Detecting Socket Disconnect Using TCP KeepAlive

Question

I'm developing a server that hosts 3rd party devices over TCP/IP and have been experiencing sudden connection drops (the devices are connecting via cellular). I need to find a way to detect a disconnect without having to write data to the device itself.

I've looked at using the TCP keepalive functionality but Java doesn't appear to allow any adjustment of the timing of the keepalive operations.

Is there any suggested method for doing this?

My simplified socket code is as follows:

public class Test2Socket {
    public static void main(String[] args) {
        try {
            ServerSocket skt = new ServerSocket(1111);

            Socket clientSocket = skt.accept();

            clientSocket.setKeepAlive(true);

            System.out.println("Connected..");

            BufferedReader input = new BufferedReader(new InputStreamReader(clientSocket.getInputStream()));

            String inputLine;

            while((inputLine = input.readLine()) != null)
            {
                System.out.println(inputLine);
            }

        } catch (IOException e) {
            e.printStackTrace();
        }
    }
}

Any feedback would be greatly appreciated.

Answer 1

You will not get far with the built-in keep-alives of the TCP stack. That's because the keep-alive interval cannot be tuned by your application, it is set by the OS, and the defaults are rather high (hours). This is not specific to Java.

If you need to time out in a reasonable time, you have to implement some kind of keep alive in the protocol to be used. Most of the high-level protocols I have seen have some kind of NOP functionality, where you send an "Are you there?" message and the other party sends a "Yes, I'm here" reply without doing anything else.