I wouldn't call it trivial - you'd need a stateful firewall (not necessarily a NAT) to keep track of which network flows are active. It would also be exploitabl

I wouldn't call it trivial - you'd need a stateful firewall (not necessarily a NAT) to keep track of which network flows are active. It would also be exploitable - you could open a TCP control connection to host X, and have host Y stream you data over UDP, to which you never reply directly, but rather use the host X control connection to ack the data.