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Internet Engineering Task Force (IETF)                  R. Fielding, Ed.
Request for Comments: 9112                                         Adobe
STD: 99                                               M. Nottingham, Ed.
Obsoletes: 7230                                                   Fastly
Category: Standards Track                                J. Reschke, Ed.
ISSN: 2070-1721                                               greenbytes
                                                               June 2022


                                HTTP/1.1

Abstract

   The Hypertext Transfer Protocol (HTTP) is a stateless application-
   level protocol for distributed, collaborative, hypertext information
   systems.  This document specifies the HTTP/1.1 message syntax,
   message parsing, connection management, and related security
   concerns.

   This document obsoletes portions of RFC 7230.

Status of This Memo

   This is an Internet Standards Track document.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   Internet Standards is available in Section 2 of RFC 7841.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   https://www.rfc-editor.org/info/rfc9112.

Copyright Notice

   Copyright (c) 2022 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (https://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Revised BSD License text as described in Section 4.e of the
   Trust Legal Provisions and are provided without warranty as described
   in the Revised BSD License.

   This document may contain material from IETF Documents or IETF
   Contributions published or made publicly available before November
   10, 2008.  The person(s) controlling the copyright in some of this
   material may not have granted the IETF Trust the right to allow
   modifications of such material outside the IETF Standards Process.
   Without obtaining an adequate license from the person(s) controlling
   the copyright in such materials, this document may not be modified
   outside the IETF Standards Process, and derivative works of it may
   not be created outside the IETF Standards Process, except to format
   it for publication as an RFC or to translate it into languages other
   than English.

Table of Contents

   1.  Introduction
     1.1.  Requirements Notation
     1.2.  Syntax Notation
   2.  Message
     2.1.  Message Format
     2.2.  Message Parsing
     2.3.  HTTP Version
   3.  Request Line
     3.1.  Method
     3.2.  Request Target
       3.2.1.  origin-form
       3.2.2.  absolute-form
       3.2.3.  authority-form
       3.2.4.  asterisk-form
     3.3.  Reconstructing the Target URI
   4.  Status Line
   5.  Field Syntax
     5.1.  Field Line Parsing
     5.2.  Obsolete Line Folding
   6.  Message Body
     6.1.  Transfer-Encoding
     6.2.  Content-Length
     6.3.  Message Body Length
   7.  Transfer Codings
     7.1.  Chunked Transfer Coding
       7.1.1.  Chunk Extensions
       7.1.2.  Chunked Trailer Section
       7.1.3.  Decoding Chunked
     7.2.  Transfer Codings for Compression
     7.3.  Transfer Coding Registry
     7.4.  Negotiating Transfer Codings
   8.  Handling Incomplete Messages
   9.  Connection Management
     9.1.  Establishment
     9.2.  Associating a Response to a Request
     9.3.  Persistence
       9.3.1.  Retrying Requests
       9.3.2.  Pipelining
     9.4.  Concurrency
     9.5.  Failures and Timeouts
     9.6.  Tear-down
     9.7.  TLS Connection Initiation
     9.8.  TLS Connection Closure
   10. Enclosing Messages as Data
     10.1.  Media Type message/http
     10.2.  Media Type application/http
   11. Security Considerations
     11.1.  Response Splitting
     11.2.  Request Smuggling
     11.3.  Message Integrity
     11.4.  Message Confidentiality
   12. IANA Considerations
     12.1.  Field Name Registration
     12.2.  Media Type Registration
     12.3.  Transfer Coding Registration
     12.4.  ALPN Protocol ID Registration
   13. References
     13.1.  Normative References
     13.2.  Informative References
   Appendix A.  Collected ABNF
   Appendix B.  Differences between HTTP and MIME
     B.1.  MIME-Version
     B.2.  Conversion to Canonical Form
     B.3.  Conversion of Date Formats
     B.4.  Conversion of Content-Encoding
     B.5.  Conversion of Content-Transfer-Encoding
     B.6.  MHTML and Line Length Limitations
   Appendix C.  Changes from Previous RFCs
     C.1.  Changes from HTTP/0.9
     C.2.  Changes from HTTP/1.0
       C.2.1.  Multihomed Web Servers
       C.2.2.  Keep-Alive Connections
       C.2.3.  Introduction of Transfer-Encoding
     C.3.  Changes from RFC 7230
   Acknowledgements
   Index
   Authors' Addresses

1.  Introduction

   The Hypertext Transfer Protocol (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/1.1 is defined by:

   *  This document

   *  "HTTP Semantics" [HTTP]

   *  "HTTP Caching" [CACHING]

   This document specifies how HTTP semantics are conveyed using the
   HTTP/1.1 message syntax, framing, and connection management
   mechanisms.  Its goal is to define the complete set of requirements
   for HTTP/1.1 message parsers and message-forwarding intermediaries.

   This document obsoletes the portions of RFC 7230 related to HTTP/1.1
   messaging and connection management, with the changes being
   summarized in Appendix C.3.  The other parts of RFC 7230 are
   obsoleted by "HTTP Semantics" [HTTP].

1.1.  Requirements Notation

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in
   BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

   Conformance criteria and considerations regarding error handling are
   defined in Section 2 of [HTTP].

1.2.  Syntax Notation

   This specification uses the Augmented Backus-Naur Form (ABNF)
   notation of [RFC5234], extended with the notation for case-
   sensitivity in strings defined in [RFC7405].

   It also uses a list extension, defined in Section 5.6.1 of [HTTP],
   that allows for compact definition of comma-separated lists using a
   "#" operator (similar to how the "*" operator indicates repetition).
   Appendix A shows the collected grammar with all list operators
   expanded to standard ABNF notation.

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

   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).

   The rules below are defined in [HTTP]:

     BWS           = <BWS, see [HTTP], Section 5.6.3>
     OWS           = <OWS, see [HTTP], Section 5.6.3>
     RWS           = <RWS, see [HTTP], Section 5.6.3>
     absolute-path = <absolute-path, see [HTTP], Section 4.1>
     field-name    = <field-name, see [HTTP], Section 5.1>
     field-value   = <field-value, see [HTTP], Section 5.5>
     obs-text      = <obs-text, see [HTTP], Section 5.6.4>
     quoted-string = <quoted-string, see [HTTP], Section 5.6.4>
     token         = <token, see [HTTP], Section 5.6.2>
     transfer-coding =
                     <transfer-coding, see [HTTP], Section 10.1.4>

   The rules below are defined in [URI]:

     absolute-URI  = <absolute-URI, see [URI], Section 4.3>
     authority     = <authority, see [URI], Section 3.2>
     uri-host      = <host, see [URI], Section 3.2.2>
     port          = <port, see [URI], Section 3.2.3>
     query         = <query, see [URI], Section 3.4>

2.  Message

   HTTP/1.1 clients and servers communicate by sending messages.  See
   Section 3 of [HTTP] for the general terminology and core concepts of
   HTTP.

2.1.  Message Format

   An HTTP/1.1 message consists of a start-line followed by a CRLF and a
   sequence of octets in a format similar to the Internet Message Format
   [RFC5322]: zero or more header field lines (collectively referred to
   as the "headers" or the "header section"), an empty line indicating
   the end of the header section, and an optional message body.

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

   A message can be either a request from client to server or a response
   from server to client.  Syntactically, the two types of messages
   differ only in the start-line, which is either a request-line (for
   requests) or a status-line (for responses), and in the algorithm for
   determining the length of the message body (Section 6).

     start-line     = request-line / status-line

   In theory, a client could receive requests and a server could receive
   responses, distinguishing them by their different start-line formats.
   In practice, servers are implemented to only expect a request (a
   response is interpreted as an unknown or invalid request method), and
   clients are implemented to only expect a response.

