You are using TCP/IP protocol every time you connect to the internet. The port opened shouldn't expose you to anything. However you could protect yourself by scanning your ports using "nmap localhost" command. This will reveal all the ports of your computer that are opened to the outer world. You can change the ports actions when they receive a packet from the internet using iptables (there are some tutorials on the net). The best thing to do, if you are connecting to a fixed IP is to allow only connections to and from that IP to your computers 3689 port
However doing this is all up to you.