocated system resources to be reclaimed.

   A client, server, or proxy MAY close the response, then the client
   cannot assume that meaning has been conveyed; the transport connection at any
   time.  For example, a client might need have started to repeat the send a new request in order to determine what action to take next.

   A message body that uses the chunked transfer coding is incomplete if
   at the zero-sized chunk same time that terminates the encoding server has not been
   received.  A message that uses a valid Content-Length is incomplete
   if decided to close the size "idle"
   connection.  From the server's point of view, the message body received (in octets) connection is less than being
   closed while it was idle, but from the
   value given by Content-Length.  A response that has neither chunked
   transfer coding nor Content-Length is terminated by closure client's point of the
   connection and, thus, view, a
   request is considered complete regardless of in progress.

   A server SHOULD sustain persistent connections, when possible, and
   allow the number
   of underlying transport's flow-control mechanisms to resolve
   temporary overloads, rather than terminate connections with the
   expectation that clients will retry.  The latter technique can
   exacerbate network congestion.

   A client sending a message body octets received, provided that SHOULD monitor the header section was
   received intact.

3.5.  Message Parsing Robustness

   Older HTTP/1.0 user agent implementations might send network connection
   for an extra CRLF
   after a POST request as error response while it is transmitting the request.  If the
   client sees a workaround for some early server
   applications that failed to read message body content response that was indicates the server does not
   terminated by a line-ending.  An HTTP/1.1 user agent MUST NOT preface
   or follow a request with an extra CRLF.  If terminating wish to
   receive the request message body with a line-ending and is desired, then closing the user agent MUST
   count connection, the terminating CRLF octets as part of client
   SHOULD immediately cease transmitting the message body length.

   In the interest and close its side of robustness,
   the connection.

9.6.  Tear-down

   The Connection header field (Section 9.1) provides a server "close"
   connection option that is expecting to receive
   and parse a request-line sender SHOULD ignore at least one empty line (CRLF)
   received prior send when it wishes to close
   the request-line.

   Although the line terminator for the start-line and header fields is connection after the sequence CRLF, a recipient MAY recognize current request/response pair.

   A client that sends a single LF as a line
   terminator and ignore any preceding CR.

   Although the request-line and status-line grammar rules require that
   each of the component elements be separated by a single SP octet,
   recipients MAY instead parse "close" connection option MUST NOT send further
   requests on whitespace-delimited word boundaries
   and, aside from the CRLF terminator, treat any form of whitespace as that connection (after the SP separator while ignoring preceding or trailing whitespace;
   such whitespace includes one or more of containing "close") and
   MUST close the following octets: SP,
   HTAB, VT (%x0B), FF (%x0C), or bare CR.  However, lenient parsing can
   result in security vulnerabilities if there are multiple recipients
   of connection after reading the final response message and each has its own unique interpretation of
   robustness (see Section 9.5).

   When a server listening only for HTTP request messages, or processing
   what appears from the start-line
   corresponding to be an HTTP request message, this request.

   A server that receives a sequence "close" connection option MUST initiate a
   close of octets that does not match the HTTP-message
   grammar aside from connection (see below) after it sends the robustness exceptions listed above, final response
   to the request that contained "close".  The server SHOULD respond with send a 400 (Bad Request) response.

4.  Transfer Codings

   Transfer coding names are used to indicate an encoding transformation
   "close" connection option in its final response on that has been, can be, or might need to be applied to connection.
   The server MUST NOT process any further requests received on that
   connection.

   A server that sends a payload body
   in order to ensure "safe transport" through "close" connection option MUST initiate a close
   of the network.  This
   differs from connection (see below) after it sends the response containing
   "close".  The server MUST NOT process any further requests received
   on that connection.

   A client that receives a content coding in "close" connection option MUST cease sending
   requests on that connection and close the transfer coding is a
   property of connection after reading
   the response message rather than a property of containing the representation "close"; if additional pipelined
   requests had been sent on the connection, the client SHOULD NOT
   assume that is being transferred.

     transfer-coding    = "chunked" ; Section 4.1
                        / "compress" ; Section 4.2.1
                        / "deflate" ; Section 4.2.2
                        / "gzip" ; Section 4.2.3
                        / transfer-extension
     transfer-extension = token *( OWS ";" OWS transfer-parameter )

   Parameters are in they will be processed by the form server.

   If a server performs an immediate close of a name or name=value pair.

     transfer-parameter = token BWS "=" BWS ( token / quoted-string )

   All transfer-coding names are case-insensitive and ought to TCP connection, there is
   a significant risk that the client will not be
   registered within able to read the last
   HTTP Transfer Coding registry, as defined in
   Section 8.4.  They are used in response.  If the TE (Section 4.3) and Transfer-
   Encoding (Section 3.3.1) header fields.

4.1.  Chunked Transfer Coding

   The chunked transfer coding wraps server receives additional data from the payload body in order to
   transfer it as
   client on a series of chunks, each with its own size indicator,
   followed by an OPTIONAL trailer containing header fields.  Chunked
   enables content streams of unknown size to be transferred fully closed connection, such as a
   sequence of length-delimited buffers, which enables another request that was
   sent by the sender client before receiving the server's response, the
   server's TCP stack will send a reset packet to
   retain connection persistence the client;
   unfortunately, the reset packet might erase the client's
   unacknowledged input buffers before they can be read and interpreted
   by the recipient to know when it has
   received client's HTTP parser.

   To avoid the entire message.

     chunked-body   = *chunk
                      last-chunk
                      trailer-part
                      CRLF

     chunk          = chunk-size [ chunk-ext ] CRLF
                      chunk-data CRLF
     chunk-size     = 1*HEXDIG
     last-chunk     = 1*("0") [ chunk-ext ] CRLF

     chunk-data     = 1*OCTET ; TCP reset problem, servers typically close a sequence connection
   in stages.  First, the server performs a half-close by closing only
   the write side of chunk-size octets the read/write connection.  The chunk-size field is server then
   continues to read from the connection until it receives a string of hex digits indicating
   corresponding close by the size client, or until the server is reasonably
   certain that its own TCP stack has received the client's
   acknowledgement of the chunk-data packet(s) containing the server's last
   response.  Finally, the server fully closes the connection.

   It is unknown whether the reset problem is exclusive to TCP or might
   also be found in octets. other transport connection protocols.

9.7.  Upgrade

   The chunked transfer coding "Upgrade" header field is complete
   when intended to provide a chunk with simple mechanism
   for transitioning from HTTP/1.1 to some other protocol on the same
   connection.  A client MAY send a chunk-size list of protocols in the Upgrade
   header field of zero is received, possibly followed
   by a trailer, and finally terminated by an empty line.

   A recipient MUST be able request to parse and decode invite the chunked transfer
   coding.

4.1.1.  Chunk Extensions

   The chunked encoding allows each chunk server to include zero switch to one or
   more chunk
   extensions, immediately following of those protocols, in order of descending preference, before
   sending the chunk-size, for final response.  A server MAY ignore a received Upgrade
   header field if it wishes to continue using the sake of
   supplying per-chunk metadata (such as current protocol on
   that connection.  Upgrade cannot be used to insist on a signature or hash), mid-
   message control information, or randomization of message body size.

     chunk-ext protocol
   change.

     Upgrade          = *( ";" chunk-ext-name [ "=" chunk-ext-val ] )

     chunk-ext-name 1#protocol

     protocol         = protocol-name ["/" protocol-version]
     protocol-name    = token
     chunk-ext-val
     protocol-version = token / quoted-string

   The chunked encoding is specific

   A server that sends a 101 (Switching Protocols) response MUST send an
   Upgrade header field to each indicate the new protocol(s) to which the
   connection and is likely being switched; if multiple protocol layers are being
   switched, the sender MUST list the protocols in layer-ascending
   order.  A server MUST NOT switch to
   be removed or recoded by each recipient (including intermediaries)
   before any higher-level application would have a chance to inspect protocol that was not indicated
   by the extensions.  Hence, use of chunk extensions is generally limited
   to specialized HTTP services such as "long polling" (where client and
   server can have shared expectations regarding in the use of chunk
   extensions) or for padding within an end-to-end secured connection.

   A recipient MUST ignore unrecognized chunk extensions. corresponding request's Upgrade header field.  A
   server
   ought MAY choose to limit ignore the total length order of chunk extensions received in a
   request to an amount reasonable for preference indicated by the services provided, in
   client and select the
   same way that it applies length limitations and timeouts for new protocol(s) based on other
   parts factors, such as
   the nature of the request or the current load on the server.

   A server that sends a message, and generate an appropriate 4xx (Client Error) 426 (Upgrade Required) response if that amount is exceeded.

4.1.2.  Chunked Trailer Part

   A trailer allows the sender MUST send an
   Upgrade header field to include additional fields at indicate the end
   of a chunked message acceptable protocols, in order
   of descending preference.

   A server MAY send an Upgrade header field in any other response to supply metadata
   advertise that might be
   dynamically generated while the message body is sent, such as a
   message integrity check, digital signature, or post-processing
   status.  The trailer fields are identical it implements support for upgrading to header fields, except
   they are sent the listed
   protocols, in a chunked trailer instead order of the message's header
   section.

     trailer-part   = *( header-field CRLF )

   A sender MUST NOT generate descending preference, when appropriate for a trailer that contains
   future request.

   The following is a field necessary
   for message framing (e.g., Transfer-Encoding and Content-Length),
   routing (e.g., Host), request modifiers (e.g., controls and
   conditionals in Section 5 of [SEMNTCS]), authentication (e.g., see
   [AUTHFRM] hypothetical example sent by a client:

     GET /hello.txt HTTP/1.1
     Host: www.example.com
     Connection: upgrade
     Upgrade: HTTP/2.0, SHTTP/1.3, IRC/6.9, RTA/x11

   The capabilities and [RFC6265]), response control data (e.g., see
   Section 7.1 nature of [SEMNTCS]), or determining how to process the payload
   (e.g., Content-Encoding, Content-Type, Content-Range, and Trailer).

   When a chunked message containing a non-empty trailer is received, application-level communication
   after the recipient MAY process protocol change is entirely dependent upon the fields (aside from those forbidden
   above) as if they were appended new
   protocol(s) chosen.  However, immediately after sending the 101
   (Switching Protocols) response, the server is expected to continue
   responding to the message's header section.  A
   recipient MUST ignore (or consider original request as if it had received its
   equivalent within the new protocol (i.e., the server still has an error) any fields that are
   forbidden
   outstanding request to be sent in a trailer, since processing them as if they
   were present in satisfy after the header section might bypass external security
   filters.

   Unless protocol has been changed,
   and is expected to do so without requiring the request includes a TE to be
   repeated).

   For example, if the Upgrade header field indicating "trailers" is acceptable, as described received in Section 4.3, a server SHOULD NOT
   generate trailer fields that it believes are necessary for the user
   agent to receive.  Without a TE containing "trailers", GET request
   and the server
   ought decides to assume switch protocols, it first responds with a
   101 (Switching Protocols) message in HTTP/1.1 and then immediately
   follows that with the trailer fields might be silently discarded
   along the path new protocol's equivalent of a response to a
   GET on the user agent. target resource.  This requirement allows
   intermediaries to forward a de-chunked message connection to an HTTP/1.0
   recipient be upgraded
   to protocols with the same semantics as HTTP without buffering the entire response.

4.1.3.  Decoding Chunked latency cost
   of an additional round trip.  A process for decoding server MUST NOT switch protocols
   unless the chunked transfer coding received message semantics can be represented
   in pseudo-code as:

     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, chunk-ext (if any), and CRLF
     }
     read trailer field
     while (trailer field is not empty) {
        if (trailer field is allowed to be sent in a trailer) {
            append trailer field to existing header fields
        }
        read trailer-field
     }
     Content-Length := length
     Remove "chunked" from Transfer-Encoding
     Remove Trailer from existing header fields

4.2.  Compression Codings

   The codings defined below honored by the new
   protocol; an OPTIONS request can be used to compress the payload of a
   message.

4.2.1.  Compress Coding honored by any protocol.

   The "compress" coding following is an adaptive Lempel-Ziv-Welch (LZW) coding
   [Welch] that is commonly produced by example response to the UNIX file compression
   program "compress".  A recipient SHOULD consider "x-compress" above hypothetical
   request:

     HTTP/1.1 101 Switching Protocols
     Connection: upgrade
     Upgrade: HTTP/2.0

     [... data stream switches to be
   equivalent HTTP/2.0 with an appropriate response
     (as defined by new protocol) to "compress".

4.2.2.  Deflate Coding

   The "deflate" coding the "GET /hello.txt" request ...]

   When Upgrade is a "zlib" data format [RFC1950] containing a
   "deflate" compressed data stream [RFC1951] that uses a combination of sent, the Lempel-Ziv (LZ77) compression algorithm and Huffman coding.

      Note: Some non-conformant implementations sender MUST also send the "deflate"
      compressed data without the zlib wrapper.

4.2.3.  Gzip Coding

   The "gzip" coding is an LZ77 coding with a 32-bit Cyclic Redundancy
   Check (CRC) Connection header
   field (Section 9.1) that is commonly produced contains an "upgrade" connection option, in
   order to prevent Upgrade from being accidentally forwarded by
   intermediaries that might not implement the gzip file compression
   program [RFC1952]. listed protocols.  A recipient SHOULD consider "x-gzip" to be
   equivalent to "gzip".

4.3.  TE

   The "TE"
   server MUST ignore an Upgrade header field in a request indicates what transfer codings,
   besides chunked, the client that is willing to accept received in response, and
   whether or not the an
   HTTP/1.0 request.

   A client is willing to accept trailer fields cannot begin using an upgraded protocol on the connection
   until it has completely sent the request message (i.e., the client
   can't change the protocol it is sending in a
   chunked transfer coding.

   The TE field-value consists the middle of a comma-separated list message).
   If a server receives both an Upgrade and an Expect header field with
   the "100-continue" expectation (Section 8.1.1 of transfer
   coding names, each allowing for optional parameters (as described in
   Section 4), and/or [Semantics]), the keyword "trailers".  A client
   server MUST NOT send
   the chunked transfer coding name in TE; chunked is always acceptable
   for HTTP/1.1 recipients.

