why is icmp needed? afaik any nat will open udp things for an amount of time to allow replies to come back , since its stateless. thats normal nat punching. I am curious why ICMP is used and what benefits it might have. (nat punching sometimes used for things like game servers to enable p2p by using central server to connect 2 natted parties using this property of nat+udp)
This is not standard hole punching. Hole punching requires two way communication. But this method uses ICMP Destination Unreachable packets to send data one way through an existing UDP NAT mapping.