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

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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).