     TE        = #t-codings
     t-codings = "trailers" / ( transfer-coding [ t-ranking ] )
     t-ranking = OWS ";" OWS "q=" rank
     rank      = ( "0" [ "." 0*3DIGIT ] )
                / ( "1" [ "." 0*3("0") ] )

   Three examples of TE use are below.

     TE: deflate
     TE:
     TE: trailers, deflate;q=0.5 a 100 (Continue) response before sending a 101
   (Switching Protocols) response.

   The presence Upgrade header field only applies to switching protocols on top
   of the keyword "trailers" indicates that existing connection; it cannot be used to switch the client
   underlying connection (transport) protocol, nor to switch the
   existing communication to a different connection.  For those
   purposes, it is
   willing more appropriate to accept trailer fields in use a chunked transfer coding, 3xx (Redirection) response
   (Section 9.4 of [Semantics]).

9.7.1.  Upgrade Protocol Names

   This specification only defines the protocol name "HTTP" for use by
   the family of Hypertext Transfer Protocols, as defined in by the HTTP
   version rules of Section 4.1.2, on behalf 3.5 of itself [Semantics] and any downstream
   clients.  For requests from an intermediary, future updates to
   this implies that
   either: (a) all downstream clients are willing specification.  Additional protocol names ought to accept trailer
   fields in be registered
   using the forwarded response; or, (b) registration procedure defined in Section 9.7.2.

   +------+-------------------+--------------------+-------------------+
   | Name | Description       | Expected Version   | Reference         |
   |      |                   | Tokens             |                   |
   +------+-------------------+--------------------+-------------------+
   | HTTP | Hypertext         | any DIGIT.DIGIT    | Section 3.5 of    |
   |      | Transfer Protocol | (e.g, "2.0")       | [Semantics]       |
   +------+-------------------+--------------------+-------------------+

9.7.2.  Upgrade Token Registry

   The "Hypertext Transfer Protocol (HTTP) Upgrade Token Registry"
   defines the intermediary will
   attempt namespace for protocol-name tokens used to buffer identify
   protocols in the response on behalf Upgrade header field.  The registry is maintained at
   <https://www.iana.org/assignments/http-upgrade-tokens>.

   Each registered protocol name is associated with contact information
   and an optional set of downstream recipients.
   Note specifications that HTTP/1.1 does not define any means to limit details how the size of a
   chunked response such that an intermediary can connection
   will be assured processed after it has been upgraded.

   Registrations happen on a "First Come First Served" basis (see
   Section 4.1 of
   buffering the entire response.

   When multiple transfer codings [RFC5226]) and are acceptable, the client MAY rank
   the codings by preference using a case-insensitive "q" parameter
   (similar subject to the qvalues used in content negotiation fields,
   Section 5.3.1 of [SEMNTCS]). following rules:

   1.  A protocol-name token, once registered, stays registered forever.

   2.  The rank value is registration MUST name a real number in the
   range 0 through 1, where 0.001 is the least preferred and 1 is responsible party for the
   most preferred;
       registration.

   3.  The registration MUST name a value point of 0 means "not acceptable".

   If the TE field-value is empty or if no TE field is present, the only
   acceptable transfer coding is chunked.  A message with no transfer
   coding is always acceptable.

   Since the TE header field only applies to the immediate connection, contact.

   4.  The registration MAY name a
   sender set of TE MUST also send a "TE" connection option within the
   Connection header field (Section 6.1) in order to prevent the TE
   field from being forwarded by intermediaries specifications associated with
       that do token.  Such specifications need not support its
   semantics.

4.4.  Trailer

   When a message includes be publicly available.

   5.  The registration SHOULD name a message body encoded with the chunked
   transfer coding and the sender desires to send metadata in the form set of trailer fields expected "protocol-version"
       tokens associated with that token at the end time of registration.

   6.  The responsible party MAY change the message, the sender SHOULD
   generate registration at any time.
       The IANA will keep a Trailer header field before the message body to indicate
   which fields record of all such changes, and make them
       available upon request.

   7.  The IESG MAY reassign responsibility for a protocol token.  This
       will normally only be present used in the trailers.  This allows the
   recipient case when a responsible party
       cannot be contacted.

10.  Enclosing Messages as Data

10.1.  Media Type message/http

   The message/http media type can be used to prepare for receipt of enclose a single HTTP
   request or response message, provided that metadata before it starts
   processing the body, which is useful if obeys the message is being streamed MIME
   restrictions for all "message" types regarding line length and the recipient wishes to confirm an integrity check on the fly.

     Trailer = 1#field-name

5.  Message Routing

   HTTP request
   encodings.

   Type name:  message routing is determined by each client based on
   the target resource, the client's proxy configuration, and
   establishment or reuse of an inbound connection.

   Subtype name:  http

   Required parameters:  N/A

   Optional parameters:  version, msgtype

      version:  The corresponding
   response routing follows HTTP-version number of the same connection chain back to enclosed message (e.g.,
         "1.1").  If not present, the
   client.

5.1.  Identifying a Target Resource

   HTTP is used in a wide variety of applications, ranging version can be determined from general-
   purpose computers to home appliances.  In some cases, communication
   options are hard-coded in a client's configuration.  However, most
   HTTP clients rely on the same resource identification mechanism and
   configuration techniques as general-purpose Web browsers.

   HTTP communication is initiated by a user agent for some purpose.
   The purpose is a combination
         first line of request semantics, which are defined
   in [SEMNTCS], and a target resource upon which to apply those
   semantics.  A URI reference (Section 2.7) is typically used as an
   identifier for the "target resource", which a user agent would
   resolve to its absolute form in order to obtain the "target URI". body.

      msgtype:  The target URI excludes the reference's fragment component, if any,
   since fragment identifiers are reserved for client-side processing
   ([RFC3986], Section 3.5).

5.2.  Connecting Inbound

   Once the target URI is determined, a client needs to decide whether a
   network request is necessary to accomplish the desired semantics and,
   if so, where that request is to be directed. message type -- "request" or "response".  If not
         present, the client has a cache [CACHING] and the request type can be satisfied
   by it, then the request is usually directed there first.

   If determined from the request is not satisfied by a cache, then a typical client
   will check its configuration to determine whether a proxy is to be
   used to satisfy first line of the request.  Proxy configuration is implementation-
   dependent, but is often based on URI prefix matching, selective
   authority matching, or both, and the proxy itself is usually
   identified by an "http"
         body.

   Encoding considerations:  only "7bit", "8bit", or "https" URI.  If a proxy is applicable,
   the client connects inbound by establishing (or reusing) a connection
   to "binary" are
      permitted

   Security considerations:  see Section 11

   Interoperability considerations:  N/A

   Published specification:  This specification (see Section 10.1).

   Applications that proxy.

   If no proxy is applicable, a typical client will invoke a handler
   routine, usually specific to the target URI's scheme, to connect
   directly to an authority use this media type:  N/A

   Fragment identifier considerations:  N/A

   Additional information:

      Magic number(s):  N/A

      Deprecated alias names for the target resource.  How that is
   accomplished is dependent on the target URI scheme this type:  N/A

      File extension(s):  N/A

      Macintosh file type code(s):  N/A

   Person and defined by its
   associated specification, similar email address to how this specification defines
   origin server access contact for resolution further information:
      See Authors' Addresses section.

   Intended usage:  COMMON

   Restrictions on usage:  N/A

   Author:  See Authors' Addresses section.

   Change controller:  IESG

10.2.  Media Type application/http

   The application/http media type can be used to enclose a pipeline of the "http" (Section 2.7.1) and
   "https" (Section 2.7.2) schemes.

   HTTP requirements regarding connection management are defined in
   Section 6.

5.3.  Request Target

   Once an inbound connection is obtained, the client sends an
   one or more HTTP request message (Section 3) with a request-target derived from or response messages (not intermixed).

   Type name:  application

   Subtype name:  http

   Required parameters:  N/A

   Optional parameters:  version, msgtype

      version:  The HTTP-version number of the
   target URI.  There are four distinct formats for enclosed messages (e.g.,
         "1.1").  If not present, the request-target,
   depending on both version can be determined from the method being requested and whether
         first line of the request
   is to a proxy.

     request-target = origin-form
                    / absolute-form
                    / authority-form
                    / asterisk-form

5.3.1.  origin-form body.

      msgtype:  The most common form of request-target is the origin-form.

     origin-form    = absolute-path [ "?" query ]

   When making a request directly to an origin server, other than a
   CONNECT message type -- "request" or server-wide OPTIONS request (as detailed below), a client
   MUST send only the absolute path and query components of the target
   URI as the request-target. "response".  If not
         present, the target URI's path component is
   empty, the client MUST send "/" as type can be determined from the path within first line of the origin-form
         body.

   Encoding considerations:  HTTP messages enclosed by this type are in
      "binary" format; use of
   request-target.  A Host header field an appropriate Content-Transfer-Encoding
      is also sent, as defined in required when transmitted via email.

   Security considerations:  see Section 5.4.

   For example, a client wishing 11

   Interoperability considerations:  N/A
   Published specification:  This specification (see Section 10.2).

   Applications that use this media type:  N/A

   Fragment identifier considerations:  N/A

   Additional information:

      Deprecated alias names for this type:  N/A

      Magic number(s):  N/A

      File extension(s):  N/A

      Macintosh file type code(s):  N/A

   Person and email address to retrieve a representation of the
   resource identified as

     http://www.example.org/where?q=now

   directly from the origin server would open (or reuse) a TCP
   connection contact for further information:
      See Authors' Addresses section.

   Intended usage:  COMMON

   Restrictions on usage:  N/A

   Author:  See Authors' Addresses section.

   Change controller:  IESG

11.  Security Considerations

   This section is meant to port 80 inform developers, information providers,
   and users of the host "www.example.org" known security considerations relevant to HTTP message
   syntax, parsing, and send the
   lines:

     GET /where?q=now HTTP/1.1
     Host: www.example.org

   followed by routing.  Security considerations about HTTP
   semantics and payloads are addressed in [Semantics].

11.1.  Response Splitting

   Response splitting (a.k.a, CRLF injection) is a common technique,
   used in various attacks on Web usage, that exploits the remainder line-based
   nature of HTTP message framing and the request message.

5.3.2.  absolute-form

   When making a request ordered association of
   requests to responses on persistent connections [Klein].  This
   technique can be particularly damaging when the requests pass through
   a proxy, other than a CONNECT or server-wide
   OPTIONS request (as detailed below), shared cache.

   Response splitting exploits a client MUST vulnerability in servers (usually
   within an application server) where an attacker can send encoded data
   within some parameter of the target
   URI in absolute-form as request that is later decoded and echoed
   within any of the request-target.

     absolute-form  = absolute-URI

   The proxy response header fields of the response.  If the
   decoded data is requested crafted to either service that request from a valid
   cache, if possible, or make look like the same request on response has ended and a
   subsequent response has begun, the client's behalf
   to either response has been split and the next inbound proxy server or directly to
   content within the origin
   server indicated apparent second response is controlled by the request-target.  Requirements
   attacker.  The attacker can then make any other request on such
   "forwarding" of messages are defined in Section 5.7.

   An example absolute-form the same
   persistent connection and trick the recipients (including
   intermediaries) into believing that the second half of request-line would be:

     GET http://www.example.org/pub/WWW/TheProject.html HTTP/1.1

   To allow for transition the split is
   an authoritative answer to the absolute-form for all requests in some
   future version of HTTP, second request.

   For example, a server MUST accept parameter within the absolute-form request-target might be read by
   an application server and reused within a redirect, resulting in
   requests, even though HTTP/1.1 clients will only send them the
   same parameter being echoed in
   requests to proxies.

5.3.3.  authority-form

   The authority-form the Location header field of request-target the
   response.  If the parameter is only used for CONNECT
   requests (Section 4.3.6 of [SEMNTCS]).

     authority-form = authority

   When making a CONNECT request to establish a tunnel through one or
   more proxies, a client MUST send only decoded by the target URI's authority
   component (excluding any userinfo application and its "@" delimiter) as not
   properly encoded when placed in the
   request-target.  For example,

     CONNECT www.example.com:80 HTTP/1.1

5.3.4.  asterisk-form

   The asterisk-form of request-target response field, the attacker can
   send encoded CRLF octets and other content that will make the
   application's single response look like two or more responses.

   A common defense against response splitting is only used for a server-wide
   OPTIONS request (Section 4.3.7 of [SEMNTCS]).

     asterisk-form  = "*"

   When a client wishes to request OPTIONS filter requests for
   data that looks like encoded CR and LF (e.g., "%0D" and "%0A").
   However, that assumes the application server as a whole, as
   opposed is only performing URI
   decoding, rather than more obscure data transformations like charset
   transcoding, XML entity translation, base64 decoding, sprintf
   reformatting, etc.  A more effective mitigation is to prevent
   anything other than the server's core protocol libraries from sending
   a specific named resource of that server, CR or LF within the client MUST
   send only "*" (%x2A) as header section, which means restricting the request-target.  For example,

     OPTIONS * HTTP/1.1

   If
   output of header fields to APIs that filter for bad octets and not
   allowing application servers to write directly to the protocol
   stream.

11.2.  Request Smuggling

   Request smuggling ([Linhart]) is a proxy receives technique that exploits
   differences in protocol parsing among various recipients to hide
   additional requests (which might otherwise be blocked or disabled by
   policy) within an OPTIONS apparently harmless request.  Like response
   splitting, request with an absolute-form smuggling can lead to a variety of
   request-target in which the URI attacks on HTTP
   usage.

   This specification has an empty path and no query
   component, then the last proxy introduced new requirements on request
   parsing, particularly with regard to message framing in Section 6.3,
   to reduce the effectiveness of request chain MUST send smuggling.

11.3.  Message Integrity

   HTTP does not define a
   request-target specific mechanism for ensuring message
   integrity, instead relying on the error-detection ability of "*" when it forwards
   underlying transport protocols and the request use of length or chunk-
   delimited framing to detect completeness.  Additional integrity
   mechanisms, such as hash functions or digital signatures applied to
   the indicated
   origin server.

   For example, the request

     OPTIONS http://www.example.org:8001 HTTP/1.1

   would content, can be forwarded by the final proxy as

     OPTIONS * HTTP/1.1
     Host: www.example.org:8001

   after connecting selectively added to port 8001 of host "www.example.org".

