Is it an external attack, as in- someone attacking you? Or is it some software (prob virus) in your machine that is attacking someone else?
You can check active connections using netstat (-n parameter to get IPs)
If it's an external attack, you could just disconnect for a little bit and the attacker will probably get bored and stop, or you can lower the timeout rating on connections. Though your firewall should be dropping any excessive packets.
If, for some reason, this person (or machine) really wants to attack you persistently, you should contact your ISP and let them know this is happening or maybe use a hardware firewall.
I should also mention, if it is an attack from your own machine to another, you should disconnect until this is resolved. ISPs will suspend your service if it looks like you are attacking someone else.