   HTTP makes use of some protocol elements similar to the Multipurpose
   Internet Mail Extensions (MIME) [RFC2045].  See Appendix B for the
   differences between HTTP and MIME messages.

2.2.  Message Parsing

   The normal procedure for parsing an HTTP message is to read the
   start-line into a structure, read each header field line into a hash
   table by field name until the empty line, and then use the parsed
   data to determine if a message body is expected.  If a message body
   has been indicated, then it is read as a stream until an amount of
   octets equal to the message body length is read or the connection is
   closed.

   A recipient MUST parse an HTTP message as a sequence of octets in an
   encoding that is a superset of US-ASCII [USASCII].  Parsing an HTTP
   message as a stream of Unicode characters, without regard for the
   specific encoding, creates security vulnerabilities due to the
   varying ways that string processing libraries handle invalid
   multibyte character sequences 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 line value after message parsing has delineated the
   individual field lines.

   Although the line terminator for the start-line and fields is the
   sequence CRLF, a recipient MAY recognize a single LF as a line
   terminator and ignore any preceding CR.

   A sender MUST NOT generate a bare CR (a CR character not immediately
   followed by LF) within any protocol elements other than the content.
   A recipient of such a bare CR MUST consider that element to be
   invalid or replace each bare CR with SP before processing the element
   or forwarding the message.

   Older HTTP/1.0 user agent implementations might send an extra CRLF
   after a POST request as a workaround for some early server
   applications that failed to read message body content that was not
   terminated by a line-ending.  An HTTP/1.1 user agent MUST NOT preface
   or follow a request with an extra CRLF.  If terminating the request
   message body with a line-ending is desired, then the user agent MUST
   count the terminating CRLF octets as part of the message body length.

   In the interest of robustness, a server that is expecting to receive
   and parse a request-line SHOULD ignore at least one empty line (CRLF)
   received prior to the request-line.

   A sender MUST NOT send whitespace between the start-line and the
   first header field.

   A recipient that 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 header section is terminated).  Rejection or removal
   of invalid whitespace-preceded lines is necessary to prevent their
   misinterpretation by downstream recipients that might be vulnerable
   to request smuggling (Section 11.2) or response splitting
   (Section 11.1) attacks.

   When a server listening only for HTTP request messages, or processing
   what appears from the start-line to be an HTTP request message,
   receives a sequence of octets that does not match the HTTP-message
   grammar aside from the robustness exceptions listed above, the server
   SHOULD respond with a 400 (Bad Request) response and close the
   connection.

2.3.  HTTP Version

   HTTP uses a "<major>.<minor>" numbering scheme to indicate versions
   of the protocol.  This specification defines version "1.1".
   Section 2.5 of [HTTP] specifies the semantics of HTTP version
   numbers.

   The version of an HTTP/1.x message is indicated by an HTTP-version
   field in the start-line.  HTTP-version is case-sensitive.

     HTTP-version  = HTTP-name "/" DIGIT "." DIGIT
     HTTP-name     = %s"HTTP"

   When an HTTP/1.1 message is sent to an HTTP/1.0 recipient [HTTP/1.0]
   or a recipient whose version is unknown, the HTTP/1.1 message is
   constructed such that it can be interpreted as a valid HTTP/1.0
   message if all of the newer features are ignored.  This specification
   places recipient-version requirements on some new features so that a
   conformant sender will only use compatible features until it has
   determined, through configuration or the receipt of a message, that
   the recipient supports HTTP/1.1.

   Intermediaries that process HTTP messages (i.e., all intermediaries
   other than those acting as tunnels) MUST send their own HTTP-version
   in forwarded messages, unless it is purposefully downgraded as a
   workaround for an upstream issue.  In other words, an intermediary is
   not allowed to blindly forward the start-line without ensuring that
   the protocol version in that message matches a version to which that
   intermediary is conformant for both the receiving and sending of
   messages.  Forwarding an HTTP message without rewriting the HTTP-
   version might result in communication errors when downstream
   recipients use the message sender's version to determine what
   features are safe to use for later communication with that sender.

   A server MAY send an HTTP/1.0 response to an HTTP/1.1 request if it
   is known or suspected that the client incorrectly implements the HTTP
   specification and is incapable of correctly processing later version
   responses, such as when a client fails to parse the version number
   correctly or when an intermediary is known to blindly forward the
   HTTP-version even when it doesn't conform to the given minor version
   of the protocol.  Such protocol downgrades SHOULD NOT be performed
   unless triggered by specific client attributes, such as when one or
   more of the request header fields (e.g., User-Agent) uniquely match
   the values sent by a client known to be in error.

3.  Request Line

   A request-line begins with a method token, followed by a single space
   (SP), the request-target, and another single space (SP), and ends
   with the protocol version.

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

   Although the request-line grammar rule requires that each of the
   component elements be separated by a single SP octet, recipients MAY
   instead parse on whitespace-delimited word boundaries and, aside from
   the CRLF terminator, treat any form of whitespace as the SP separator
   while ignoring preceding or trailing whitespace; such whitespace
   includes one or more of the following octets: SP, HTAB, VT (%x0B), FF
   (%x0C), or bare CR.  However, lenient parsing can result in request
   smuggling security vulnerabilities if there are multiple recipients
   of the message and each has its own unique interpretation of
   robustness (see Section 11.2).

   HTTP does not place a predefined limit on the length of a request-
   line, as described in Section 2.3 of [HTTP].  A server that 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 15.5.15 of [HTTP]).

   Various ad hoc limitations on request-line length are found in
   practice.  It is RECOMMENDED that all HTTP senders and recipients
   support, at a minimum, request-line lengths of 8000 octets.

3.1.  Method

   The method token indicates the request method to be performed on the
   target resource.  The request method is case-sensitive.

     method         = token

   The request methods defined by this specification can be found in
   Section 9 of [HTTP], along with information regarding the HTTP method
   registry and considerations for defining new methods.

3.2.  Request Target

   The request-target identifies the target resource upon which to apply
   the request.  The client derives a request-target from its desired
   target URI.  There are four distinct formats for the request-target,
   depending on both the method being requested and whether the request
   is to a proxy.

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

   No whitespace is allowed in the request-target.  Unfortunately, some
   user agents fail to properly encode or exclude whitespace found in
   hypertext references, resulting in those disallowed characters being
   sent as the request-target in a malformed request-line.

   Recipients of an invalid request-line SHOULD respond with either a
   400 (Bad Request) error or a 301 (Moved Permanently) redirect with
   the request-target properly encoded.  A recipient SHOULD NOT attempt
   to autocorrect and then process the request without a redirect, since
   the invalid request-line might be deliberately crafted to bypass
   security filters along the request chain.

   A client MUST send a Host header field (Section 7.2 of [HTTP]) in all
   HTTP/1.1 request messages.  If the target URI includes an authority
   component, then a client MUST send a field value for Host that is
   identical to that authority component, excluding any userinfo
   subcomponent and its "@" delimiter (Section 4.2 of [HTTP]).  If the
   authority component is missing or undefined for the target URI, then
   a client MUST send a Host header field with an empty field value.

   A server MUST respond with a 400 (Bad Request) status code to any
   HTTP/1.1 request message that lacks a Host header field and to any
   request message that contains more than one Host header field line or
   a Host header field with an invalid field value.

3.2.1.  origin-form

   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 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.  If the target URI's path component is
   empty, the client MUST send "/" as the path within the origin-form of
   request-target.  A Host header field is also sent, as defined in
   Section 7.2 of [HTTP].

   For example, a client wishing 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 to port 80 of the host "www.example.org" and send the
   lines:

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

   followed by the remainder of the request message.