5.4.  Host

   The "Host" messages via extensible
   metadata header field in a request provides the host and port
   information from the target URI, enabling fields.  Historically, the origin server to
   distinguish among resources while servicing requests for multiple
   host names on lack of a single IP address.

     Host = uri-host [ ":" port ] ; Section 2.7.1

   A client MUST send a Host header field in all HTTP/1.1 request
   messages.  If integrity
   mechanism has been justified by the target URI includes informal nature of most HTTP
   communication.  However, the prevalence of HTTP as an authority component, then a
   client MUST send a field-value for Host that information
   access mechanism has resulted in its increasing use within
   environments where verification of message integrity is identical crucial.

   User agents are encouraged to that
   authority component, excluding any userinfo subcomponent implement configurable means for
   detecting and its "@"
   delimiter (Section 2.7.1).  If the authority component is missing or
   undefined reporting failures of message integrity such that those
   means can be enabled within environments for the target URI, then a client MUST send a Host header
   field with an empty field-value.

   Since the Host field-value which integrity is critical information for handling a
   request, a user agent SHOULD generate Host as the first header field
   following the request-line.
   necessary.  For example, a GET request browser being used to view medical history
   or drug interaction information needs to indicate to the origin server for
   <http://www.example.org/pub/WWW/> would begin with:

     GET /pub/WWW/ HTTP/1.1
     Host: www.example.org

   A client MUST send a Host header field in an HTTP/1.1 request even if
   the request-target user when
   such information is in the absolute-form, since this allows detected by the
   Host information protocol to be forwarded through ancient HTTP/1.0 proxies
   that incomplete,
   expired, or corrupted during transfer.  Such mechanisms might not have implemented Host.

   When a proxy receives a request with an absolute-form of request-
   target, the proxy MUST ignore the received Host header field (if any)
   and instead replace it with be
   selectively enabled via user agent extensions or the host information presence of the request-
   target.  A proxy that forwards such
   message integrity metadata in a request MUST generate a new
   Host field-value based on the received request-target rather than
   forward the received Host field-value.

   Since the Host header field acts as an application-level routing
   mechanism, it is response.  At a frequent target for malware seeking minimum, user agents
   ought to poison a
   shared cache or redirect provide some indication that allows a request user to an unintended server.  An
   interception proxy distinguish
   between a complete and incomplete response message (Section 8) when
   such verification is particularly vulnerable if it desired.

11.4.  Message Confidentiality

   HTTP relies on the
   Host 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.

   A server MUST respond with a 400 (Bad Request) status code underlying transport protocols to any
   HTTP/1.1 request provide message
   confidentiality when that lacks a Host header field and is desired.  HTTP has been specifically
   designed to any
   request message be independent of the transport protocol, such that contains more than one Host header field or a
   Host header field it
   can be used over many different forms of encrypted connection, with an invalid field-value.

5.5.  Effective Request URI

   Since
   the request-target often contains only part selection of such transports being identified by the choice of
   URI scheme or within user agent's
   target URI, agent configuration.

   The "https" scheme can be used to identify resources that require a server reconstructs the intended target
   confidential connection, as an
   "effective request URI" to properly service the request.  This
   reconstruction involves both the server's local configuration and
   information communicated described in the request-target, Host header field,
   and connection context.

   For a user agent, the effective request URI Section 2.5.2 of
   [Semantics].

12.  IANA Considerations

   This section is the target URI.

   If the request-target is in absolute-form, the effective request URI
   is the same as the request-target.  Otherwise, the effective request
   URI is constructed to be removed before publishing as follows:

      If the server's configuration (or outbound gateway) provides a
      fixed URI scheme, that scheme is used an RFC.

   The change controller for the effective request
      URI.  Otherwise, if the request is received over a TLS-secured TCP
      connection, following registrations is: "IETF
   (iesg@ietf.org) - Internet Engineering Task Force".

12.1.  Header Field Registration

   Please update the effective request URI's scheme is "https"; if not, "Message Headers" registry of "Permanent Message
   Header Field Names" at <https://www.iana.org/assignments/message-
   headers> with the scheme is "http".

      If header field names listed in the server's configuration (or outbound gateway) provides a
      fixed URI authority component, that authority is used for two tables of
   Section 5.

12.2.  Media Type Registration

   Please update the
      effective request URI.  If not, then if "Media Types" registry at
   <https://www.iana.org/assignments/media-types> with the request-target is registration
   information in
      authority-form, the effective request URI's authority component is Section 10.1 and Section 10.2 for the same as media types
   "message/http" and "application/http", respectively.

12.3.  Transfer Coding Registration

   Please update the request-target.  If not, then if a Host header
      field is supplied "HTTP Transfer Coding Registry" at
   <https://www.iana.org/assignments/http-parameters/> with a non-empty field-value, the authority
      component is the same as
   registration procedure of Section 7.3 and the Host field-value.  Otherwise, content coding names
   summarized in the
      authority component is assigned table of Section 7.

12.4.  Upgrade Token Registration

   Please update the default name configured for "Hypertext Transfer Protocol (HTTP) Upgrade Token
   Registry" at <https://www.iana.org/assignments/http-upgrade-tokens>
   with the server and, if registration procedure of Section 9.7.2 and the connection's incoming TCP port number
      differs from upgrade
   token names summarized in the default port for the effective request URI's
      scheme, then a colon (":") table of Section 9.7.1.

13.  References

13.1.  Normative References

   [Caching]  Fielding, R., Ed., Nottingham, M., Ed., and the incoming port number (in
      decimal form) are appended to the authority component.

      If the request-target is J. Reschke,
              Ed., "HTTP Caching", draft-ietf-httpbis-cache-01 (work in authority-form or asterisk-form, the
      effective request URI's combined path and query component is
      empty.  Otherwise, the combined path and query component is the
      same as the request-target.

      The components of the effective request URI, once determined as
      above, can be combined into absolute-URI form by concatenating the
      scheme, "://", authority,
              progress), May 2018.

   [RFC1950]  Deutsch, L. and combined path J-L. Gailly, "ZLIB Compressed Data Format
              Specification version 3.3", RFC 1950,
              DOI 10.17487/RFC1950, May 1996,
              <https://www.rfc-editor.org/info/rfc1950>.

   [RFC1951]  Deutsch, P., "DEFLATE Compressed Data Format Specification
              version 1.3", RFC 1951, DOI 10.17487/RFC1951, May 1996,
              <https://www.rfc-editor.org/info/rfc1951>.

   [RFC1952]  Deutsch, P., Gailly, J-L., Adler, M., Deutsch, L., and query component.

   Example 1: the following message received over an insecure TCP
   connection

     GET /pub/WWW/TheProject.html HTTP/1.1
     Host: www.example.org:8080

   has an effective request URI of

     http://www.example.org:8080/pub/WWW/TheProject.html

   Example 2: the following message received over a TLS-secured TCP
   connection

     OPTIONS * HTTP/1.1
     Host: www.example.org

   has an effective request URI of

     https://www.example.org

   Recipients of an HTTP/1.0 request that lacks a Host header field
   might need to use heuristics (e.g., examination of the URI path G.
              Randers-Pehrson, "GZIP file format specification version
              4.3", RFC 1952, DOI 10.17487/RFC1952, May 1996,
              <https://www.rfc-editor.org/info/rfc1952>.

   [RFC2119]  Bradner, S., "Key words for
   something unique to a particular host) use in order RFCs to guess the
   effective request URI's authority component.

   Once the effective request URI has been constructed, an origin server
   needs to decide whether or not to provide service Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <https://www.rfc-editor.org/info/rfc2119>.

   [RFC3986]  Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
              Resource Identifier (URI): Generic Syntax", STD 66,
              RFC 3986, DOI 10.17487/RFC3986, January 2005,
              <https://www.rfc-editor.org/info/rfc3986>.

   [RFC5234]  Crocker, D., Ed. and P. Overell, "Augmented BNF for that URI via
   the connection Syntax
              Specifications: ABNF", STD 68, RFC 5234,
              DOI 10.17487/RFC5234, January 2008,
              <https://www.rfc-editor.org/info/rfc5234>.

   [Semantics]
              Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke,
              Ed., "HTTP Semantics", draft-ietf-httpbis-semantics-01
              (work in which the request was received.  For example, the
   request might have been misdirected, deliberately or accidentally,
   such that the information within a received request-target or Host
   header field differs from the host or port upon which the connection
   has been made.  If the connection is from a trusted gateway, that
   inconsistency might be expected; otherwise, it might indicate an
   attempt to bypass security filters, trick the server into delivering
   non-public content, or poison a cache.  See Section 9 progress), May 2018.

   [USASCII]  American National Standards Institute, "Coded Character
              Set -- 7-bit American Standard Code for security
   considerations regarding message routing.

5.6.  Associating a Information
              Interchange", ANSI X3.4, 1986.

   [Welch]    Welch, T., "A Technique for High-Performance Data
              Compression", IEEE Computer 17(6), June 1984.

13.2.  Informative References

   [Klein]    Klein, A., "Divide and Conquer - HTTP Response to a Splitting,
              Web Cache Poisoning Attacks, and Related Topics", March
              2004, <http://packetstormsecurity.com/papers/general/
              whitepaper_httpresponse.pdf>.

   [Linhart]  Linhart, C., Klein, A., Heled, R., and S. Orrin, "HTTP
              Request

   HTTP does not include a request identifier for associating a given
   request message with its corresponding one or more response messages.
   Hence, it relies on the order of response arrival to correspond
   exactly to the order in which requests are made on the same
   connection.  More than one response message per request only occurs
   when one or more informational responses (1xx, see Section 6.2 of
   [SEMNTCS]) precede a final response to the same request.

   A client that has more than one outstanding request on a connection
   MUST maintain a list of outstanding requests in the order sent Smuggling", June 2005,
              <http://www.watchfire.com/news/whitepapers.aspx>.

   [RFC1945]  Berners-Lee, T., Fielding, R., and
   MUST associate each received response message on that connection to
   the highest ordered request that has not yet received a final (non-
   1xx) response.

5.7.  Message Forwarding

   As described in Section 2.3, intermediaries can serve a variety H. Nielsen, "Hypertext
              Transfer Protocol -- HTTP/1.0", RFC 1945,
              DOI 10.17487/RFC1945, May 1996,
              <https://www.rfc-editor.org/info/rfc1945>.

   [RFC2045]  Freed, N. and N. Borenstein, "Multipurpose Internet Mail
              Extensions (MIME) Part One: Format of
   roles in the processing Internet Message
              Bodies", RFC 2045, DOI 10.17487/RFC2045, November 1996,
              <https://www.rfc-editor.org/info/rfc2045>.

   [RFC2046]  Freed, N. and N. Borenstein, "Multipurpose Internet Mail
              Extensions (MIME) Part Two: Media Types", RFC 2046,
              DOI 10.17487/RFC2046, November 1996,
              <https://www.rfc-editor.org/info/rfc2046>.

   [RFC2049]  Freed, N. and N. Borenstein, "Multipurpose Internet Mail
              Extensions (MIME) Part Five: Conformance Criteria and
              Examples", RFC 2049, DOI 10.17487/RFC2049, November 1996,
              <https://www.rfc-editor.org/info/rfc2049>.

   [RFC2068]  Fielding, R., Gettys, J., Mogul, J., Nielsen, H., and T.
              Berners-Lee, "Hypertext Transfer Protocol -- HTTP/1.1",
              RFC 2068, DOI 10.17487/RFC2068, January 1997,
              <https://www.rfc-editor.org/info/rfc2068>.

   [RFC2557]  Palme, F., Hopmann, A., Shelness, N., and E. Stefferud,
              "MIME Encapsulation of HTTP requests Aggregate Documents, such as HTML
              (MHTML)", RFC 2557, DOI 10.17487/RFC2557, March 1999,
              <https://www.rfc-editor.org/info/rfc2557>.

   [RFC5226]  Narten, T. and responses.  Some
   intermediaries are used to improve performance or availability.
   Others are used H. Alvestrand, "Guidelines for access control or to filter content.  Since an
   HTTP stream has characteristics similar to a pipe-and-filter
   architecture, there are no inherent limits to the extent Writing an
   intermediary can enhance (or interfere) with either direction of the
   stream.

   An intermediary not acting as a tunnel MUST implement the Connection
   header field, as specified in
              IANA Considerations Section 6.1, and exclude fields from
   being forwarded that are only intended for the incoming connection.

   An intermediary MUST NOT forward a message to itself unless it is
   protected from an infinite request loop.  In general, an intermediary
   ought to recognize its own server names, including any aliases, local
   variations, or literal IP addresses, and respond to such requests
   directly.

5.7.1.  Via

   The "Via" header field indicates the presence of intermediate
   protocols in RFCs", BCP 26, RFC 5226,
              DOI 10.17487/RFC5226, May 2008,
              <https://www.rfc-editor.org/info/rfc5226>.

   [RFC5322]  Resnick, P., "Internet Message Format", RFC 5322,
              DOI 10.17487/RFC5322, October 2008,
              <https://www.rfc-editor.org/info/rfc5322>.

   [RFC6265]  Barth, A., "HTTP State Management Mechanism", RFC 6265,
              DOI 10.17487/RFC6265, April 2011,
              <https://www.rfc-editor.org/info/rfc6265>.

   [RFC7230]  Fielding, R., Ed. and recipients between the user agent J. Reschke, Ed., "Hypertext Transfer
              Protocol (HTTP/1.1): Message Syntax and the server (on
   requests) or between the origin server Routing",
              RFC 7230, DOI 10.17487/RFC7230, June 2014,
              <https://www.rfc-editor.org/info/rfc7230>.

   [RFC7231]  Fielding, R., Ed. and the client (on responses),
   similar to the "Received" header field in email (Section 3.6.7 of
   [RFC5322]).  Via can be used for tracking message forwards, avoiding
   request loops, J. Reschke, Ed., "Hypertext Transfer
              Protocol (HTTP/1.1): Semantics and identifying Content", RFC 7231,
              DOI 10.17487/RFC7231, June 2014,
              <https://www.rfc-editor.org/info/rfc7231>.

Appendix A.  Collected ABNF

   In the protocol capabilities collected ABNF below, list rules are expanded as per
   Section 11 of senders
   along the request/response chain.

