Socket closed but sends SYN_SENT after a few seconds

[ttcds]

After closing a ZMQ.Socket like below (simplified code) a few seconds later a SYN_SENT is send which causes an underlying tcp socket to connect again. And when starting a new thread with a new ZMQ.Socket there are 2 tcp sockets open to the endpoint while the first one should have never reconnected again by itself.

Code

public class Main
{
    static ZContext context = new ZContext();

    public static void main(String[] args) throws InterruptedException
    {
        var runner = new RemoteDealer.Runner();
        var runnerThread = new Thread(runner);
        runnerThread.start();

        Thread.sleep(5000);

        runnerThread.interrupt();

        runner = new Runner.Runner();
        runnerThread = new Thread(runner);
        runnerThread.start();
        Thread.sleep(6000);
    }

    static class Runner implements Runnable
    {
        ZMQ.Socket socket;

        @Override
        public void run()
        {
            String endpoint = "tcp://" + remoteIp + ":60000";

            try
            {
                socket = context.createSocket(SocketType.DEALER);
                socket.setTCPKeepAlive(1);
                socket.setTCPKeepAliveCount(1);
                socket.setTCPKeepAliveIdle(8);
                socket.setTCPKeepAliveInterval(5);
                socket.setLinger(0);
                socket.setHandshakeIvl(0);
                socket.setReconnectIVL(30000);
                socket.setSendTimeOut(5000);
                socket.setReceiveTimeOut(5000);
                socket.connect(endpoint);

                ZMQ.Poller poller = context.createPoller(6);
                poller.register(socket, ZMQ.Poller.POLLIN);

                while (!Thread.currentThread().isInterrupted())
                {
                    poller.poll();
                    try
                    {
                        if (poller.pollin(0))
                        {
                            ...
                        }
                    } catch (Exception ex)
                    {
                        ex.printStackTrace();
                    }
                }
            }
            catch (Exception ex)
            {
                ex.printStackTrace();
            }
            finally
            {
                socket.disconnect(endpoint);
                socket.close();
            }
        }
    }
}

public static void main(String[] args)
{
    var router = "tcp://*:" + 60000;

    var context = new ZContext();

    ZMQ.Socket routerSocket = context.createSocket(SocketType.ROUTER);
    routerSocket.setTCPKeepAlive(1);
    routerSocket.setTCPKeepAliveCount(1);
    routerSocket.setTCPKeepAliveIdle(8);
    routerSocket.setTCPKeepAliveInterval(5);
    routerSocket.setLinger(0);
    routerSocket.setHandshakeIvl(0);
    routerSocket.setSendTimeOut(5000);
    routerSocket.setReceiveTimeOut(5000);
    routerSocket.bind(router);

    var poller = context.createPoller(6);
    poller.register(routerSocket, ZMQ.Poller.POLLIN);

    var runnerThread = new Thread(() -> {
        try
        {
            while (!Thread.currentThread().isInterrupted())
            {
                poller.poll();

                if (poller.pollin(0))
                {
                    ...
                }
            }
        } catch (Exception ex)
        {
            ex.printStackTrace();
        } finally
        {
            routerSocket.close();
        }
    });

    runnerThread.start();
    Thread.sleep(1000000);
}

After a few disconnects and reconnects the output of netstat looks like this:

tcp        0      0 src:35476    dst:60000 ESTABLISHED
tcp        0      0 src:35480    dst:60000 ESTABLISHED
tcp        0      0 src:35482    dst:60000 ESTABLISHED
tcp        0      0 src:35484    dst:60000 ESTABLISHED

[trevorbernard]

@ttcds Thank you for the test case but we'll need more information about the where the server and client is running on. What version of Linux and the kernel are you running?

[ttcds]

@trevorbernard I am using a Yocto build (dunfell) and using kernel 5.4.147.

The situation is as follows:

• There is a disconnect (plugging out ethernet cable) so I interrupt the thread
• Socket disconnects and closes properly
• Socket gone from netstat also
• When plugging the ethernet cable back in and restarting the thread there are now multiple connections

[trevorbernard]

What version of the Java SDK?

[ttcds]

Openjdk 17.0.10

[trevorbernard]

@ttcds is the issue that there are extraneous SYN packets sent or that the ROUTER maintains an established connection with a closed socket?