Why client disconnect auto every 3 or 5 minutes

[zhaoyou]

I use webrtc.io module in server. use webrtc.io.js in client.

Case one:
first two client work well , but every 3 or 5 minutes later. server received client emit socket close event. but both client work well (In fact, do not disconnect ， network normal) . when other client receive remove_peer_connected event . video is black screen.

Case two:
first two client work well. I kill server app. 20 minutes later or longer. clients both work well also. (I know clients not need signale server exchange message when client connected)

so:

Why server received socket close event from client 3 or 5 minutes later?

[zhaoyou]

Maybe client heartbeat is not emit ?