SecWebSocketProtocol

The Sec-WebSocket-Protocol header field is used in the WebSocket opening handshake. It is sent from the client to the server and back from the server to the client to confirm the subprotocol of the connection. This enables scripts to both select a subprotocol and be sure that the server agreed to serve that subprotocol.

See: https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Sec-WebSocket-Protocol