     Via [Semantics].

   BWS = 1#( received-protocol RWS received-by <BWS, see [Semantics], Section 4.3>

   Connection = *( "," OWS ) connection-option *( OWS "," [ RWS comment OWS
    connection-option ] )

     received-protocol

   HTTP-message = start-line *( header-field CRLF ) CRLF [ protocol-name "/" message-body
    ] protocol-version
   HTTP-name = %x48.54.54.50 ; HTTP
   HTTP-version = HTTP-name "/" DIGIT "." DIGIT

   OWS = <OWS, see [Semantics], Section 6.7
     received-by 4.3>

   RWS = <RWS, see [Semantics], Section 4.3>

   TE = [ ( uri-host "," / t-codings ) *( OWS "," [ ":" port OWS t-codings ] ) / pseudonym
     pseudonym ]
   Transfer-Encoding = token

   Multiple Via field values represent each proxy or gateway that has
   forwarded the message.  Each intermediary appends its own information
   about how the message was received, such that the end result is
   ordered according to the sequence of forwarding recipients.

   A proxy MUST send an appropriate Via header field, as described
   below, in each message that it forwards.  An HTTP-to-HTTP gateway
   MUST send an appropriate Via header field in each inbound request
   message and MAY send a Via header field in forwarded response
   messages.

   For each intermediary, the received-protocol indicates the protocol
   and *( "," OWS ) transfer-coding *( OWS "," [ OWS
    transfer-coding ] )

   Upgrade = *( "," OWS ) protocol version used by the upstream sender of the message.
   Hence, the Via field value records the advertised *( OWS "," [ OWS protocol
   capabilities of the request/response chain such that they remain
   visible to downstream recipients; this can be useful for determining
   what backwards-incompatible features might be safe to use in
   response, or within a later request, as described in Section 2.6.
   For brevity, the protocol-name is omitted when the received protocol
   is HTTP.

   The received-by portion of the field value 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, a sender MAY replace it with
   a pseudonym.  If a port is not provided, a recipient MAY interpret
   that as meaning it was received on the default TCP port, if any, for
   the received-protocol.

   A sender MAY generate comments 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 a recipient MAY remove them prior to forwarding the
   message.

   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:

     Via: 1.0 fred, 1.1 p.example.net

   An intermediary 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, such an
   intermediary SHOULD replace each received-by host of any host behind
   the firewall by an appropriate pseudonym for that host.

   An intermediary MAY combine an ordered subsequence of Via header
   field entries into a single such entry if the entries have identical
   received-protocol values.  For example,

     Via: 1.0 ricky, 1.1 ethel, 1.1 fred, 1.0 lucy

   could be collapsed to

     Via: 1.0 ricky, 1.1 mertz, 1.0 lucy

   A sender SHOULD NOT combine multiple entries unless they are all
   under the same organizational control and the hosts have already been
   replaced by pseudonyms.  A sender MUST NOT combine entries that have
   different received-protocol values.

5.7.2.  Transformations

   Some intermediaries include features for transforming messages and
   their payloads.  A proxy might, for example, convert between image
   formats in order to save cache space or to reduce the amount of
   traffic on a slow link.  However, operational problems might occur
   when these transformations are applied to payloads intended for
   critical applications, such as medical imaging or scientific data
   analysis, particularly when integrity checks or digital signatures
   are used to ensure that the payload received is identical to the
   original.

   An HTTP-to-HTTP proxy is called a "transforming proxy" if it is
   designed or configured to modify 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 a privacy filter.  Such transformations are
   presumed to be desired by whichever client (or client organization)
   selected the proxy.

   If a proxy receives a request-target with a host name that is not a
   fully qualified domain name, it MAY add its own domain to the host
   name it received when forwarding the request.  A proxy MUST NOT
   change the host name if the request-target contains a fully qualified
   domain name.

   A proxy MUST NOT modify the "absolute-path" and "query" parts of the
   received request-target when forwarding it to the next inbound
   server, except as noted above to replace an empty path with "/" or
   "*".

   A proxy MAY modify the message body through application or removal of
   a transfer coding (Section 4).

   A proxy MUST NOT transform the payload (Section 3.3 of [SEMNTCS]) of
   a message that contains a no-transform cache-control directive
   (Section 5.2 of [CACHING]).

   A proxy MAY transform the payload of a message that does not contain
   a no-transform cache-control directive.  A proxy that transforms a
   payload MUST add a Warning header field with the warn-code of 214
   ("Transformation Applied") if one is not already in the message (see
   Section 5.5 of [CACHING]).  A proxy that transforms the payload of a
   200 (OK) response can further inform downstream recipients that a
   transformation has been applied by changing the response status code
   to 203 (Non-Authoritative Information) (Section 6.3.4 of [SEMNTCS]).

   A proxy SHOULD NOT modify header fields that provide information
   about the endpoints of the communication chain, the resource state,
   or the selected representation (other than the payload) unless the
   field's definition specifically allows such modification or the
   modification is deemed necessary for privacy or security.

6.  Connection Management

   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 an underlying transport
   protocol is outside the scope of this specification.

   As described in Section 5.2, the specific connection protocols to be
   used for an HTTP interaction are determined by client configuration
   and the target URI.  For example, the "http" URI scheme
   (Section 2.7.1) 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.

   HTTP implementations are expected to engage in connection management,
   which includes maintaining the state of current connections,
   establishing a new connection or reusing an existing connection,
   processing messages received on a connection, detecting connection
   failures, and closing each connection.  Most clients maintain
   multiple connections in parallel, including more than one connection
   per server endpoint.  Most servers are designed to maintain thousands
   of concurrent connections, while controlling request queues to enable
   fair use and detect denial-of-service attacks.

6.1.  Connection

   The "Connection" header field allows the sender to indicate desired
   control options for the current connection.  In order to avoid
   confusing downstream recipients, a proxy or gateway MUST remove or
   replace any received connection options before forwarding the
   message.

   When a header field aside from Connection is used to supply control
   information for or about the current connection, the sender MUST list
   the corresponding field-name within the Connection header field.  A
   proxy or gateway MUST parse a received Connection header field before
   a message is forwarded and, for each connection-option in this field,
   remove any header field(s) from the message with the same name as the
   connection-option, and then remove the Connection header field itself
   (or replace it with the intermediary's own connection options for the
   forwarded message).

   Hence, the Connection header field provides a declarative way of
   distinguishing header fields that are only intended for the immediate
   recipient ("hop-by-hop") from those fields that are intended for all
   recipients on the chain ("end-to-end"), enabling the message to be
   self-descriptive and allowing future connection-specific extensions
   to be deployed without fear that they will be blindly forwarded by
   older intermediaries.

   The Connection header field's value has the following grammar:

     Connection        = 1#connection-option
     connection-option = token

   Connection options are case-insensitive.

   A sender MUST NOT send a connection option corresponding to a header
   field that is intended for all recipients of the payload.  For
   example, Cache-Control is never appropriate as a connection option
   (Section 5.2 of [CACHING]).

   The connection options do not always 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 a
   connection option.  In contrast, a connection-specific header field
   that is received without a corresponding connection option usually
   indicates that the field has been improperly forwarded by an
   intermediary and ought to be ignored by the recipient.

   When defining new connection options, specification authors ought to
   survey existing header field names and ensure that the new connection
   option does not share the same name as an already deployed header
   field.  Defining a new connection option 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.

   The "close" connection option is defined for a sender to signal that
   this connection will be closed after completion of the response.  For
   example,

     Connection: close

   in either the request or the response header fields indicates that
   the sender is going to close the connection after the current
   request/response is complete (Section 6.6).

   A client that does not support persistent connections MUST send the
   "close" connection option in every request message.

   A server that does not support persistent connections MUST send the
   "close" connection option in every response message that does not
   have a 1xx (Informational) status code.

6.2.  Establishment

   It is beyond the scope of this specification to describe how
   connections are established via various transport- or session-layer
   protocols.  Each connection applies to only one transport link.

6.3.  Persistence

   HTTP/1.1 defaults to the use of "persistent connections", allowing
   multiple requests and responses to be carried over a single
   connection.  The "close" connection option is used to signal that a
   connection will not persist after the current request/response.  HTTP
   implementations SHOULD support persistent connections.

   A recipient determines whether a connection is persistent or not
   based on the most recently received message's protocol version and
   Connection header field (if any):

   o  If the "close" connection option is present, the connection will
      not persist after the current response; else,

   o  If the received protocol is HTTP/1.1 (or later), the connection
      will persist after the current response; else,

   o  If the received protocol is HTTP/1.0, the "keep-alive" connection
      option is present, the recipient is not a proxy, and the recipient
      wishes to honor the HTTP/1.0 "keep-alive" mechanism, the
      connection will persist after the current response; otherwise,

   o  The connection will close after the current response.

   A client MAY send additional requests on a persistent connection
   until it sends or receives a "close" connection option or receives an
   HTTP/1.0 response without a "keep-alive" connection option.

   In order to remain persistent, all messages on a connection need to
   have a self-defined message length (i.e., one not defined by closure
   of the connection), as described in Section 3.3.  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.

   A proxy server MUST NOT maintain a persistent connection with an
   HTTP/1.0 client (see Section 19.7.1 of [RFC2068] for information and
   discussion of the problems with the Keep-Alive header field
   implemented by many HTTP/1.0 clients).

   See Appendix A.1.2 for more information on backwards compatibility
   with HTTP/1.0 clients.

6.3.1.  Retrying Requests

   Connections can be closed at any time, with or without intention.
   Implementations ought to anticipate the need to recover from
   asynchronous close events.

   When an inbound connection is closed prematurely, a client MAY open a
   new connection and automatically retransmit an aborted sequence of
   requests if all of those requests have idempotent methods
   (Section 4.2.2 of [SEMNTCS]).  A proxy MUST NOT automatically retry
   non-idempotent requests.

   A user agent MUST NOT automatically retry a request with a non-
   idempotent method unless it has some means to know that the request
   semantics are actually idempotent, regardless of the method, or some
   means to detect that the original request was never applied.  For
   example, a user agent that knows (through design or configuration)
   that a POST request to a given resource is safe can repeat that
   request automatically.  Likewise, a user agent designed specifically
   to operate on a version control repository might be able to recover
   from partial failure conditions by checking the target resource
   revision(s) after a failed connection, reverting or fixing any
   changes that were partially applied, and then automatically retrying
   the requests that failed.

   A client SHOULD NOT automatically retry a failed automatic retry.

6.3.2.  Pipelining

   A client that supports persistent connections MAY "pipeline" its
   requests (i.e., send multiple requests without waiting for each
   response).  A server MAY process a sequence of pipelined requests in
   parallel if they all have safe methods (Section 4.2.1 of [SEMNTCS]),
   but it MUST send the corresponding responses in the same order that
   the requests were received.

   A client that pipelines requests SHOULD retry unanswered requests if
   the connection closes before it receives all of the corresponding
   responses.  When retrying pipelined requests after a failed
   connection (a connection not explicitly closed by the server in its
   last complete response), a client MUST NOT pipeline immediately after
   connection establishment, since the first remaining request in the
   prior pipeline might have caused an error response that can be lost
   again if multiple requests are sent on a prematurely closed
   connection (see the TCP reset problem described in Section 6.6).

   Idempotent methods (Section 4.2.2 of [SEMNTCS]) are significant to
   pipelining because they can be automatically retried after a
   connection failure.  A user agent SHOULD NOT pipeline requests after
   a non-idempotent method, until the final response status code for
   that method has been received, unless the user agent has a means to
   detect and recover from partial failure conditions involving the
   pipelined sequence.

   An intermediary that receives pipelined requests MAY pipeline those
   requests when forwarding them inbound, since it can rely on the
   outbound user agent(s) to determine what requests can be safely
   pipelined.  If the inbound connection fails before receiving a
   response, the pipelining intermediary MAY attempt to retry a sequence
   of requests that have yet to receive a response if the requests all
   have idempotent methods; otherwise, the pipelining intermediary
   SHOULD forward any received responses and then close the
   corresponding outbound connection(s) so that the outbound user
   agent(s) can recover accordingly.

6.4.  Concurrency

   A client ought to limit the number of simultaneous open connections
   that it maintains to a given server.

   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.

   Multiple connections are typically used to avoid the "head-of-line
   blocking" problem, wherein a request that takes significant server-
   side processing and/or has a large payload blocks subsequent requests
   on the same connection.  However, each connection consumes server
   resources.  Furthermore, using multiple connections can cause
   undesirable side effects in congested networks.

   Note that a server might reject traffic that it deems abusive or
   characteristic of a denial-of-service attack, such as an excessive
   number of open connections from a single client.

6.5.  Failures and Timeouts

   Servers will usually have some timeout 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 proxy server.  The use of
   persistent connections places no requirements on the length (or
   existence) of this timeout for either the client or the server.

   A client or server that wishes to time out SHOULD issue a graceful
   close on the connection.  Implementations SHOULD constantly monitor
   open connections for a received closure signal and respond to it as
   appropriate, since prompt closure of both sides of a connection
   enables allocated system resources to be reclaimed.

   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.

   A server SHOULD sustain persistent connections, when possible, and
   allow the underlying transport's flow-control mechanisms to resolve
   temporary overloads, rather than terminate connections with the
   expectation that clients will retry.  The latter technique can
   exacerbate network congestion.

   A client sending a message body SHOULD monitor the network connection
   for an error response while it is transmitting the request.  If the
   client sees a response that indicates the server does not wish to
   receive the message body and is closing the connection, the client
   SHOULD immediately cease transmitting the body and close its side of
   the connection.

6.6.  Tear-down

   The Connection header field (Section 6.1) provides a "close"
   connection option that a sender SHOULD send when it wishes to close
   the connection after the current request/response pair.

   A client that sends a "close" connection option MUST NOT send further
   requests on that connection (after the one containing "close") and
   MUST close the connection after reading the final response message
   corresponding to this request.

   A server that receives a "close" connection option MUST initiate a
   close of the connection (see below) after it sends the final response
   to the request that contained "close".  The server SHOULD send a
   "close" connection option in its final response on that connection.
   The server MUST NOT process any further requests received on that
   connection.

