Socket Disconnet Detection (ZZ)

Disconnet Detection

Since TCP/IP sends no packets except when there is traffic, without Socket.setKeepAlive( true ), it has no way of noticing a disconnect until you start trying to send (or to a certain extent receive) traffic again. Java has the Socket.setKeepAlive( true ) method to ask TCP/IP to handle heartbeat probing without any data packets or application programming. Unfortunately, you can't tell it how frequently to send the heartbeat probes. If the other end does not respond in time, you will get a socket exception on your pending read. Heartbeat packets in both directions let the other end know you are still there. A heartbeat packet is just an ordinary TCP/IP ack packet without any piggybacking data.

When the applications are idling, your applications could periodically send tiny heartbeat messages to each other. The receiver could just ignore them. However, they force the TCP/IP protocol to check if the other end is still alive. These are not part of the TCP/IP protocol. You would have to build them into your application protocols. They act as are-you-still-alive? messages. I have found Java's connection continuity testing to be less that 100% reliable. My bullet-proof technique to detect disconnect is to have the server send an application-level heartbeat packet if it has not sent some packet in the last 30 seconds. It has to send some message every 30 seconds, not necessarily a dummy heartbeat packet. The heartbeat packets thus only appear when the server is idling. Otherwise normal traffic acts as the heartbeat. The Applet detects the lack of traffic on disconnect and automatically restarts the connection. The downside is your applications have to be aware of these heartbeats and they have to fit into whatever other protocol you are using, unlike relying on TCP/IP level heartbeats.

However, it is simpler to use the built-in Socket.setKeepAlive( true ) method to ask TCP/IP to handle the heartbeat probing without any data packets or application programming. Each end with nothing to say just periodically sends an empty data packet with its current sequence, acknowledgement and window numbers.

The advantage of application level heartbeats is they let you know the applications at both ends are alive, not just the communications software.