Question

I have a Java TCP game server, I use java.net.ServerSocket and everything runs just fine, but recently my ISP did a some kind of an upgrade, where, if you send two packets very fast for the same TCP connexion, they close it by force.

This is why a lot of my players are disconnected randomly when there’s a lot of traffic in game (when there is a lot of chance that the server will send 2 packets at same time for the same person)

Here is an example of what I mean:
If I do something like this, my ISP will close the connexion for no reason to both client and server side:

tcpOut.print("Hello.");
tcpOut.flush();

tcpOut.print("How are you?");
tcpOut.flush();

But it will work just fine if i do something like this:

tcpOut.print("Hello.");
tcpOut.flush();

Thread.sleep(200);

tcpOut.print("How are you?");
tcpOut.flush();

Or this:

tcpOut.print("Hello.");
tcpOut.print("How are you?");
tcpOut.flush();

This only started a couple of weeks ago when they (the ISP) did some changes to the service and the network. I noticed using Wireshark that you have to have at least ~150ms time between two packets for same TCP connexion or else it will close.

1)Do you guys know what is this called ? does is it even have a name ? Is it legal ?

Now I have to re-write my game server knowing that I use a method called: send(PrintWriter out, String packetData);

2)Is there any easy solution to ask java to buffer the data before it sends it to clients ? Or wait 150ms before each sending without having to rewrite the whole thing ? I did some googling but I can’t find anything that deals with this problem. Any tips or information to help about this would be really appreciated, btw speed optimisation is very crucial. Thank you.

Answer 1

You may create a Writer wrapper implementation to keep track of last flush call timestamp. A quick implementation is to add a wait call to honor the 150 ms delay between two consecutive flushes.

public class ControlledFlushWriter extends Writer {
    private long enforcedDelay = 150;
    private long lastFlush = 0;
    private Writer delegated;

    public ControlledFlushWriter(Writer writer, long flushDelay) {
        this.delegated = writer:
        this.enforcedDelay = flushDelay;
    }

    /* simple delegation for other abstract methods... */

    public void flush() {
        long now = System.currentTimeMillis();
        if (now < lastFlush + enforcedDelay) {
            try {
                Thread.sleep(lastFlush + enforcedDelay - now);
            } catch (InterruptedException e) {
                // probably prefer to give up flushing 
                // instead of risking a connection reset !
                return;
            }
        }
        lastFlush = System.currentTimeMillis();
        this.delegated.flush();
    }

}

It now should be enough to wrap your existing PrintWriter with this ControlledFlushWriter to work-around your ISP QoS without re-writing all your application.

After all, it sounds reasonable to prevent a connection to flag any of its packet as urgent… In such a condition, it is difficult to implement a fair QoS link sharing.