3.2.2.  absolute-form

   When making a request to a proxy, other than a CONNECT or server-wide
   OPTIONS request (as detailed below), a client MUST send the target
   URI in "absolute-form" as the request-target.

     absolute-form  = absolute-URI

   The proxy is requested to either service that request from a valid
   cache, if possible, or make the same request on the client's behalf
   either to the next inbound proxy server or directly to the origin
   server indicated by the request-target.  Requirements on such
   "forwarding" of messages are defined in Section 7.6 of [HTTP].

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

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

   A client MUST send a Host header field in an HTTP/1.1 request even if
   the request-target is in the absolute-form, since this allows the
   Host information to be forwarded through ancient HTTP/1.0 proxies
   that 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 the host information of the request-
   target.  A proxy that forwards such a request MUST generate a new
   Host field value based on the received request-target rather than
   forward the received Host field value.

   When an origin server receives a request with an absolute-form of
   request-target, the origin server MUST ignore the received Host
   header field (if any) and instead use the host information of the
   request-target.  Note that if the request-target does not have an
   authority component, an empty Host header field will be sent in this
   case.

   A server MUST accept the absolute-form in requests even though most
   HTTP/1.1 clients will only send the absolute-form to a proxy.

3.2.3.  authority-form

   The "authority-form" of request-target is only used for CONNECT
   requests (Section 9.3.6 of [HTTP]).  It consists of only the uri-host
   and port number of the tunnel destination, separated by a colon
   (":").

     authority-form = uri-host ":" port

   When making a CONNECT request to establish a tunnel through one or
   more proxies, a client MUST send only the host and port of the tunnel
   destination as the request-target.  The client obtains the host and
   port from the target URI's authority component, except that it sends
   the scheme's default port if the target URI elides the port.  For
   example, a CONNECT request to "http://www.example.com" looks like the
   following:

   CONNECT www.example.com:80 HTTP/1.1
   Host: www.example.com

3.2.4.  asterisk-form

   The "asterisk-form" of request-target is only used for a server-wide
   OPTIONS request (Section 9.3.7 of [HTTP]).

     asterisk-form  = "*"

   When a client wishes to request OPTIONS for the server as a whole, as
   opposed to a specific named resource of that server, the client MUST
   send only "*" (%x2A) as the request-target.  For example,

   OPTIONS * HTTP/1.1

   If a proxy receives an OPTIONS request with an absolute-form of
   request-target in which the URI has an empty path and no query
   component, then the last proxy on the request chain MUST send a
   request-target of "*" when it forwards the request to the indicated
   origin server.

   For example, the request

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

   would be forwarded by the final proxy as

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

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

3.3.  Reconstructing the Target URI

   The target URI is the request-target when the request-target is in
   absolute-form.  In that case, a server will parse the URI into its
   generic components for further evaluation.

   Otherwise, the server reconstructs the target URI from the connection
   context and various parts of the request message in order to identify
   the target resource (Section 7.1 of [HTTP]):

   *  If the server's configuration provides for a fixed URI scheme, or
      a scheme is provided by a trusted outbound gateway, that scheme is
      used for the target URI.  This is common in large-scale
      deployments because a gateway server will receive the client's
      connection context and replace that with their own connection to
      the inbound server.  Otherwise, if the request is received over a
      secured connection, the target URI's scheme is "https"; if not,
      the scheme is "http".

   *  If the request-target is in authority-form, the target URI's
      authority component is the request-target.  Otherwise, the target
      URI's authority component is the field value of the Host header
      field.  If there is no Host header field or if its field value is
      empty or invalid, the target URI's authority component is empty.

   *  If the request-target is in authority-form or asterisk-form, the
      target URI's combined path and query component is empty.
      Otherwise, the target URI's combined path and query component is
      the request-target.

   *  The components of a reconstructed target URI, once determined as
      above, can be recombined into absolute-URI form by concatenating
      the scheme, "://", authority, and combined path and query
      component.

   Example 1: The following message received over a secure connection

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

   has a target URI of

     https://www.example.org/pub/WWW/TheProject.html

   Example 2: The following message received over an insecure connection

   OPTIONS * HTTP/1.1
   Host: www.example.org:8080

   has a target URI of

     http://www.example.org:8080

   If the target URI's authority component is empty and its URI scheme
   requires a non-empty authority (as is the case for "http" and
   "https"), the server can reject the request or determine whether a
   configured default applies that is consistent with the incoming
   connection's context.  Context might include connection details like
   address and port, what security has been applied, and locally defined
   information specific to that server's configuration.  An empty
   authority is replaced with the configured default before further
   processing of the request.

   Supplying a default name for authority within the context of a
   secured connection is inherently unsafe if there is any chance that
   the user agent's intended authority might differ from the default.  A
   server that can uniquely identify an authority from the request
   context MAY use that identity as a default without this risk.
   Alternatively, it might be better to redirect the request to a safe
   resource that explains how to obtain a new client.

   Note that reconstructing the client's target URI is only half of the
   process for identifying a target resource.  The other half is
   determining whether that target URI identifies a resource for which
   the server is willing and able to send a response, as defined in
   Section 7.4 of [HTTP].

4.  Status Line

   The first line of a response message is the status-line, consisting
   of the protocol version, a space (SP), the status code, and another
   space and ending with an OPTIONAL textual phrase describing the
   status code.

     status-line = HTTP-version SP status-code SP [ reason-phrase ]

   Although the status-line grammar rule requires that each of the
   component elements be separated by 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 the SP separator
   while ignoring preceding or trailing whitespace; such whitespace
   includes one or more of the following octets: SP, HTAB, VT (%x0B), FF
   (%x0C), or bare CR.  However, lenient parsing can result in response
   splitting security vulnerabilities if there are multiple recipients
   of the message and each has its own unique interpretation of
   robustness (see Section 11.1).

   The status-code element is a 3-digit integer code describing the
   result of the server's attempt to understand and satisfy the client's
   corresponding request.  A recipient parses and interprets the
   remainder of the response message in light of the semantics defined
   for that status code, if the status code is recognized by that
   recipient, or in accordance with the class of that status code when
   the specific code is unrecognized.

     status-code    = 3DIGIT

   HTTP's core status codes are defined in Section 15 of [HTTP], along
   with the classes of status codes, considerations for the definition
   of new status codes, and the IANA registry for collecting such
   definitions.

   The reason-phrase element exists for the sole purpose of providing a
   textual description associated with the numeric status code, mostly
   out of deference to earlier Internet application protocols that were
   more frequently used with interactive text clients.

     reason-phrase  = 1*( HTAB / SP / VCHAR / obs-text )

   A client SHOULD ignore the reason-phrase content because it is not a
   reliable channel for information (it might be translated for a given
   locale, overwritten by intermediaries, or discarded when the message
   is forwarded via other versions of HTTP).  A server MUST send the
   space that separates the status-code from the reason-phrase even when
   the reason-phrase is absent (i.e., the status-line would end with the
   space).

5.  Field Syntax

   Each field line consists of a case-insensitive field name followed by
   a colon (":"), optional leading whitespace, the field line value, and
   optional trailing whitespace.

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

   Rules for parsing within field values are defined in Section 5.5 of
   [HTTP].  This section covers the generic syntax for header field
   inclusion within, and extraction from, HTTP/1.1 messages.

5.1.  Field Line Parsing

   Messages are parsed using a generic algorithm, independent of the
   individual field names.  The contents within a given field line value
   are not parsed until a later stage of message interpretation (usually
   after the message's entire field section has been processed).

   No whitespace is allowed between the field name and colon.  In the
   past, differences in the handling of such whitespace have led to
   security vulnerabilities in request routing and response handling.  A
   server MUST reject, with a response status code of 400 (Bad Request),
   any received request message that contains whitespace between a
   header field name and colon.  A proxy MUST remove any such whitespace
   from a response message before forwarding the message downstream.