   A server that sends a "close" connection option MUST initiate a close
   of the connection (see below) after it sends the response containing
   "close".  The server MUST NOT process any further requests received
   on that connection.

   A client that receives a "close" connection option MUST cease sending
   requests on that connection and close the connection after reading
   the response message containing the "close"; if additional pipelined
   requests had been sent on the connection, the client SHOULD NOT
   assume that they will be processed by the server.

   If a server performs an immediate close of a TCP connection, there is
   a significant risk that the client will not be able to read the last
   HTTP response.  If the server receives additional data from the
   client on a fully closed connection, such as another request that was
   sent by the client before receiving the server's response, the
   server's TCP stack will send a reset packet to the client;
   unfortunately, the reset packet might erase the client's
   unacknowledged input buffers before they can be read and interpreted
   by the client's HTTP parser.

   To avoid the TCP reset problem, servers typically close a connection
   in stages.  First, the server performs a half-close by closing only
   the write side of the read/write connection.  The server then
   continues to read from the connection until it receives a
   corresponding close by the client, or until the server is reasonably
   certain that its own TCP stack has received the client's
   acknowledgement of the packet(s) containing the server's last
   response.  Finally, the server fully closes the connection.

   It is unknown whether the reset problem is exclusive to TCP or might
   also be found in other transport connection protocols.

6.7.  Upgrade

   The "Upgrade" header field is intended to provide a simple mechanism
   for transitioning from HTTP/1.1 to some other protocol on the same
   connection.  A client MAY send a list of protocols in the Upgrade
   header field of a request to invite the server to switch to one or
   more of those protocols, in order of descending preference, before
   sending the final response.  A server MAY ignore a received Upgrade
   header field if it wishes to continue using the current protocol on
   that connection.  Upgrade cannot be used to insist on a protocol
   change.

     Upgrade          = 1#protocol

     protocol         = protocol-name ["/" protocol-version]
     protocol-name    = token
     protocol-version = token

   A server that sends a 101 (Switching Protocols) response MUST send an
   Upgrade header field to indicate the new protocol(s) to which the
   connection is being switched; if multiple protocol layers are being
   switched, the sender MUST list the protocols in layer-ascending
   order.  A server MUST NOT switch to a protocol that was not indicated
   by the client in the corresponding request's Upgrade header field.  A
   server MAY choose to ignore the order of preference indicated by the
   client and select the new protocol(s) based on other factors, such as
   the nature of the request or the current load on the server.

   A server that sends a 426 (Upgrade Required) response MUST send an
   Upgrade header field to indicate the acceptable protocols, in order
   of descending preference.

   A server MAY send an Upgrade header field in any other response to
   advertise that it implements support for upgrading to the listed
   protocols, in order of descending preference, when appropriate for a
   future request.

   The following is a hypothetical example sent by a client:

     GET /hello.txt HTTP/1.1
     Host: www.example.com
     Connection: upgrade
     Upgrade: HTTP/2.0, SHTTP/1.3, IRC/6.9, RTA/x11

   The capabilities and nature of the application-level communication
   after the protocol change is entirely dependent upon the new
   protocol(s) chosen.  However, immediately after sending the 101
   (Switching Protocols) response, the server is expected to continue
   responding to the original request as if it had received its
   equivalent within the new protocol (i.e., the server still has an
   outstanding request to satisfy after the protocol has been changed,
   and is expected to do so without requiring the request to be
   repeated).

   For example, if the Upgrade header field is received in a GET request
   and the server decides to switch protocols, it first responds with a
   101 (Switching Protocols) message in HTTP/1.1 and then immediately
   follows that with the new protocol's equivalent of a response to a
   GET on the target resource.  This allows a connection to be upgraded
   to protocols with the same semantics as HTTP without the latency cost
   of an additional round trip.  A server MUST NOT switch protocols
   unless the received message semantics can be honored by the new
   protocol; an OPTIONS request can be honored by any protocol.

   The following is an example response to the above hypothetical
   request:

     HTTP/1.1 101 Switching Protocols
     Connection: upgrade
     Upgrade: HTTP/2.0

     [... data stream switches to HTTP/2.0 with an appropriate response
     (as defined by new protocol) to the "GET /hello.txt" request ...]

   When Upgrade is sent, the sender MUST also send a Connection header
   field (Section 6.1) that contains an "upgrade" connection option, in
   order to prevent Upgrade from being accidentally forwarded by
   intermediaries that might not implement the listed protocols.  A
   server MUST ignore an Upgrade header field that is received in an
   HTTP/1.0 request.

   A client cannot begin using an upgraded protocol on the connection
   until it has completely sent the request message (i.e., the client
   can't change the protocol it is sending in the middle of a message).
   If a server receives both an Upgrade and an Expect header field with
   the "100-continue" expectation (Section 5.1.1 of [SEMNTCS]), the
   server MUST send a 100 (Continue) response before sending a 101
   (Switching Protocols) response.

   The Upgrade header field only applies to switching protocols on top
   of the existing connection; it cannot be used to switch the
   underlying connection (transport) protocol, nor to switch the
   existing communication to a different connection.  For those
   purposes, it is more appropriate to use a 3xx (Redirection) response
   (Section 6.4 of [SEMNTCS]).

   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 Section 2.6 and future updates to this
   specification.  Additional tokens ought to be registered with IANA
   using the registration procedure defined in Section 8.6.

7.  ABNF List Extension: #rule

   A #rule extension to the ABNF rules of [RFC5234] is used to improve
   readability in the definitions of some header field values.

   A construct "#" is defined, similar to "*", for defining comma-
   delimited lists of elements.  The full form is "<n>#<m>element"
   indicating at least <n> and at most <m> elements, each separated by a
   single comma (",") and optional whitespace (OWS).

   In any production that uses the list construct, a sender MUST NOT
   generate empty list elements.  In other words, a sender MUST generate
   lists that satisfy the following syntax:

     1#element => element *( OWS "," OWS element )

   and:

     #element => [ 1#element ]

   and for n >= 1 and m > 1:

     <n>#<m>element => element <n-1>*<m-1>( OWS "," OWS element )

   For compatibility with legacy list rules, a recipient MUST parse and
   ignore a reasonable number of empty list elements: enough to handle
   common mistakes by senders that merge values, but not so much that
   they could be used as a denial-of-service mechanism.  In other words,
   a recipient MUST accept lists that satisfy the following syntax:

     #element => [ ( "," / element ) *( OWS "," [ OWS element ] ) ]

     1#element => *( "," OWS ) element *( OWS "," [ OWS element ] )

   Empty elements do not contribute to the count of elements present.
   For example, given these ABNF productions:

     example-list      = 1#example-list-elmt
     example-list-elmt = token ; see Section 3.2.6

   Then the following are valid values for example-list (not including
   the double quotes, which are present for delimitation only):

     "foo,bar"
     "foo ,bar,"
     "foo , ,bar,charlie   "

   In contrast, the following values would be invalid, since at least
   one non-empty element is required by the example-list production:

     ""
     ","
     ",   ,"

   Appendix B shows the collected ABNF for recipients after the list
   constructs have been expanded.

8.  IANA Considerations

8.1.  Header Field Registration

   HTTP header fields are registered within the "Message Headers"
   registry maintained at <http://www.iana.org/assignments/message-
   headers/>.

   This document defines the following HTTP header fields, so the
   "Permanent Message Header Field Names" registry has been updated
   accordingly (see [BCP90]).

   +-------------------+----------+----------+----------------+
   | Header Field Name | Protocol | Status   | Reference      |
   +-------------------+----------+----------+----------------+
   | Connection        | http     | standard | Section 6.1    |
   | Content-Length    | http     | standard | Section 3.3.2  |
   | Host              | http     | standard | Section 5.4    |
   | TE                | http     | standard | Section 4.3    |
   | Trailer           | http     | standard | Section 4.4    |
   | Transfer-Encoding | http     | standard | Section 3.3.1  |
   | Upgrade           | http     | standard | Section 6.7    |
   | Via               | http     | standard | Section 5.7.1  |
   +-------------------+----------+----------+----------------+

   Furthermore, the header field-name "Close" has been registered as
   "reserved", since using that name as an HTTP header field might
   conflict with the "close" connection option of the Connection header
   field (Section 6.1).

   +-------------------+----------+----------+--------------+
   | Header Field Name | Protocol | Status   | Reference    |
   +-------------------+----------+----------+--------------+
   | Close             | http     | reserved | Section 8.1  |
   +-------------------+----------+----------+--------------+

   The change controller is: "IETF (iesg@ietf.org) - Internet
   Engineering Task Force".

8.2.  URI Scheme Registration

   IANA maintains the registry of URI Schemes [BCP115] at
   <http://www.iana.org/assignments/uri-schemes/>.

   This document defines the following URI schemes, so the "Permanent
   URI Schemes" registry has been updated accordingly.

   +------------+------------------------------------+---------------+
   | URI Scheme | Description                        | Reference     |
   +------------+------------------------------------+---------------+
   | http       | Hypertext Transfer Protocol        | Section 2.7.1 |
   | https      | Hypertext Transfer Protocol Secure | Section 2.7.2 |
   +------------+------------------------------------+---------------+

8.3.  Internet Media Type Registration

   IANA maintains the registry of Internet media types [BCP13] at
   <http://www.iana.org/assignments/media-types>.

   This document serves as the specification for the Internet media
   types "message/http" and "application/http".  The following has been
   registered with IANA.

8.3.1.  Internet Media Type message/http

   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.

   Type name:  message

   Subtype name:  http

   Required parameters:  N/A

   Optional parameters:  version, msgtype

      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.

      msgtype:  The message type -- "request" or "response".  If not
         present, the type can be determined from the first line of the
         body.

   Encoding considerations:  only "7bit", "8bit", or "binary" are
      permitted

   Security considerations:  see Section 9

   Interoperability considerations:  N/A

   Published specification:  This specification (see Section 8.3.1).

   Applications that use this media type:  N/A

   Fragment identifier considerations:  N/A

   Additional information:

      Magic number(s):  N/A

      Deprecated alias names for this type:  N/A

      File extension(s):  N/A

      Macintosh file type code(s):  N/A

   Person and email address to contact for further information:
      See Authors' Addresses section.

   Intended usage:  COMMON

   Restrictions on usage:  N/A

   Author:  See Authors' Addresses section.

   Change controller:  IESG

8.3.2.  Internet Media Type application/http

   The application/http type can be used to enclose a pipeline of one or
   more HTTP request or response messages (not intermixed).

   Type name:  application

   Subtype name:  http

   Required parameters:  N/A

   Optional parameters:  version, msgtype

      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.

      msgtype:  The message type -- "request" or "response".  If not
         present, the type can be determined from the first line of the
         body.

   Encoding considerations:  HTTP messages enclosed by this type are in
      "binary" format; use of an appropriate Content-Transfer-Encoding
      is required when transmitted via email.

   Security considerations:  see Section 9

   Interoperability considerations:  N/A

   Published specification:  This specification (see Section 8.3.2).

   Applications that use this media type:  N/A

   Fragment identifier considerations:  N/A

   Additional information:

      Deprecated alias names for this type:  N/A

      Magic number(s):  N/A

      File extension(s):  N/A

      Macintosh file type code(s):  N/A

   Person and email address to contact for further information:
      See Authors' Addresses section.

   Intended usage:  COMMON

   Restrictions on usage:  N/A

   Author:  See Authors' Addresses section.

   Change controller:  IESG

8.4.  Transfer Coding Registry

   The "HTTP Transfer Coding Registry" defines the namespace for
   transfer coding names.  It is maintained at
   <http://www.iana.org/assignments/http-parameters>.

8.4.1.  Procedure

   Registrations MUST include the following fields:

   o  Name

   o  Description

   o  Pointer to specification text

   Names of transfer codings MUST NOT overlap with names of content
   codings (Section 3.1.2.1 of [SEMNTCS]) unless the encoding
   transformation is identical, as is the case for the compression
   codings defined in Section 4.2.

   Values to be added to this namespace require IETF Review (see
   Section 4.1 of [RFC5226]), and MUST conform to the purpose of
   transfer coding defined in this specification.

   Use of program names for the identification of encoding formats is
   not desirable and is discouraged for future encodings.

8.4.2.  Registration

   The "HTTP Transfer Coding Registry" has been updated with the
   registrations below:

   +------------+------------------------------------------+-----------+
   | Name       | Description                              | Reference |
   +------------+------------------------------------------+-----------+
   | chunked    | Transfer in a series of chunks           | Section 4 |
   |            |                                          | .1        |
   | compress   | UNIX "compress" data format [Welch]      | Section 4 |
   |            |                                          | .2.1      |
   | deflate    | "deflate" compressed data ([RFC1951])    | Section 4 |
   |            | inside the "zlib" data format            | .2.2      |
   |            | ([RFC1950])                              |           |
   | gzip       | GZIP file format [RFC1952]               | Section 4 |
   |            |                                          | .2.3      |
   | x-compress | Deprecated (alias for compress)          | Section 4 |
   |            |                                          | .2.1      |
   | x-gzip     | Deprecated (alias for gzip)              | Section 4 |
   |            |                                          | .2.3      |
   +------------+------------------------------------------+-----------+

8.5.  Content Coding Registration

   IANA maintains the "HTTP Content Coding Registry" at
   <http://www.iana.org/assignments/http-parameters>.

   The "HTTP Content Coding Registry" has been updated with the
   registrations below:

   +------------+------------------------------------------+-----------+
   | Name       | Description                              | Reference |
   +------------+------------------------------------------+-----------+
   | compress   | UNIX "compress" data format [Welch]      | Section 4 |
   |            |                                          | .2.1      |
   | deflate    | "deflate" compressed data ([RFC1951])    | Section 4 |
   |            | inside the "zlib" data format            | .2.2      |
   |            | ([RFC1950])                              |           |
   | gzip       | GZIP file format [RFC1952]               | Section 4 |
   |            |                                          | .2.3      |
   | x-compress | Deprecated (alias for compress)          | Section 4 |
   |            |                                          | .2.1      |
   | x-gzip     | Deprecated (alias for gzip)              | Section 4 |
   |            |                                          | .2.3      |
   +------------+------------------------------------------+-----------+

8.6.  Upgrade Token Registry

   The "Hypertext Transfer Protocol (HTTP) Upgrade Token Registry"
   defines the namespace for protocol-name tokens used to identify
   protocols in the Upgrade header field.  The registry is maintained at
   <http://www.iana.org/assignments/http-upgrade-tokens>.

