Not TCP. UDP - there are no connection in UDP, so the routers / firewalls don't really differentiate between a new connection and packets that happen to have th

Not TCP. UDP - there are no connection in UDP, so the routers / firewalls don't really differentiate between a new connection and packets that happen to have the correct ip+port. Some are trickier than others, as there are 2 IPs and 2 ports in a packet (at any point), and it can be hard to match all of them for the stricter devices, esp. on NATs that do source port randomization.