   A field line value might be preceded and/or followed by optional
   whitespace (OWS); a single SP preceding the field line value is
   preferred for consistent readability by humans.  The field line value
   does not include that leading or trailing whitespace: OWS occurring
   before the first non-whitespace octet of the field line value, or
   after the last non-whitespace octet of the field line value, is
   excluded by parsers when extracting the field line value from a field
   line.

5.2.  Obsolete Line Folding

   Historically, HTTP/1.x 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 except within the "message/http" media type (Section 10.1).

     obs-fold     = OWS CRLF RWS
                  ; obsolete line folding

   A sender MUST NOT generate a message that includes line folding
   (i.e., that has any field line value that contains a match to the
   obs-fold rule) unless the message is intended for packaging within
   the "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 message by
   sending 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 the field value or forwarding the message downstream.

   A proxy or gateway that receives an obs-fold in a response message
   that is not 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 that receives an obs-fold in a response message that is
   not within a "message/http" container MUST replace each received
   obs-fold with one or more SP octets prior to interpreting the field
   value.

6.  Message Body

   The message body (if any) of an HTTP/1.1 message is used to carry
   content (Section 6.4 of [HTTP]) for the request or response.  The
   message body is identical to the content unless a transfer coding has
   been applied, as described in Section 6.1.

     message-body = *OCTET

   The rules for determining when a message body is present in an
   HTTP/1.1 message differ for requests and responses.

   The presence of a message body in a request is signaled by a
   Content-Length or Transfer-Encoding header field.  Request message
   framing is independent of method semantics.

   The presence of a message body in a response, as detailed in
   Section 6.3, depends on both the request method to which it is
   responding and the response status code.  This corresponds to when
   response content is allowed by HTTP semantics (Section 6.4.1 of
   [HTTP]).

6.1.  Transfer-Encoding

   The Transfer-Encoding header field lists the transfer coding names
   corresponding to the sequence of transfer codings that have been (or
   will be) applied to the content in order to form the message body.
   Transfer codings are defined in Section 7.

     Transfer-Encoding = #transfer-coding
                          ; defined in [HTTP], Section 10.1.4

   Transfer-Encoding is analogous to the Content-Transfer-Encoding field
   of MIME, which was designed to enable safe transport 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 dynamically generated content.  It also serves to distinguish
   encodings that are only applied in transit from the encodings that
   are a characteristic of the selected representation.

   A recipient MUST be able to parse the chunked transfer coding
   (Section 7.1) because it plays a crucial role in framing messages
   when the content size is not known in advance.  A sender MUST NOT
   apply the chunked transfer coding more than once to a message body
   (i.e., chunking an already chunked message is not allowed).  If any
   transfer coding other than chunked is applied to a request's content,
   the sender MUST apply chunked as the final transfer coding to ensure
   that the message is properly framed.  If any transfer coding other
   than chunked is applied to a response's content, the sender MUST
   either apply chunked as the final transfer coding or terminate the
   message by closing the connection.

   For example,

   Transfer-Encoding: gzip, chunked

   indicates that the content has been compressed using the gzip coding
   and then chunked using the chunked coding while forming the message
   body.

   Unlike Content-Encoding (Section 8.4.1 of [HTTP]), Transfer-Encoding
   is a property of the message, not of the representation.  Any
   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 fields not
   defined by this specification.

   Transfer-Encoding MAY be sent in a response to a HEAD request or in a
   304 (Not Modified) response (Section 15.4.5 of [HTTP]) to a GET
   request, neither of which includes a message body, to indicate that
   the origin server would have applied a transfer coding to the message
   body if the request 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 MUST NOT send a Transfer-Encoding header field in any
   response 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 a CONNECT request (Section 9.3.6 of
   [HTTP]).

   A server that receives a request message with a transfer coding it
   does not understand SHOULD respond with 501 (Not Implemented).

   Transfer-Encoding was added in HTTP/1.1.  It is generally assumed
   that implementations advertising only HTTP/1.0 support will not
   understand how to process transfer-encoded content, and that an
   HTTP/1.0 message received with a Transfer-Encoding is likely to have
   been forwarded without proper handling of the chunked transfer coding
   in transit.

   A client MUST NOT send a request containing Transfer-Encoding unless
   it knows the server will handle HTTP/1.1 requests (or later minor
   revisions); such knowledge might be in the form of specific user
   configuration or by remembering the version of a prior received
   response.  A server MUST NOT send a response containing Transfer-
   Encoding unless the corresponding request indicates HTTP/1.1 (or
   later minor revisions).

   Early implementations of Transfer-Encoding would occasionally send
   both a chunked transfer coding for message framing and an estimated
   Content-Length header field for use by progress bars.  This is why
   Transfer-Encoding is defined as overriding Content-Length, as opposed
   to them being mutually incompatible.  Unfortunately, forwarding such
   a message can lead to vulnerabilities regarding request smuggling
   (Section 11.2) or response splitting (Section 11.1) attacks if any
   downstream recipient fails to parse the message according to this
   specification, particularly when a downstream recipient only
   implements HTTP/1.0.

   A server MAY reject a request that contains both Content-Length and
   Transfer-Encoding or process such a request in accordance with the
   Transfer-Encoding alone.  Regardless, the server MUST close the
   connection after responding to such a request to avoid the potential
   attacks.

   A server or client that receives an HTTP/1.0 message containing a
   Transfer-Encoding header field MUST treat the message as if the
   framing is faulty, even if a Content-Length is present, and close the
   connection after processing the message.  The message sender might
   have retained a portion of the message, in buffer, that could be
   misinterpreted by further use of the connection.

6.2.  Content-Length

   When a message does not have a Transfer-Encoding header field, a
   Content-Length header field (Section 8.6 of [HTTP]) can provide the
   anticipated size, as a decimal number of octets, for potential
   content.  For messages that do include content, the Content-Length
   field value provides the framing information necessary for
   determining where the data (and message) ends.  For messages that do
   not include content, the Content-Length indicates the size of the
   selected representation (Section 8.6 of [HTTP]).

   A sender MUST NOT send a Content-Length header field in any message
   that contains a Transfer-Encoding header field.

      |  *Note:* HTTP's use of Content-Length for message framing
      |  differs significantly from the same field's use in MIME, where
      |  it is an optional field used only within the "message/external-
      |  body" media-type.

6.3.  Message Body Length

   The length of a message body is determined by one of the following
   (in order of precedence):

   1.  Any response to a HEAD request and any response with a 1xx
       (Informational), 204 (No Content), or 304 (Not Modified) status
       code is always terminated by the first empty line after the
       header fields, regardless of the header fields present in the
       message, and thus cannot contain a message body or trailer
       section.

   2.  Any 2xx (Successful) response to a CONNECT request implies that
       the connection will become a 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 message.

   3.  If a message is received 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 to
       perform request smuggling (Section 11.2) or response splitting
       (Section 11.1) and ought to be handled as an error.  An
       intermediary that chooses to forward the message MUST first
       remove the received Content-Length field and process the
       Transfer-Encoding (as described below) prior to forwarding the
       message downstream.

   4.  If a Transfer-Encoding header field is present and the chunked
       transfer coding (Section 7.1) is the final encoding, the message
       body length is determined by reading and decoding the chunked
       data until the transfer coding indicates the data is complete.

       If a Transfer-Encoding header field is present in a response and
       the chunked transfer coding is not the final encoding, the
       message body length is determined by reading the connection until
       it is closed by the server.

       If a Transfer-Encoding header field is present in a request and
       the chunked transfer coding is not the final encoding, the
       message body length cannot be determined reliably; the server
       MUST respond with the 400 (Bad Request) status code and then
       close the connection.