8.6.1.  Procedure

   Each registered protocol name is associated with contact information
   and an optional set of specifications that details how the connection
   will be processed after it has been upgraded.

   Registrations happen on a "First Come First Served" basis (see
   Section 4.1 of [RFC5226]) and are subject to the following rules:

   1.  A protocol-name token, once registered, stays registered forever.

   2.  The registration MUST name a responsible party for the
       registration.

   3.  The registration MUST name a point of contact.

   4.  The registration MAY name a set of specifications associated with
       that token.  Such specifications need not be publicly available.

   5.  The registration SHOULD name a set of expected "protocol-version"
       tokens associated with that token at the time of registration.

   6.  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.

   7.  The IESG MAY reassign responsibility for a protocol token.  This
       will normally only be used in the case when a responsible party
       cannot be contacted.

8.6.2.  Upgrade Token Registration

   The "HTTP" entry in the upgrade token registry has been updated with
   the registration below:

   +-------+----------------------+------------------------+-----------+
   | Value | Description          | Expected Version       | Reference |
   |       |                      | Tokens                 |           |
   +-------+----------------------+------------------------+-----------+
   | HTTP  | Hypertext Transfer   | any DIGIT.DIGIT (e.g,  | Section 2 |
   |       | Protocol             | "2.0")                 | .6        |
   +-------+----------------------+------------------------+-----------+

   The responsible party is: "IETF (iesg@ietf.org) - Internet
   Engineering Task Force".

9.  Security Considerations

   This section is meant to inform developers, information providers,
   and users of known security considerations relevant to HTTP message
   syntax, parsing, and routing.  Security considerations about HTTP
   semantics and payloads are addressed in [SEMNTCS].

9.1.  Establishing Authority

   HTTP relies on the notion of an authoritative response: a response
   that has been determined by (or at the direction of) the authority
   identified within the target URI to be the most appropriate response
   for that request given the state of the target resource at the time
   of response message origination.  Providing a response from a non-
   authoritative source, such as a shared cache, is often useful to
   improve performance and availability, but only to the extent that the
   source can be trusted or the distrusted response can be safely used.

   Unfortunately, establishing authority can be difficult.  For example,
   phishing is an attack on the user's perception of authority, where
   that perception can be misled by presenting similar branding in
   hypertext, possibly aided by userinfo obfuscating the authority
   component (see Section 2.7.1).  User agents can reduce the impact of
   phishing attacks by enabling users to easily inspect a target URI
   prior to making an action, by prominently distinguishing (or
   rejecting) userinfo when present, and by not sending stored
   credentials and cookies when the referring document is from an
   unknown or untrusted source.

   When a registered name is used in the authority component, the "http"
   URI scheme (Section 2.7.1) relies on the user's local name resolution
   service to determine where it can find authoritative responses.  This
   means that any attack on a user's network host table, cached names,
   or name resolution libraries becomes an avenue for attack on
   establishing authority.  Likewise, the user's choice of server for
   Domain Name Service (DNS), and the hierarchy of servers from which it
   obtains resolution results, could impact the authenticity of address
   mappings; DNS Security Extensions (DNSSEC, [RFC4033]) are one way to
   improve authenticity.

   Furthermore, after an IP address is obtained, establishing authority
   for an "http" URI is vulnerable to attacks on Internet Protocol
   routing.

   The "https" scheme (Section 2.7.2) is intended to prevent (or at
   least reveal) many of these potential attacks on establishing
   authority, provided that the negotiated TLS connection is secured and
   the client properly verifies that the communicating server's identity
   matches the target URI's authority component (see [RFC2818]).
   Correctly implementing such verification can be difficult (see
   [Georgiev]).

9.2.  Risks of Intermediaries

   By their very nature, HTTP intermediaries are men-in-the-middle and,
   thus, represent an opportunity for man-in-the-middle attacks.
   Compromise of the systems on which the intermediaries run can result
   in serious security and privacy problems.  Intermediaries might have
   access to security-related information, personal information about
   individual users and organizations, and proprietary information
   belonging to users and content providers.  A compromised
   intermediary, or an intermediary implemented or configured without
   regard to security and privacy considerations, might be used in the
   commission of a wide range of potential attacks.

   Intermediaries that contain a shared cache are especially vulnerable
   to cache poisoning attacks, as described in Section 8 of [CACHING].

   Implementers need to consider the privacy and security implications
   of their design and coding decisions, and of the configuration
   options they provide to operators (especially the default
   configuration).

   Users need to be aware that intermediaries are no more trustworthy
   than the people who run them; HTTP itself cannot solve this problem.

9.3.  Attacks via Protocol Element Length

   Because HTTP uses mostly textual, character-delimited fields, parsers
   are often vulnerable to attacks based on sending very long (or very
   slow) streams of data, particularly where an implementation is
   expecting a protocol element with no predefined length.

   To promote interoperability, specific recommendations are made for
   minimum size limits on request-line (Section 3.1.1) and header fields
   (Section 3.2).  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.

   A server can reject a message that has a request-target that is too
   long (Section 6.5.12 of [SEMNTCS]) or a request payload that is too
   large (Section 6.5.11 of [SEMNTCS]).  Additional status codes related
   to capacity limits have been defined by extensions to HTTP [RFC6585].

   Recipients ought to carefully limit the extent to which they process
   other protocol elements, including (but not limited to) request
   methods, response status phrases, header field-names, numeric values,
   and body chunks.  Failure to limit such processing can result in
   buffer overflows, arithmetic overflows, or increased vulnerability to
   denial-of-service attacks.

9.4.  Response Splitting

   Response splitting (a.k.a, CRLF injection) is a common technique,
   used in various attacks on Web usage, that exploits the line-based
   nature of HTTP message framing and the ordered association of
   requests to responses on persistent connections [Klein].  This
   technique can be particularly damaging when the requests pass through
   a shared cache.

   Response splitting exploits a vulnerability in servers (usually
   within an application server) where an attacker can send encoded data
   within some parameter of the request that is later decoded and echoed
   within any of the response header fields of the response.  If the
   decoded data is crafted to look like the response has ended and a
   subsequent response has begun, the response has been split and the
   content within the apparent second response is controlled by the
   attacker.  The attacker can then make any other request on the same
   persistent connection and trick the recipients (including
   intermediaries) into believing that the second half of the split is
   an authoritative answer to the second request.

   For example, a parameter within the request-target might be read by
   an application server and reused within a redirect, resulting in the
   same parameter being echoed in the Location header field of the
   response.  If the parameter is decoded by the application and not
   properly encoded when placed in the response field, the attacker can
   send encoded CRLF octets and other content that will make the
   application's single response look like two or more responses.

   A common defense against response splitting is to filter requests for
   data that looks like encoded CR and LF (e.g., "%0D" and "%0A").

   However, that assumes the application server is only performing URI
   decoding, rather than more obscure data transformations like charset
   transcoding, XML entity translation, base64 decoding, sprintf
   reformatting, etc.  A more effective mitigation is to prevent
   anything other than the server's core protocol libraries from sending
   a CR or LF within the header section, which means restricting the
   output of header fields to APIs that filter for bad octets and not
   allowing application servers to write directly to the protocol
   stream.

9.5.  Request Smuggling

   Request smuggling ([Linhart]) is a technique that exploits
   differences in protocol parsing among various recipients to hide
   additional requests (which might otherwise be blocked or disabled by
   policy) within an apparently harmless request.  Like response
   splitting, request smuggling can lead to a variety of attacks on HTTP
   usage.

   This specification has introduced new requirements on request
   parsing, particularly with regard to message framing in
   Section 3.3.3, to reduce the effectiveness of request smuggling.

9.6.  Message Integrity

   HTTP does not define a specific mechanism for ensuring message
   integrity, instead relying on the error-detection ability of
   underlying transport protocols and the use of length or chunk-
   delimited framing to detect completeness.  Additional integrity
   mechanisms, such as hash functions or digital signatures applied to
   the content, can be selectively added to messages via extensible
   metadata header fields.  Historically, the lack of a single integrity
   mechanism has been justified by the informal nature of most HTTP
   communication.  However, the prevalence of HTTP as an information
   access mechanism has resulted in its increasing use within
   environments where verification of message integrity is crucial.

   User agents are encouraged to implement configurable means for
   detecting and reporting failures of message integrity such that those
   means can be enabled within environments for which integrity is
   necessary.  For example, a browser being used to view medical history
   or drug interaction information needs to indicate to the user when
   such information is detected by the protocol to be incomplete,
   expired, or corrupted during transfer.  Such mechanisms might be
   selectively enabled via user agent extensions or the presence of
   message integrity metadata in a response.  At a minimum, user agents
   ought to provide some indication that allows a user to distinguish
   between a complete and incomplete response message (Section 3.4) when
   such verification is desired.

9.7.  Message Confidentiality

   HTTP relies on underlying transport protocols to provide message
   confidentiality when that is desired.  HTTP has been specifically
   designed to be independent of the transport protocol, such that it
   can be used over many different forms of encrypted connection, with
   the selection of such transports being identified by the choice of
   URI scheme or within user agent configuration.

   The "https" scheme can be used to identify resources that require a
   confidential connection, as described in Section 2.7.2.

9.8.  Privacy of Server Log Information

   A server is in the position to save personal data about a user's
   requests over time, which might identify their reading patterns or
   subjects of interest.  In particular, log information gathered at an
   intermediary often contains a history of user agent interaction,
   across a multitude of sites, that can be traced to individual users.

   HTTP log information is confidential in nature; its handling is often
   constrained by laws and regulations.  Log information needs to be
   securely stored and appropriate guidelines followed for its analysis.
   Anonymization of personal information within individual entries
   helps, but it is generally not sufficient to prevent real log traces
   from being re-identified based on correlation with other access
   characteristics.  As such, access traces that are keyed to a specific
   client are unsafe to publish even if the key is pseudonymous.

   To minimize the risk of theft or accidental publication, log
   information ought to be purged of personally identifiable
   information, including user identifiers, IP addresses, and user-
   provided query parameters, as soon as that information is no longer
   necessary to support operational needs for security, auditing, or
   fraud control.

10.  References

10.1.  Normative References

   [AUTHFRM]  Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke,
              Ed., "Hypertext Transfer Protocol (HTTP): Authentication",
              draft-ietf-httpbis-auth-00 (work in progress), April 2018.

   [CACHING]  Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke,
              Ed., "Hypertext Transfer Protocol (HTTP): Caching", draft-
              ietf-httpbis-cache-00 (work in progress), April 2018.

   [CONDTNL]  Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke,
              Ed., "Hypertext Transfer Protocol (HTTP): Conditional
              Requests", draft-ietf-httpbis-conditional-00 (work in
              progress), April 2018.

   [RANGERQ]  Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke,
              Ed., "Hypertext Transfer Protocol (HTTP): Range Requests",
              draft-ietf-httpbis-range-00 (work in progress), April
              2018.

   [RFC0793]  Postel, J., "Transmission Control Protocol", STD 7,
              RFC 793, DOI 10.17487/RFC0793, September 1981,
              <https://www.rfc-editor.org/info/rfc793>.

   [RFC1950]  Deutsch, L. and J-L. Gailly, "ZLIB Compressed Data Format
              Specification version 3.3", RFC 1950,
              DOI 10.17487/RFC1950, May 1996,
              <https://www.rfc-editor.org/info/rfc1950>.

   [RFC1951]  Deutsch, P., "DEFLATE Compressed Data Format Specification
              version 1.3", RFC 1951, DOI 10.17487/RFC1951, May 1996,
              <https://www.rfc-editor.org/info/rfc1951>.

   [RFC1952]  Deutsch, P., Gailly, J-L., Adler, M., Deutsch, L., and G.
              Randers-Pehrson, "GZIP file format specification version
              4.3", RFC 1952, DOI 10.17487/RFC1952, May 1996,
              <https://www.rfc-editor.org/info/rfc1952>.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <https://www.rfc-editor.org/info/rfc2119>.

   [RFC3986]  Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
              Resource Identifier (URI): Generic Syntax", STD 66,
              RFC 3986, DOI 10.17487/RFC3986, January 2005,
              <https://www.rfc-editor.org/info/rfc3986>.

   [RFC5234]  Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
              Specifications: ABNF", STD 68, RFC 5234,
              DOI 10.17487/RFC5234, January 2008,
              <https://www.rfc-editor.org/info/rfc5234>.

   [SEMNTCS]  Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke,
              Ed., "Hypertext Transfer Protocol (HTTP): Semantics and
              Content", draft-ietf-httpbis-semantics-00 (work in
              progress), April 2018.

   [USASCII]  American National Standards Institute, "Coded Character
              Set -- 7-bit American Standard Code for Information
              Interchange", ANSI X3.4, 1986.

   [Welch]    Welch, T., "A Technique for High-Performance Data
              Compression", IEEE Computer 17(6), June 1984.

10.2.  Informative References

   [BCP115]   Hansen, T., Hardie, T., and L. Masinter, "Guidelines and
              Registration Procedures for New URI Schemes", BCP 115,
              RFC 4395, February 2006,
              <https://www.rfc-editor.org/info/bcp115>.

   [BCP13]    Freed, N., Klensin, J., and T. Hansen, "Media Type
              Specifications and Registration Procedures", BCP 13,
              RFC 6838, January 2013,
              <https://www.rfc-editor.org/info/bcp13>.

   [BCP90]    Klyne, G., Nottingham, M., and J. Mogul, "Registration
              Procedures for Message Header Fields", BCP 90, RFC 3864,
              September 2004, <https://www.rfc-editor.org/info/bcp90>.

   [Georgiev]
              Georgiev, M., Iyengar, S., Jana, S., Anubhai, R., Boneh,
              D., and V. Shmatikov, "The Most Dangerous Code in the
              World: Validating SSL Certificates in Non-browser
              Software", In Proceedings of the 2012 ACM Conference on
              Computer and Communications Security (CCS '12), pp. 38-49,
              October 2012,
              <http://doi.acm.org/10.1145/2382196.2382204>.

