It would pretty much have to someone other than the person you are fighting in PK executing a DoS attack on you. The basic idea of something like this is to overwhelm your modem/router with traffic such that you essentially have zero bandwidth left at your disposal, which takes a lot of bandwidth not only on your end, but the attacker's end as well. In the world of broadband connections, it would be highly unlikely that a single machine could attack any other single machine in this manner and still be running normally itself.

Generally for these sorts of things to be effective, the attacker must be able to command more bandwidth than the victim, which typical will involve having the ability to make several machines with different connections assault the victim at once.

Still like I said... if they have your IP and the knowledge of who you're playing such that they could do something like this, you fucked yourself somewhere along the line.