   5.  If a message is received without Transfer-Encoding and with an
       invalid Content-Length header field, then the message framing is
       invalid and the recipient MUST treat it as an unrecoverable
       error, unless the field value can be successfully parsed as a
       comma-separated list (Section 5.6.1 of [HTTP]), all values in the
       list are valid, and all values in the list are the same (in which
       case, the message is processed with that single value used as the
       Content-Length field value).  If the unrecoverable error is in a
       request message, the server MUST respond with a 400 (Bad Request)
       status code and then close the connection.  If it is in a
       response message received by a proxy, the proxy MUST close the
       connection to the server, discard the received response, and send
       a 502 (Bad Gateway) response to the client.  If it is in a
       response message received by a user agent, the user agent MUST
       close the connection to the server and discard the received
       response.

   6.  If a valid Content-Length header field is present without
       Transfer-Encoding, its decimal value defines the expected message
       body length in octets.  If the sender closes the connection or
       the recipient times out before the indicated number of octets are
       received, the recipient MUST consider the message to be
       incomplete and close the connection.

   7.  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).

   8.  Otherwise, this is a response message without a declared message
       body length, so the message body length is determined by the
       number of octets received prior to the server closing the
       connection.

   Since there is no way to distinguish a successfully completed, close-
   delimited response message from a partially received message
   interrupted by network failure, a server SHOULD generate encoding or
   length-delimited messages whenever possible.  The close-delimiting
   feature exists primarily for backwards compatibility with HTTP/1.0.

      |  *Note:* Request messages are never close-delimited because they
      |  are always explicitly framed by length or transfer coding, with
      |  the absence of both implying the request ends immediately after
      |  the header section.

   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 sends a request containing a message body SHOULD use a
   valid Content-Length header field if the message body length is known
   in advance, rather than the chunked transfer coding, since some
   existing services respond to chunked with a 411 (Length Required)
   status code even though they understand the chunked transfer coding.
   This is typically because such services are implemented via a gateway
   that requires a content length in advance of being called, and the
   server is unable or unwilling to buffer the entire request before
   processing.

   A user agent that sends a request that contains a message body MUST
   send either a valid Content-Length header field or use the chunked
   transfer coding.  A client MUST NOT use the chunked transfer coding
   unless it knows the server will handle HTTP/1.1 (or later) requests;
   such knowledge can be in the form of specific user configuration or
   by remembering the version of a prior received response.

   If the final response to the last request on a connection has been
   completely received and there remains additional data to read, a user
   agent MAY discard the remaining data or attempt to determine if that
   data belongs as part of the prior message body, which might be the
   case if 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.

7.  Transfer Codings

   Transfer coding names are used to indicate an encoding transformation
   that has been, can be, or might need to be applied to a message's
   content in order to ensure "safe transport" through the network.
   This differs from a content coding in that the transfer coding is a
   property of the message rather than a property of the representation
   that is being transferred.

   All transfer-coding names are case-insensitive and ought to be
   registered within the HTTP Transfer Coding registry, as defined in
   Section 7.3.  They are used in the Transfer-Encoding (Section 6.1)
   and TE (Section 10.1.4 of [HTTP]) header fields (the latter also
   defining the "transfer-coding" grammar).

7.1.  Chunked Transfer Coding

   The chunked transfer coding wraps content in order to transfer it as
   a series of chunks, each with its own size indicator, followed by an
   OPTIONAL trailer section containing trailer fields.  Chunked enables
   content streams of unknown size to be transferred as a sequence of
   length-delimited buffers, which enables the sender to retain
   connection persistence and the recipient to know when it has received
   the entire message.

     chunked-body   = *chunk
                      last-chunk
                      trailer-section
                      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 ; a sequence of chunk-size octets

   The chunk-size field is a string of hex digits indicating the size of
   the chunk-data in octets.  The chunked transfer coding is complete
   when a chunk with a chunk-size of zero is received, possibly followed
   by a trailer section, and finally terminated by an empty line.

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

   HTTP/1.1 does not define any means to limit the size of a chunked
   response such that an intermediary can be assured of buffering the
   entire response.  Additionally, very large chunk sizes may cause
   overflows or loss of precision if their values are not represented
   accurately in a receiving implementation.  Therefore, recipients MUST
   anticipate potentially large hexadecimal numerals and prevent parsing
   errors due to integer conversion overflows or precision loss due to
   integer representation.

   The chunked coding does not define any parameters.  Their presence
   SHOULD be treated as an error.

7.1.1.  Chunk Extensions

   The chunked coding allows each chunk to include zero or more chunk
   extensions, immediately following the chunk-size, for the sake of
   supplying per-chunk metadata (such as a signature or hash), mid-
   message control information, or randomization of message body size.

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

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

   The chunked coding is specific to each connection and is likely to be
   removed or recoded by each recipient (including intermediaries)
   before any higher-level application would have a chance to inspect
   the extensions.  Hence, the use of chunk extensions is generally
   limited to specialized HTTP services such as "long polling" (where
   client and server can have shared expectations regarding the use of
   chunk extensions) or for padding within an end-to-end secured
   connection.

   A recipient MUST ignore unrecognized chunk extensions.  A server
   ought to limit the total length of chunk extensions received in a
   request to an amount reasonable for the services provided, in the
   same way that it applies length limitations and timeouts for other
   parts of a message, and generate an appropriate 4xx (Client Error)
   response if that amount is exceeded.

7.1.2.  Chunked Trailer Section

   A trailer section allows the sender to include additional fields at
   the end of a chunked message in order to supply metadata that might
   be dynamically generated while the content is sent, such as a message
   integrity check, digital signature, or post-processing status.  The
   proper use and limitations of trailer fields are defined in
   Section 6.5 of [HTTP].

     trailer-section   = *( field-line CRLF )

   A recipient that removes the chunked coding from a message MAY
   selectively retain or discard the received trailer fields.  A
   recipient that retains a received trailer field MUST either store/
   forward the trailer field separately from the received header fields
   or merge the received trailer field into the header section.  A
   recipient MUST NOT merge a received trailer field into the header
   section unless its corresponding header field definition explicitly
   permits and instructs how the trailer field value can be safely
   merged.

7.1.3.  Decoding Chunked

   A process for decoding the chunked transfer coding 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 content
        length := length + chunk-size
        read chunk-size, chunk-ext (if any), and CRLF
     }
     read trailer field
     while (trailer field is not empty) {
        if (trailer fields are stored/forwarded separately) {
            append trailer field to existing trailer fields
        }
        else if (trailer field is understood and defined as mergeable) {
            merge trailer field with existing header fields
        }
        else {
            discard trailer field
        }
        read trailer field
     }
     Content-Length := length
     Remove "chunked" from Transfer-Encoding

7.2.  Transfer Codings for Compression

   The following transfer coding names for compression are defined by
   the same algorithm as their corresponding content coding:

   compress (and x-compress)
      See Section 8.4.1.1 of [HTTP].

   deflate
      See Section 8.4.1.2 of [HTTP].

   gzip (and x-gzip)
      See Section 8.4.1.3 of [HTTP].

   The compression codings do not define any parameters.  The presence
   of parameters with any of these compression codings SHOULD be treated
   as an error.

7.3.  Transfer Coding Registry

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

   Registrations MUST include the following fields:

   *  Name

   *  Description

   *  Pointer to specification text

   Names of transfer codings MUST NOT overlap with names of content
   codings (Section 8.4.1 of [HTTP]) unless the encoding transformation
   is identical, as is the case for the compression codings defined in
   Section 7.2.

   The TE header field (Section 10.1.4 of [HTTP]) uses a pseudo-
   parameter named "q" as the rank value when multiple transfer codings
   are acceptable.  Future registrations of transfer codings SHOULD NOT
   define parameters called "q" (case-insensitively) in order to avoid
   ambiguities.