   [ISO-8859-1]
              International Organization for Standardization,
              "Information technology -- 8-bit single-byte coded graphic
              character sets -- Part 1: Latin alphabet No. 1", ISO/
              IEC 8859-1:1998, 1998.

   [Klein]    Klein, A., "Divide and Conquer - HTTP Response Splitting,
              Web Cache Poisoning Attacks, and Related Topics", March
              2004, <http://packetstormsecurity.com/papers/general/
              whitepaper_httpresponse.pdf>.

   [Kri2001]  Kristol, D., "HTTP Cookies: Standards, Privacy, and
              Politics", ACM Transactions on Internet Technology 1(2),
              November 2001, <http://arxiv.org/abs/cs.SE/0105018>.

   [Linhart]  Linhart, C., Klein, A., Heled, R., and S. Orrin, "HTTP
              Request Smuggling", June 2005,
              <http://www.watchfire.com/news/whitepapers.aspx>.

   [RFC1919]  Chatel, M., "Classical versus Transparent IP Proxies",
              RFC 1919, DOI 10.17487/RFC1919, March 1996,
              <https://www.rfc-editor.org/info/rfc1919>.

   [RFC1945]  Berners-Lee, T., Fielding, R., and H. Nielsen, "Hypertext
              Transfer Protocol -- HTTP/1.0", RFC 1945,
              DOI 10.17487/RFC1945, May 1996,
              <https://www.rfc-editor.org/info/rfc1945>.

   [RFC2045]  Freed, N. and N. Borenstein, "Multipurpose Internet Mail
              Extensions (MIME) Part One: Format of Internet Message
              Bodies", RFC 2045, DOI 10.17487/RFC2045, November 1996,
              <https://www.rfc-editor.org/info/rfc2045>.

   [RFC2047]  Moore, K., "MIME (Multipurpose Internet Mail Extensions)
              Part Three: Message Header Extensions for Non-ASCII Text",
              RFC 2047, DOI 10.17487/RFC2047, November 1996,
              <https://www.rfc-editor.org/info/rfc2047>.

   [RFC2068]  Fielding, R., Gettys, J., Mogul, J., Nielsen, H., and T.
              Berners-Lee, "Hypertext Transfer Protocol -- HTTP/1.1",
              RFC 2068, DOI 10.17487/RFC2068, January 1997,
              <https://www.rfc-editor.org/info/rfc2068>.

   [RFC2145]  Mogul, J., Fielding, R., Gettys, J., and H. Nielsen, "Use
              and Interpretation of HTTP Version Numbers", RFC 2145,
              DOI 10.17487/RFC2145, May 1997,
              <https://www.rfc-editor.org/info/rfc2145>.

   [RFC2616]  Fielding, R., Gettys, J., Mogul, J., Frystyk, H.,
              Masinter, L., Leach, P., and T. Berners-Lee, "Hypertext
              Transfer Protocol -- HTTP/1.1", RFC 2616,
              DOI 10.17487/RFC2616, June 1999,
              <https://www.rfc-editor.org/info/rfc2616>.

   [RFC2818]  Rescorla, E., "HTTP Over TLS", RFC 2818,
              DOI 10.17487/RFC2818, May 2000,
              <https://www.rfc-editor.org/info/rfc2818>.

   [RFC3040]  Cooper, I., Melve, I., and G. Tomlinson, "Internet Web
              Replication and Caching Taxonomy", RFC 3040,
              DOI 10.17487/RFC3040, January 2001,
              <https://www.rfc-editor.org/info/rfc3040>.

   [RFC4033]  Arends, R., Austein, R., Larson, M., Massey, D., and S.
              Rose, "DNS Security Introduction and Requirements",
              RFC 4033, DOI 10.17487/RFC4033, March 2005,
              <https://www.rfc-editor.org/info/rfc4033>.

   [RFC4559]  Jaganathan, K., Zhu, L., and J. Brezak, "SPNEGO-based
              Kerberos and NTLM HTTP Authentication in Microsoft
              Windows", RFC 4559, DOI 10.17487/RFC4559, June 2006,
              <https://www.rfc-editor.org/info/rfc4559>.

   [RFC5226]  Narten, T. and H. Alvestrand, "Guidelines for Writing an
              IANA Considerations Section in RFCs", BCP 26, RFC 5226,
              DOI 10.17487/RFC5226, May 2008,
              <https://www.rfc-editor.org/info/rfc5226>.

   [RFC5246]  Dierks, T. and E. Rescorla, "The Transport Layer Security
              (TLS) Protocol Version 1.2", RFC 5246,
              DOI 10.17487/RFC5246, August 2008,
              <https://www.rfc-editor.org/info/rfc5246>.

   [RFC5322]  Resnick, P., "Internet Message Format", RFC 5322,
              DOI 10.17487/RFC5322, October 2008,
              <https://www.rfc-editor.org/info/rfc5322>.

   [RFC6265]  Barth, A., "HTTP State Management Mechanism", RFC 6265,
              DOI 10.17487/RFC6265, April 2011,
              <https://www.rfc-editor.org/info/rfc6265>.

   [RFC6585]  Nottingham, M. and R. Fielding, "Additional HTTP Status
              Codes", RFC 6585, DOI 10.17487/RFC6585, April 2012,
              <https://www.rfc-editor.org/info/rfc6585>.

   [RFC7230]  Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
              Protocol (HTTP/1.1): Message Syntax and Routing",
              RFC 7230, DOI 10.17487/RFC7230, June 2014,
              <https://www.rfc-editor.org/info/rfc7230>.

Appendix A.  HTTP Version History

   HTTP has been in use since 1990.  The first version, later referred
   to as HTTP/0.9, was a simple protocol for hypertext data transfer
   across the Internet, using only a single request method (GET) and no
   metadata.  HTTP/1.0, as defined by [RFC1945], added a range of
   request methods and MIME-like messaging, allowing for metadata to be
   transferred and modifiers placed 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.

   HTTP/1.1 remains compatible with HTTP/1.0 by including more stringent
   requirements that enable reliable implementations, adding only those
   features that can either be safely ignored by an HTTP/1.0 recipient
   or only be sent when communicating with a party advertising
   conformance with HTTP/1.1.

   HTTP/1.1 has been designed to make supporting previous versions easy.
   A general-purpose HTTP/1.1 server ought to be able to understand any
   valid request in the format of HTTP/1.0, responding appropriately
   with an HTTP/1.1 message that only uses features understood (or
   safely ignored) by HTTP/1.0 clients.  Likewise, an HTTP/1.1 client
   can be expected to understand any valid HTTP/1.0 response.

   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 caused by a client failing to
   properly encode the request-target.

A.1.  Changes from HTTP/1.0

   This section summarizes major differences between versions HTTP/1.0
   and HTTP/1.1.

A.1.1.  Multihomed Web Servers

   The requirements that clients and servers support the Host header
   field (Section 5.4), report an error if it is missing from an
   HTTP/1.1 request, and accept absolute URIs (Section 5.3) are among
   the most important changes defined by HTTP/1.1.

   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.

A.1.2.  Keep-Alive Connections

   In 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 explicitly negotiated
   ("Keep-Alive") version of persistent connections described in
   Section 19.7.1 of [RFC2068].

   Some clients and servers might wish to be compatible with these
   previous approaches to persistent connections, by explicitly
   negotiating for them with a "Connection: keep-alive" request header
   field.  However, some experimental implementations of HTTP/1.0
   persistent connections are faulty; for example, if an HTTP/1.0 proxy
   server doesn't understand Connection, it will erroneously forward
   that header field to the next inbound server, which would result in a
   hung connection.

   One attempted solution was the introduction of a Proxy-Connection
   header field, targeted specifically at proxies.  In practice, this
   was also unworkable, because proxies are often deployed in multiple
   layers, bringing about the same problem discussed above.

   As a result, clients are encouraged not to send the Proxy-Connection
   header field in any requests.

   Clients are also encouraged to consider the use of Connection: keep-
   alive in requests carefully; while they can enable persistent
   connections with HTTP/1.0 servers, clients using them will need to
   monitor the connection for "hung" requests (which indicate that the
   client ought stop sending the header field), and this mechanism ought
   not be used by clients at all when a proxy is being used.

A.1.3.  Introduction of Transfer-Encoding

   HTTP/1.1 introduces the Transfer-Encoding header field
   (Section 3.3.1).  Transfer codings need to be decoded prior to
   forwarding an HTTP message over a MIME-compliant protocol.

A.2.  Changes from RFC 7230

   None yet.

Appendix B.  Collected ABNF

   BWS = OWS

   Connection = *( "," OWS ) connection-option *( OWS "," [ OWS
    connection-option ] )
   Content-Length = 1*DIGIT

   HTTP-message = start-line *( header-field CRLF ) CRLF [ message-body
    ]
   HTTP-name = %x48.54.54.50 ; HTTP
   HTTP-version = HTTP-name "/" DIGIT "." DIGIT
   Host = uri-host [ ":" port ]

   OWS = *( SP / HTAB )

   RWS = 1*( SP / HTAB )

   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, see [RFC3986], Section 4.1>
   Upgrade = *( "," OWS ) protocol *( OWS "," [ OWS protocol ] )

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

   absolute-URI = <absolute-URI, see [RFC3986], ] )

   absolute-URI = <absolute-URI, see [RFC3986], Section 4.3>
   absolute-form = absolute-URI
   absolute-path = 1*( "/" segment ) <absolute-path, see [Semantics], Section 2.4>
   asterisk-form = "*"
   authority = <authority, see [RFC3986], Section 3.2>
   authority-form = authority

   chunk = chunk-size [ chunk-ext ] CRLF chunk-data CRLF
   chunk-data = 1*OCTET
   chunk-ext = *( ";" chunk-ext-name [ "=" chunk-ext-val ] )
   chunk-ext-name = token
   chunk-ext-val = token / quoted-string
   chunk-size = 1*HEXDIG
   chunked-body = *chunk last-chunk trailer-part CRLF
   comment = "(" *( ctext / quoted-pair / comment ) ")" <comment, see [Semantics], Section 4.2.3>
   connection-option = token
   ctext = HTAB / SP / %x21-27 ; '!'-'''
    / %x2A-5B ; '*'-'['
    / %x5D-7E ; ']'-'~'
    / obs-text

   field-content = field-vchar [ 1*( SP / HTAB ) field-vchar ]

   field-name = token <field-name, see [Semantics], Section 4.2>
   field-value = *( field-content / obs-fold )
   field-vchar = VCHAR / obs-text
   fragment = <fragment, <field-value, see [RFC3986], [Semantics], Section 3.5> 4.2>

   header-field = field-name ":" OWS field-value OWS
   http-URI

   last-chunk = "http://" authority path-abempty [ "?" query ] 1*"0" [ "#"
    fragment chunk-ext ]
   https-URI CRLF
   message-body = "https://" authority path-abempty *OCTET
   method = token

   obs-fold = CRLF 1*( SP / HTAB )
   obs-text = <obs-text, see [Semantics], Section 4.2.3>
   origin-form = absolute-path [ "?" query ]

   port = <port, see [RFC3986], Section 3.2.3>
   protocol = protocol-name [ "#"
    fragment "/" protocol-version ]

   last-chunk
   protocol-name = 1*"0" token
   protocol-version = token

   query = <query, see [RFC3986], Section 3.4>
   quoted-string = <quoted-string, see [Semantics], Section 4.2.3>

   rank = ( "0" [ chunk-ext "." *3DIGIT ] CRLF

   message-body ) / ( "1" [ "." *3"0" ] )
   reason-phrase = *( HTAB / SP / VCHAR / obs-text )
   request-line = *OCTET method SP request-target SP HTTP-version CRLF
   request-target = origin-form / absolute-form / authority-form /
    asterisk-form

   start-line = request-line / status-line
   status-code = 3DIGIT
   status-line = HTTP-version SP status-code SP reason-phrase CRLF

   t-codings = "trailers" / ( transfer-coding [ t-ranking ] )
   t-ranking = OWS ";" OWS "q=" rank
   token

   obs-fold = <token, see [Semantics], Section 4.2.3>
   trailer-part = *( header-field CRLF 1*( SP / HTAB )
   obs-text
   transfer-coding = %x80-FF
   origin-form "chunked" / "compress" / "deflate" / "gzip" /
    transfer-extension
   transfer-extension = absolute-path [ "?" query ]

   partial-URI token *( OWS ";" OWS transfer-parameter )
   transfer-parameter = relative-part [ "?" query ]
   path-abempty token BWS "=" BWS ( token / quoted-string )

   uri-host = <path-abempty, <host, see [RFC3986], Section 3.3>
   port = <port, see [RFC3986], 3.2.2>

Appendix B.  Differences between HTTP and MIME

   HTTP/1.1 uses many of the constructs defined for the Internet Message
   Format [RFC5322] and the Multipurpose Internet Mail Extensions (MIME)
   [RFC2045] to allow a message body to be transmitted in an open
   variety of representations and with extensible header fields.
   However, RFC 2045 is focused only on email; applications of HTTP have
   many characteristics that differ from email; hence, HTTP has features
   that differ from MIME.  These differences were carefully chosen to
   optimize performance over binary connections, to allow greater
   freedom in the use of new media types, to make date comparisons
   easier, and to acknowledge the practice of some early HTTP servers
   and clients.

   This appendix describes specific areas where HTTP differs from MIME.
   Proxies and gateways to and from strict MIME environments need to be
   aware of these differences and provide the appropriate conversions
   where necessary.

B.1.  MIME-Version

   HTTP is not a MIME-compliant protocol.  However, messages can include
   a single MIME-Version header field to indicate what version of the
   MIME protocol was used to construct the message.  Use of the MIME-
   Version header field indicates that the message is in full
   conformance with the MIME protocol (as defined in [RFC2045]).
   Senders are responsible for ensuring full conformance (where
   possible) when exporting HTTP messages to strict MIME environments.

B.2.  Conversion to Canonical Form

   MIME requires that an Internet mail body part be converted to
   canonical form prior to being transferred, as described in Section 4
   of [RFC2049].  Section 3.2.3> 6.1.1.2 of [Semantics] describes the forms
   allowed for subtypes of the "text" media type when transmitted over
   HTTP.  [RFC2046] requires that content with a type of "text"
   represent line breaks as CRLF and forbids the use of CR or LF outside
   of line break sequences.  HTTP allows CRLF, bare CR, and bare LF to
   indicate a line break within text content.

