Comment by abcd_f

> It’s the same assumption is required for any hole punching handshake (including STUN).

This is incorrect.

Hole punching requires being able to predict external port. That's it. If the port remains the same, it certainly simplifies things, but ports going up (or down) by 1 (or 2, or 5) with each new mapping is quite common, trivial to detect and to punch through.