   Values to be added to this namespace require IETF Review (see
   Section 4.8 of [RFC8126]) 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.

7.4.  Negotiating Transfer Codings

   The TE field (Section 10.1.4 of [HTTP]) is used in HTTP/1.1 to
   indicate what transfer codings, besides chunked, the client is
   willing to accept in the response and whether the client is willing
   to preserve trailer fields in a chunked transfer coding.

   A client MUST NOT send the chunked transfer coding name in TE;
   chunked is always acceptable for HTTP/1.1 recipients.

   Three examples of TE use are below.

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

   When multiple transfer codings are acceptable, the client MAY rank
   the codings by preference using a case-insensitive "q" parameter
   (similar to the qvalues used in content negotiation fields; see
   Section 12.4.2 of [HTTP]).  The rank value is a real number in the
   range 0 through 1, where 0.001 is the least preferred and 1 is the
   most preferred; a value 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.

   The keyword "trailers" indicates that the sender will not discard
   trailer fields, as described in Section 6.5 of [HTTP].

   Since the TE header field only applies to the immediate connection, a
   sender of TE MUST also send a "TE" connection option within the
   Connection header field (Section 7.6.1 of [HTTP]) in order to prevent
   the TE header field from being forwarded by 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 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 for incomplete responses are defined
   in Section 3.3 of [CACHING].

   If a response terminates in the middle of the header section (before
   the empty line is received) and the status code might rely on header
   fields to convey the full meaning of the response, then the client
   cannot assume that meaning has been conveyed; the client might need
   to repeat the request in order to determine what action to take next.

   A message body that uses the chunked transfer coding is incomplete if
   the zero-sized chunk that terminates the encoding has not been
   received.  A message that uses a valid Content-Length is incomplete
   if the size of the message body received (in octets) is less than the
   value given by Content-Length.  A response that has neither chunked
   transfer coding nor Content-Length is terminated by closure of the
   connection and, if the header section was received intact, is
   considered complete unless an error was indicated by the underlying
   connection (e.g., an "incomplete close" in TLS would leave the
   response incomplete, as described in Section 9.8).

9.  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 7.3 of [HTTP], 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 4.2.1 of [HTTP]) 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.

9.1.  Establishment

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

9.2.  Associating a Response to a Request

   HTTP/1.1 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 15.2 of [HTTP]) 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 and
   MUST associate each received response message on that connection to
   the first outstanding request that has not yet received a final (non-
   1xx) response.

   If a client receives data on a connection that doesn't have
   outstanding requests, the client MUST NOT consider that data to be a
   valid response; the client SHOULD close the connection, since message
   delimitation is now ambiguous, unless the data consists only of one
   or more CRLF (which can be discarded per Section 2.2).

9.3.  Persistence

   HTTP/1.1 defaults to the use of "persistent connections", allowing
   multiple requests and responses to be carried over a single
   connection.  HTTP implementations SHOULD support persistent
   connections.

   A recipient determines whether a connection is persistent or not
   based on the protocol version and Connection header field
   (Section 7.6.1 of [HTTP]) in the most recently received message, if
   any:

   *  If the "close" connection option is present (Section 9.6), the
      connection will not persist after the current response; else,

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

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

   *  The connection will close after the current response.

   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.

   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 6.  A server MUST read
   the entire request message body or close the connection after sending
   its response; 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 Appendix C.2.2 for information and discussion of
   the problems with the Keep-Alive header field implemented by many
   HTTP/1.0 clients).

   See Appendix C.2.2 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.  The conditions under which a client can
   automatically retry a sequence of outstanding requests are defined in
   Section 9.2.2 of [HTTP].

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 sequence of pipelined requests in
   parallel if they all have safe methods (Section 9.2.1 of [HTTP]), 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 9.6).

   Idempotent methods (Section 9.2.2 of [HTTP]) 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.

9.4.  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 transfers very large content would block
   subsequent requests on the same connection.  However, each connection
   consumes server resources.

   Furthermore, using multiple connections can cause undesirable side
   effects in congested networks.  Using larger numbers of connections
   can also cause side effects in otherwise uncongested networks,
   because their aggregate and initially synchronized sending behavior
   can cause congestion that would not have been present if fewer
   parallel connections had been used.

   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.

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 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 or server load.

   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.

9.6.  Tear-down

   The "close" connection option is defined as a signal that the sender
   will close this connection after completion of the response.  A
   sender SHOULD send a Connection header field (Section 7.6.1 of
   [HTTP]) containing the "close" connection option when it intends to
   close a connection.  For example,

   Connection: close

   as a request header field indicates that this is the last request
   that the client will send on this connection, while in a response,
   the same field indicates that the server is going to close this
   connection after the response message is complete.

   Note that the field name "Close" is reserved, since using that name
   as a header field might conflict with the "close" connection option.

   A client that sends a "close" connection option MUST NOT send further
   requests on that connection (after the one containing the "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
   closure of the connection (see below) after it sends the final
   response to the request that contained the "close" connection option.
   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 closure
   of the connection (see below) after it sends the response containing
   the "close" connection option.  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" connection option; 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 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.

   Note that a TCP connection that is half-closed by the client does not
   delimit a request message, nor does it imply that the client is no
   longer interested in a response.  In general, transport signals
   cannot be relied upon to signal edge cases, since HTTP/1.1 is
   independent of transport.

9.7.  TLS Connection Initiation

   Conceptually, HTTP/TLS is simply sending HTTP messages over a
   connection secured via TLS [TLS13].

   The HTTP client also acts as the TLS client.  It initiates a
   connection to the server on the appropriate port and sends the TLS
   ClientHello to begin the TLS handshake.  When the TLS handshake has
   finished, the client may then initiate the first HTTP request.  All
   HTTP data MUST be sent as TLS "application data" but is otherwise
   treated like a normal connection for HTTP (including potential reuse
   as a persistent connection).

9.8.  TLS Connection Closure

   TLS uses an exchange of closure alerts prior to (non-error)
   connection closure to provide secure connection closure; see
   Section 6.1 of [TLS13].  When a valid closure alert is received, an
   implementation can be assured that no further data will be received
   on that connection.

   When an implementation knows that it has sent or received all the
   message data that it cares about, typically by detecting HTTP message
   boundaries, it might generate an "incomplete close" by sending a
   closure alert and then closing the connection without waiting to
   receive the corresponding closure alert from its peer.

   An incomplete close does not call into question the security of the
   data already received, but it could indicate that subsequent data
   might have been truncated.  As TLS is not directly aware of HTTP
   message framing, it is necessary to examine the HTTP data itself to
   determine whether messages are complete.  Handling of incomplete
   messages is defined in Section 8.

   When encountering an incomplete close, a client SHOULD treat as
   completed all requests for which it has received either

   1.  as much data as specified in the Content-Length header field or

   2.  the terminal zero-length chunk (when Transfer-Encoding of chunked
       is used).

   A response that has neither chunked transfer coding nor Content-
   Length is complete only if a valid closure alert has been received.
   Treating an incomplete message as complete could expose
   implementations to attack.

   A client detecting an incomplete close SHOULD recover gracefully.

   Clients MUST send a closure alert before closing the connection.
   Clients that do not expect to receive any more data MAY choose not to
   wait for the server's closure alert and simply close the connection,
   thus generating an incomplete close on the server side.

   Servers SHOULD be prepared to receive an incomplete close from the
   client, since the client can often locate the end of server data.

   Servers MUST attempt to initiate an exchange of closure alerts with
   the client before closing the connection.  Servers MAY close the
   connection after sending the closure alert, thus generating an
   incomplete close on the client side.

