Which RFC standardized WebSocket as a protocol in 2011?

RFC 6455 standardized WebSocket as a protocol in 2011.

Published by the Internet Engineering Task Force in December 2011, RFC 6455 is titled “The WebSocket Protocol.” It defines a persistent, two-way communication channel between a client and server, allowing both sides to send messages without repeatedly opening separate HTTP requests.

WebSocket begins with an HTTP-based opening handshake. After the server agrees to upgrade the connection, communication uses WebSocket frames over a TCP connection. The protocol supports text and binary data, control frames, fragmentation, masking of client-to-server traffic, and an orderly closing handshake. Secure WebSocket connections use the wss scheme and TLS.

A common mix-up is confusing RFC 6455 with RFC 2616, which specified HTTP/1.1, or RFC 5321, which specifies SMTP. WebSocket also predates RFC 8441, which later described bootstrapping WebSockets over HTTP/2; RFC 8441 did not create the original WebSocket protocol.