To be able to do this easily, you can use SignalR which simplify all the hassle of which underlying technology to be used based on the version of the browser it will choose the best communication protocol (WebSocket, LongPolling..etc).
Behind SignalR, one of the underlying used technologies is websockets, it doesn't send anything except keep an open full duplex channels between server and client, in case any update in the server it will push this update to the client. most popular sites use websockets for keeping open channels between server and client.
SignalR uses actually websockets however it will downgrade to use long polling for example in case of old browsers that don't support websocket's connection upgrades.
You have the option to use the websockets directly in case you are assured your clients use new browsers.
One last thing Stackoverflow as a big and heavy loaded site uses websockets to update once there is a new answer or comment for example.