   A proxy or gateway from HTTP to a strict MIME environment ought to
   translate all line breaks within text media types to the RFC 2049
   canonical form of CRLF.  Note, however, this might be complicated by
   the presence of a Content-Encoding and by the fact that HTTP allows
   the use of some charsets that do not use octets 13 and 10 to
   represent CR and LF, respectively.

   Conversion will break any cryptographic checksums applied to the
   original content unless the original content is already in canonical
   form.  Therefore, the canonical form is recommended for any content
   that uses such checksums in HTTP.

B.3.  Conversion of Date Formats

   HTTP/1.1 uses a restricted set of date formats (Section 10.1.1.1 of
   [Semantics]) to simplify the process of date comparison.  Proxies and
   gateways from other protocols ought to ensure that any Date header
   field present in a message conforms to one of the HTTP/1.1 formats
   and rewrite the date if necessary.

B.4.  Conversion of Content-Encoding

   MIME does not include any concept equivalent to HTTP/1.1's Content-
   Encoding header field.  Since this acts as a modifier on the media
   type, proxies and gateways from HTTP to MIME-compliant protocols
   ought to either change the value of the Content-Type header field or
   decode the representation before forwarding the message.  (Some
   experimental applications of Content-Type for Internet mail have used
   a media-type parameter of ";conversions=<content-coding>" to perform
   a function equivalent to Content-Encoding.  However, this parameter
   is not part of the MIME standards).

B.5.  Conversion of Content-Transfer-Encoding

   HTTP does not use the Content-Transfer-Encoding field of MIME.
   Proxies and gateways from MIME-compliant protocols to HTTP need to
   remove any Content-Transfer-Encoding prior to delivering the response
   message to an HTTP client.

   Proxies and gateways from HTTP to MIME-compliant protocols are
   responsible for ensuring that the message is in the correct format
   and encoding for safe transport on that protocol, where "safe
   transport" is defined by the limitations of the protocol = protocol-name [ "/" protocol-version ]
   protocol-name = token
   protocol-version = token
   pseudonym = token

   qdtext = HTAB / SP / "!" / %x23-5B ; '#'-'['
    / %x5D-7E ; ']'-'~'
    / obs-text
   query = <query, see [RFC3986], Section 3.4>
   quoted-pair = "\" ( HTAB / SP / VCHAR / obs-text )
   quoted-string = DQUOTE *( qdtext / quoted-pair ) DQUOTE

   rank = ( "0" [ "." *3DIGIT ] ) / ( "1" [ "." *3"0" ] )
   reason-phrase = *( HTAB / SP / VCHAR / obs-text )
   received-by = ( uri-host [ ":" port ] ) / pseudonym
   received-protocol = [ protocol-name "/" ] protocol-version
   relative-part = <relative-part, see [RFC3986], Section 4.2>
   request-line = being used.
   Such a proxy or gateway ought to transform and label the data with an
   appropriate Content-Transfer-Encoding if doing so will improve the
   likelihood of safe transport over the destination protocol.

B.6.  MHTML and Line Length Limitations

   HTTP implementations that share code with MHTML [RFC2557]
   implementations need to be aware of MIME line length limitations.
   Since HTTP does not have this limitation, HTTP does not fold long
   lines.  MHTML messages being transported by HTTP follow all
   conventions of MHTML, including line length limitations and folding,
   canonicalization, etc., since HTTP transfers message-bodies as
   payload and, aside from the "multipart/byteranges" type
   (Section 6.3.4 of [Semantics]), does not interpret the content or any
   MIME header lines that might be contained therein.

Appendix C.  HTTP Version History

   HTTP has been in use since 1990.  The first version, later referred
   to as HTTP/0.9, was a simple protocol for hypertext data transfer
   across the Internet, using only a single request method SP request-target SP HTTP-version CRLF
   request-target = origin-form / absolute-form / authority-form /
    asterisk-form

   scheme = <scheme, see [RFC3986], Section 3.1>
   segment = <segment, see [RFC3986], Section 3.3>
   start-line = request-line / status-line
   status-code = 3DIGIT
   status-line = HTTP-version SP status-code SP reason-phrase CRLF

   t-codings = "trailers" / ( transfer-coding [ t-ranking ] )
   t-ranking = OWS ";" OWS "q=" rank
   tchar = "!" / "#" / "$" / "%" / "&" / "'" / "*" / "+" / "-" / "." /
    "^" / "_" / "`" / "|" / "~" / DIGIT / ALPHA
   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 = token BWS "=" BWS ( token / quoted-string )

   uri-host = <host, see [RFC3986], (GET) and no
   metadata.  HTTP/1.0, as defined by [RFC1945], added a range of
   request methods and MIME-like messaging, allowing for metadata to be
   transferred and modifiers placed 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.

   HTTP/1.1 remains compatible with HTTP/1.0 by including more stringent
   requirements that enable reliable implementations, adding only those
   features that can either be safely ignored by an HTTP/1.0 recipient
   or only be sent when communicating with a party advertising
   conformance with HTTP/1.1.

   HTTP/1.1 has been designed to make supporting previous versions easy.
   A general-purpose HTTP/1.1 server ought to be able to understand any
   valid request in the format of HTTP/1.0, responding appropriately
   with an HTTP/1.1 message that only uses features understood (or
   safely ignored) by HTTP/1.0 clients.  Likewise, an HTTP/1.1 client
   can be expected to understand any valid HTTP/1.0 response.

   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 caused by a client failing to
   properly encode the request-target.

C.1.  Changes from HTTP/1.0

   This section summarizes major differences between versions HTTP/1.0
   and HTTP/1.1.

C.1.1.  Multihomed Web Servers

   The requirements that clients and servers support the Host header
   field (Section 5.4 of [Semantics]), report an error if it is missing
   from an HTTP/1.1 request, and accept absolute URIs (Section 3.2) are
   among the most important changes defined by HTTP/1.1.

   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.

C.1.2.  Keep-Alive Connections

   In 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 explicitly negotiated
   ("Keep-Alive") version of persistent connections described in
   Section 3.2.2> 19.7.1 of [RFC2068].

   Some clients and servers might wish to be compatible with these
   previous approaches to persistent connections, by explicitly
   negotiating for them with a "Connection: keep-alive" request header
   field.  However, some experimental implementations of HTTP/1.0
   persistent connections are faulty; for example, if an HTTP/1.0 proxy
   server doesn't understand Connection, it will erroneously forward
   that header field to the next inbound server, which would result in a
   hung connection.

   One attempted solution was the introduction of a Proxy-Connection
   header field, targeted specifically at proxies.  In practice, this
   was also unworkable, because proxies are often deployed in multiple
   layers, bringing about the same problem discussed above.

   As a result, clients are encouraged not to send the Proxy-Connection
   header field in any requests.

   Clients are also encouraged to consider the use of Connection: keep-
   alive in requests carefully; while they can enable persistent
   connections with HTTP/1.0 servers, clients using them will need to
   monitor the connection for "hung" requests (which indicate that the
   client ought stop sending the header field), and this mechanism ought
   not be used by clients at all when a proxy is being used.

C.1.3.  Introduction of Transfer-Encoding

   HTTP/1.1 introduces the Transfer-Encoding header field (Section 6.1).
   Transfer codings need to be decoded prior to forwarding an HTTP
   message over a MIME-compliant protocol.

C.2.  Changes from RFC 7230

   Most of the sections introducing HTTP's design goals, history,
   architecture, conformance criteria, protocol versioning, URIs,
   message routing, and header field values have been moved to
   [Semantics].  This document has been reduced to just the messaging
   syntax and connection management requirements specific to HTTP/1.1.

Appendix C. D.  Change Log

   This section is to be removed before publishing as an RFC.

C.1.  Since RFC 7230

D.1.  Between RFC7230 and draft 00

   The changes in this draft are were purely editorial:

   o  Change boilerplate and abstract to indicate the "draft" status,
      and update references to ancestor specifications.

   o  Adjust historical notes.

   o  Update links to sibling specifications.

   o  Replace sections listing changes from RFC 2616 by new empty
      sections referring to RFC 723x.

   o  Remove acknowledgements specific to RFC 723x.

   o  Move "Acknowledgements" to the very end and make them unnumbered.

D.2.  Since draft-ietf-httpbis-messaging-00

   The changes in this draft are editorial, with respect to HTTP as a
   whole, to move all core HTTP semantics into [Semantics]:

   o  Moved introduction, architecture, conformance, and ABNF extensions
      from RFC 7230 (Messaging) to semantics [Semantics].

   o  Moved discussion of MIME differences from RFC 7231 (Semantics) to
      Appendix B since they mostly cover transforming 1.1 messages.

   o  Moved all extensibility tips, registration procedures, and
      registry tables from the IANA considerations to normative
      sections, reducing the IANA considerations to just instructions
      that will be removed prior to publication as an RFC.

Index

   A
      absolute-form (of request-target)  41
      accelerator  10
      application/http Media Type  62  38
      asterisk-form (of request-target)  42
      authoritative response  66  11
      authority-form (of request-target)  42

   B
      browser  7  11

   C
      Connection header field  50, 55  27, 33
      Content-Length header field  29
      cache  11
      cacheable  11
      captive portal  11  18
      Content-Transfer-Encoding header field  48
      chunked (Coding Format)  28, 31, 35
      client  7  17, 19
      chunked (transfer coding)  22
      close  50, 55  27, 33
      compress (Coding Format)  38
      connection  7 (transfer coding)  24

   D
      Delimiters  26
      deflate (Coding Format)  38
      downstream  10 (transfer coding)  24

   E
      effective request URI  44  12

   G
      Grammar
         absolute-form  41
         absolute-path  16
         absolute-URI  16  9-10
         ALPHA  6  5
         asterisk-form  41-42
         authority  16  9, 11
         authority-form  41-42
         BWS  24  9, 11
         chunk  35  22
         chunk-data  35  22
         chunk-ext  35-36  22
         chunk-ext-name  36  22
         chunk-ext-val  36  22
         chunk-size  35  22
         chunked-body  35-36
         comment  27  22
         Connection  50  28
         connection-option  50
         Content-Length  30  28
         CR  6  5
         CRLF  6
         ctext  27  5
         CTL  6  5
         DIGIT  6  5
         DQUOTE  6
         field-content  22  5
         field-name  22, 39  14
         field-value  22
         field-vchar  22
         fragment  16  14
         header-field  22, 36  14, 23
         HEXDIG  6
         Host  43  5
         HTAB  6  5
         HTTP-message  19  6
         HTTP-name  14
         http-URI  17  6
         HTTP-version  14
         https-URI  18  6
         last-chunk  35  22
         LF  6  5
         message-body  27  16
         method  21  9
         obs-fold  22
         obs-text  27  15
         OCTET  6  5
         origin-form  41
         OWS  24
         partial-URI  16
         port  16
         protocol-name  47
         protocol-version  47
         pseudonym  47
         qdtext  27
         query  16
         quoted-pair  27
         quoted-string  27  9-10
         rank  38  25
         reason-phrase  22
         received-by  47
         received-protocol  47  14
         request-line  21  8
         request-target  41
         RWS  24
         scheme  16
         segment  16  9
         SP  6  5
         start-line  20  6
         status-code  22  14
         status-line  22  13
         t-codings  38  25
         t-ranking  38
         tchar  26  25
         TE  38
         token  26
         Trailer  39  25
         trailer-part  35-36  22-23
         transfer-coding  35  21
         Transfer-Encoding  28  17
         transfer-extension  35  21
         transfer-parameter  35  21
         Upgrade  56
         uri-host  16
         URI-reference  16  34
         VCHAR  6
         Via  47
      gateway  10  5
      gzip (Coding Format)  38 (transfer coding)  24

   H
      Host header field  43
      header field  19  6
      header section  19  6
      headers  19
      http URI scheme  16
      https URI scheme  18

   I
      inbound  10
      interception proxy  11
      intermediary  9  6

   M
      MIME-Version header field  47
      Media Type
         application/http  62  38
         message/http  61
      message  7  37
      message/http Media Type  61  37
      method  21

   N
      non-transforming proxy  48  9

   O
      origin server  7
      origin-form (of request-target)  41
      outbound  10

   P
      phishing  66
      proxy  10

   R
      recipient  7
      request  7
      request-target  21
      resource  16
      response  7
      reverse proxy  10

   S
      sender  7
      server  7
      spider  7  9

   T
      TE header field  38
      Trailer header field  39  25
      Transfer-Encoding header field  28
      target URI  40
      target resource  40
      transforming proxy  48
      transparent proxy  11
      tunnel  10  17

   U
      URI scheme
         http  16
         https  18
      Upgrade header field  56
      upstream  10
      user agent  7

   V
      Via header field  46  34

   X
      x-compress (transfer coding)  24
      x-gzip (transfer coding)  24

Acknowledgments

   This edition of the HTTP specification builds on the many
   contributions that went into RFC 1945, RFC 2068, RFC 2145, and RFC
   2616, including substantial contributions made by the previous
   authors, editors, and Working Group Chairs: Tim Berners-Lee, Ari
   Luotonen, Roy T.  Fielding, Henrik Frystyk Nielsen, Jim Gettys,
   Jeffrey C.  Mogul, Larry Masinter, Paul J.  Leach, and Yves Lafon.

   See Section 10 Appendix "Acknowledgments" of [RFC7230] for additional acknowledgements from
   prior revisions.

   [[newacks: New acks to be added here.]] [Semantics].

Authors' Addresses

   Roy T. Fielding (editor)
   Adobe
   345 Park Ave
   San Jose, CA  95110
   USA

   EMail: fielding@gbiv.com
   URI:   http://roy.gbiv.com/   https://roy.gbiv.com/

   Mark Nottingham (editor)
   Fastly

   EMail: mnot@mnot.net
   URI:   https://www.mnot.net/

   Julian F. Reschke (editor)
   greenbytes GmbH
   Hafenweg 16
   Muenster, NW  48155
   Germany

   EMail: julian.reschke@greenbytes.de
   URI:   http://greenbytes.de/tech/webdav/   https://greenbytes.de/tech/webdav/