The ability for the server to send dataWithout the client requesting it What does it look like?var ws = new WebSocket('ws://localhost'); ws.onopen = function () { ws.onmessage = function (ev) { alert(ev.data); } ws.send('my message'); } On the serverGET /ws HTTP/1.1 Host: pmx Upgrade: websocket Origin: http://pmx Connection: Upgrade Sec-WebSocket-Version: 6 Sec-WebSocket-Extensions: deflate-stream