Me and My Friend

Me and my friend are doing kali vs kali outside LAN i gave him my public IP and he gaved me his, he is able to DoS me but in his i can't scan for open ports with nmap it says that it's filteres the TCP ir UDP, i don't remeber, and i can't even Dos Jim too.

I dont need the fast way to learn, i just want to know why he can scan me for exploiting and DoS and i can't do any.

Thanks