I've read about DDoS attacks and specially the amplification techniques. For example, the DNS amplification technique where you can send queries to open resolvers pretending to be the victim, then [if nobody is filtering packets] the victim will get all the responses and if the traffic is high enough, get overwhelmed and stop functioning the way it is supposed to.
If I understand that correctly, the DNS response will be sent from the open resolver with an arbitrary destination port that the victim is not listening to (cuz he didn't make the request, thus not waiting for a response on that port!). How does that affect the victim? Can you get overwhelmed by receiving too many packets wrongly addressed to a port where you are not listening? Is it an expensive operation to ignore packets? Or am I getting it wrong somehow?