Take a look at SSE (Server-Sent Events, aka EventSource). Unlike WebSockets, the it has the concept of an "event ID", which allows the protocol to automatically

Take a look at SSE (Server-Sent Events, aka EventSource). Unlike WebSockets, the it has the concept of an "event ID", which allows the protocol to automatically continue from the last event, no extra roundtrip needed to send a setup message to the server. Implemented by every major browser (except, of course, IE, where you can polyfill it pretty easily).