10.  Enclosing Messages as Data

10.1.  Media Type message/http

   The "message/http" media 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.  Because of the line length limitations, field values
   within "message/http" are allowed to use line folding (obs-fold), as
   described in Section 5.2, to convey the field value over multiple
   lines.  A recipient of "message/http" data MUST replace any obsolete
   line folding with one or more SP characters when the message is
   consumed.

   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 11

   Interoperability considerations:  N/A

   Published specification:  RFC 9112 (see Section 10.1).

   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 Aut
      hors' 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 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 11

   Interoperability considerations:  N/A

   Published specification:  RFC 9112 (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 contact for further information:  See Aut
      hors' 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 inform developers, information providers,
   and users about known security considerations relevant to HTTP
   message syntax and parsing.  Security considerations about HTTP
   semantics, content, and routing are addressed in [HTTP].

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 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.

11.2.  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 6.3,
   to reduce the effectiveness of request smuggling.

11.3.  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.  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.

   The mechanisms provided with the "https" scheme, such as
   authenticated encryption, provide protection against modification of
   messages.  Care is needed, however, to ensure that connection closure
   cannot be used to truncate messages (see Section 9.8).  User agents
   might refuse to accept incomplete messages or treat them specially.
   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.

   The "http" scheme provides no protection against accidental or
   malicious modification of messages.

   Extensions to the protocol might be used to mitigate the risk of
   unwanted modification of messages by intermediaries, even when the
   "https" scheme is used.  Integrity might be assured by using message
   authentication codes or digital signatures that are selectively added
   to messages via extensible metadata fields.

11.4.  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 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 4.2.2 of [HTTP].

12.  IANA Considerations

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

12.1.  Field Name Registration

   IANA has added the following field names to the "Hypertext Transfer
   Protocol (HTTP) Field Name Registry" at
   <https://www.iana.org/assignments/http-fields>, as described in
   Section 18.4 of [HTTP].

   +===================+===========+=========+============+
   | Field Name        | Status    | Section | Comments   |
   +===================+===========+=========+============+
   | Close             | permanent | 9.6     | (reserved) |
   +-------------------+-----------+---------+------------+
   | MIME-Version      | permanent | B.1     |            |
   +-------------------+-----------+---------+------------+
   | Transfer-Encoding | permanent | 6.1     |            |
   +-------------------+-----------+---------+------------+

                           Table 1

12.2.  Media Type Registration

   IANA has updated the "Media Types" registry at
   <https://www.iana.org/assignments/media-types> with the registration
   information in Sections 10.1 and 10.2 for the media types "message/
   http" and "application/http", respectively.

12.3.  Transfer Coding Registration

   IANA has updated the "HTTP Transfer Coding Registry" at
   <https://www.iana.org/assignments/http-parameters/> with the
   registration procedure of Section 7.3 and the content coding names
   summarized in the table below.

   +============+===========================================+=========+
   | Name       | Description                               | Section |
   +============+===========================================+=========+
   | chunked    | Transfer in a series of chunks            | 7.1     |
   +------------+-------------------------------------------+---------+
   | compress   | UNIX "compress" data format [Welch]       | 7.2     |
   +------------+-------------------------------------------+---------+
   | deflate    | "deflate" compressed data ([RFC1951])     | 7.2     |
   |            | inside the "zlib" data format ([RFC1950]) |         |
   +------------+-------------------------------------------+---------+
   | gzip       | GZIP file format [RFC1952]                | 7.2     |
   +------------+-------------------------------------------+---------+
   | trailers   | (reserved)                                | 12.3    |
   +------------+-------------------------------------------+---------+
   | x-compress | Deprecated (alias for compress)           | 7.2     |
   +------------+-------------------------------------------+---------+
   | x-gzip     | Deprecated (alias for gzip)               | 7.2     |
   +------------+-------------------------------------------+---------+

                                 Table 2

      |  *Note:* the coding name "trailers" is reserved because its use
      |  would conflict with the keyword "trailers" in the TE header
      |  field (Section 10.1.4 of [HTTP]).

12.4.  ALPN Protocol ID Registration

   IANA has updated the "TLS Application-Layer Protocol Negotiation
   (ALPN) Protocol IDs" registry at <https://www.iana.org/assignments/
   tls-extensiontype-values/> with the registration below:

          +==========+=============================+===========+
          | Protocol | Identification Sequence     | Reference |
          +==========+=============================+===========+
          | HTTP/1.1 | 0x68 0x74 0x74 0x70 0x2f    | RFC 9112  |
          |          | 0x31 0x2e 0x31 ("http/1.1") |           |
          +----------+-----------------------------+-----------+

                                 Table 3

13.  References

13.1.  Normative References

   [CACHING]  Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke,
              Ed., "HTTP Caching", STD 98, RFC 9111,
              DOI 10.17487/RFC9111, June 2022,
              <https://www.rfc-editor.org/info/rfc9111>.

   [HTTP]     Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke,
              Ed., "HTTP Semantics", STD 97, RFC 9110,
              DOI 10.17487/RFC9110, June 2022,
              <https://www.rfc-editor.org/info/rfc9110>.

   [RFC1950]  Deutsch, P. 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., "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>.

   [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>.

   [RFC7405]  Kyzivat, P., "Case-Sensitive String Support in ABNF",
              RFC 7405, DOI 10.17487/RFC7405, December 2014,
              <https://www.rfc-editor.org/info/rfc7405>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

   [TLS13]    Rescorla, E., "The Transport Layer Security (TLS) Protocol
              Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018,
              <https://www.rfc-editor.org/info/rfc8446>.

   [URI]      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>.

   [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),
              DOI 10.1109/MC.1984.1659158, June 1984,
              <https://ieeexplore.ieee.org/document/1659158/>.

13.2.  Informative References

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

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

   [Linhart]  Linhart, C., Klein, A., Heled, R., and S. Orrin, "HTTP
              Request Smuggling", June 2005,
              <https://www.cgisecurity.com/lib/HTTP-Request-
              Smuggling.pdf>.

   [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>.

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

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

   [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>.

   [RFC8126]  Cotton, M., Leiba, B., and T. Narten, "Guidelines for
              Writing an IANA Considerations Section in RFCs", BCP 26,
              RFC 8126, DOI 10.17487/RFC8126, June 2017,
              <https://www.rfc-editor.org/info/rfc8126>.

Appendix A.  Collected ABNF

   In the collected ABNF below, list rules are expanded per
   Section 5.6.1 of [HTTP].

   BWS = <BWS, see [HTTP], Section 5.6.3>

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

   OWS = <OWS, see [HTTP], Section 5.6.3>

   RWS = <RWS, see [HTTP], Section 5.6.3>

   Transfer-Encoding = [ transfer-coding *( OWS "," OWS transfer-coding
    ) ]

   absolute-URI = <absolute-URI, see [URI], Section 4.3>
   absolute-form = absolute-URI
   absolute-path = <absolute-path, see [HTTP], Section 4.1>
   asterisk-form = "*"
   authority = <authority, see [URI], Section 3.2>
   authority-form = uri-host ":" port

   chunk = chunk-size [ chunk-ext ] CRLF chunk-data CRLF
   chunk-data = 1*OCTET
   chunk-ext = *( BWS ";" BWS chunk-ext-name [ BWS "=" BWS chunk-ext-val
    ] )
   chunk-ext-name = token
   chunk-ext-val = token / quoted-string
   chunk-size = 1*HEXDIG
   chunked-body = *chunk last-chunk trailer-section CRLF

   field-line = field-name ":" OWS field-value OWS
   field-name = <field-name, see [HTTP], Section 5.1>
   field-value = <field-value, see [HTTP], Section 5.5>

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

   message-body = *OCTET
   method = token

   obs-fold = OWS CRLF RWS
   obs-text = <obs-text, see [HTTP], Section 5.6.4>
   origin-form = absolute-path [ "?" query ]

   port = <port, see [URI], Section 3.2.3>

   query = <query, see [URI], Section 3.4>
   quoted-string = <quoted-string, see [HTTP], Section 5.6.4>

   reason-phrase = 1*( HTAB / SP / VCHAR / obs-text )
   request-line = method SP request-target SP HTTP-version
   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 ]

   token = <token, see [HTTP], Section 5.6.2>
   trailer-section = *( field-line CRLF )
   transfer-coding = <transfer-coding, see [HTTP], Section 10.1.4>

   uri-host = <host, see [URI], Section 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 Multipurpose Internet Mail Extensions (MIME)
   [RFC2045] to allow a message body to be transmitted in an open
   variety of representations and with extensible fields.  However, some
   of these constructs have been reinterpreted to better fit the needs
   of interactive communication, leading to some differences in how MIME
   constructs are used within HTTP.  These differences were carefully
   chosen to optimize performance over binary connections, allow greater
   freedom in the use of new media types, ease date comparisons, and
   accommodate common implementations.

