Yes it is illegal however it is very hard to track the best thing you can do is contact your isp or get the gamertag of the offender and contact Microsoft also report them in game.
how am i supposed to find out who did it?
You call your ISP and tell them the time it happened. They will take care of the rest. Now if that person does have the same internet provider they can and will cancel his service permanently.
Some IPs might even take legal action, they might even have the cops knock on his door. You can never tell. Its doesnt have to be an FBI investigation but some cops and sherrifs would love to lean on them a little.
But regardless make a report to your ISP to keep it on record.
Make a record of it then if it keeps happening then threaten to go with another provider even if your bluffing.
Yes, but something so small isn't worth the FBI's time and money to pursue it. Only when it involves a big company and money loss.
am i able to close port 80 which they use and still be able to play online?
I wouldn't bother, attacks can use lots of different ports (TCP/SYN/ICMP)
Just make sure you have a decent firewall.
well how am i supposed to put a firewall on my router since all those packets go straight there?
and based on what ive looked up, they are using udp attacks from servers that they pay for, so im wondering how to track and protect against the attacks