   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], and that content with a type of "text" represents line
   breaks as CRLF, forbidding the use of CR or LF outside of line break
   sequences [RFC2046].  In contrast, HTTP does not care whether CRLF,
   bare CR, or bare LF are used to indicate a line break within 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 5.6.7 of
   [HTTP]) 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'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 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 without
   modification and, aside from the "multipart/byteranges" type
   (Section 14.6 of [HTTP]), does not interpret the content or any MIME
   header lines that might be contained therein.

Appendix C.  Changes from Previous RFCs

C.1.  Changes from HTTP/0.9

   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.2.  Changes from HTTP/1.0

C.2.1.  Multihomed Web Servers

   The requirements that clients and servers support the Host header
   field (Section 7.2 of [HTTP]), 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 established mechanism for
   distinguishing the intended server of a request other 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.2.2.  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 to stop sending the header field), and this mechanism
   ought not be used by clients at all when a proxy is being used.

C.2.3.  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.3.  Changes from RFC 7230

   Most of the sections introducing HTTP's design goals, history,
   architecture, conformance criteria, protocol versioning, URIs,
   message routing, and header fields have been moved to [HTTP].  This
   document has been reduced to just the messaging syntax and connection
   management requirements specific to HTTP/1.1.

   Bare CRs have been prohibited outside of content.  (Section 2.2)

   The ABNF definition of authority-form has changed from the more
   general authority component of a URI (in which port is optional) to
   the specific host:port format that is required by CONNECT.
   (Section 3.2.3)

   Recipients are required to avoid smuggling/splitting attacks when
   processing an ambiguous message framing.  (Section 6.1)

   In the ABNF for chunked extensions, (bad) whitespace around ";" and
   "=" has been reintroduced.  Whitespace was removed in [RFC7230], but
   that change was found to break existing implementations.
   (Section 7.1.1)

   Trailer field semantics now transcend the specifics of chunked
   transfer coding.  The decoding algorithm for chunked (Section 7.1.3)
   has been updated to encourage storage/forwarding of trailer fields
   separately from the header section, to only allow merging into the
   header section if the recipient knows the corresponding field
   definition permits and defines how to merge, and otherwise to discard
   the trailer fields instead of merging.  The trailer part is now
   called the trailer section to be more consistent with the header
   section and more distinct from a body part.  (Section 7.1.2)

   Transfer coding parameters called "q" are disallowed in order to
   avoid conflicts with the use of ranks in the TE header field.
   (Section 7.3)

Acknowledgements

   See Appendix "Acknowledgements" of [HTTP], which applies to this
   document as well.

Index

   A C D F G H M O R T X

      A

         absolute-form (of request-target)  Section 3.2.2
         application/http Media Type  *_Section 10.2_*
         asterisk-form (of request-target)  Section 3.2.4
         authority-form (of request-target)  Section 3.2.3

      C

         chunked (Coding Format)  Section 6.1; Section 6.3
         chunked (transfer coding)  *_Section 7.1_*
         close  Section 9.3; *_Section 9.6_*
         compress (transfer coding)  *_Section 7.2_*
         Connection header field  Section 9.6
         Content-Length header field  Section 6.2
         Content-Transfer-Encoding header field  Appendix B.5

      D

         deflate (transfer coding)  *_Section 7.2_*

      F

         Fields
            Close  *_Section 9.6, Paragraph 4_*
            MIME-Version  *_Appendix B.1_*
            Transfer-Encoding  *_Section 6.1_*

      G

         Grammar
            ALPHA  *_Section 1.2_*
            CR  *_Section 1.2_*
            CRLF  *_Section 1.2_*
            CTL  *_Section 1.2_*
            DIGIT  *_Section 1.2_*
            DQUOTE  *_Section 1.2_*
            HEXDIG  *_Section 1.2_*
            HTAB  *_Section 1.2_*
            HTTP-message  *_Section 2.1_*
            HTTP-name  *_Section 2.3_*
            HTTP-version  *_Section 2.3_*
            LF  *_Section 1.2_*
            OCTET  *_Section 1.2_*
            SP  *_Section 1.2_*
            Transfer-Encoding  *_Section 6.1_*
            VCHAR  *_Section 1.2_*
            absolute-form  Section 3.2; *_Section 3.2.2_*
            asterisk-form  Section 3.2; *_Section 3.2.4_*
            authority-form  Section 3.2; *_Section 3.2.3_*
            chunk  *_Section 7.1_*
            chunk-data  *_Section 7.1_*
            chunk-ext  Section 7.1; *_Section 7.1.1_*
            chunk-ext-name  *_Section 7.1.1_*
            chunk-ext-val  *_Section 7.1.1_*
            chunk-size  *_Section 7.1_*
            chunked-body  *_Section 7.1_*
            field-line  *_Section 5_*; Section 7.1.2
            field-name  Section 5
            field-value  Section 5
            last-chunk  *_Section 7.1_*
            message-body  *_Section 6_*
            method  *_Section 3.1_*
            obs-fold  *_Section 5.2_*
            origin-form  Section 3.2; *_Section 3.2.1_*
            reason-phrase  *_Section 4_*
            request-line  *_Section 3_*
            request-target  *_Section 3.2_*
            start-line  *_Section 2.1_*
            status-code  *_Section 4_*
            status-line  *_Section 4_*
            trailer-section  Section 7.1; *_Section 7.1.2_*
         gzip (transfer coding)  *_Section 7.2_*

      H

         Header Fields
            MIME-Version  *_Appendix B.1_*
            Transfer-Encoding  *_Section 6.1_*
         header line  Section 2.1
         header section  Section 2.1
         headers  Section 2.1

      M

         Media Type
            application/http  *_Section 10.2_*
            message/http  *_Section 10.1_*
         message/http Media Type  *_Section 10.1_*
         method  *_Section 3.1_*
         MIME-Version header field  *_Appendix B.1_*

      O

         origin-form (of request-target)  Section 3.2.1

      R

         request-target  *_Section 3.2_*

      T

         Transfer-Encoding header field  *_Section 6.1_*

      X

         x-compress (transfer coding)  *_Section 7.2_*
         x-gzip (transfer coding)  *_Section 7.2_*

Authors' Addresses

   Roy T. Fielding (editor)
   Adobe
   345 Park Ave
   San Jose, CA 95110
   United States of America
   Email: fielding@gbiv.com
   URI:   https://roy.gbiv.com/


   Mark Nottingham (editor)
   Fastly
   Prahran
   Australia
   Email: mnot@mnot.net
   URI:   https://www.mnot.net/


   Julian Reschke (editor)
   greenbytes GmbH
   Hafenweg 16
   48155 Münster
   Germany
   Email: julian.reschke@greenbytes.de
   URI:   https://greenbytes.de/tech/webdav/