sounds very much Tether PEER IM app - punchhole UDP etc this concept is nothing new and good approach point of failure / weakness - STUN / TURN servers which help NAT CGNAT endpoints to like hub n spoke simplex has been do this over tor TCP only using tor server for directory/signaling jami is failure in absence of signaling server - which is critical factor unless UID of endpoint behind NAT is PRE-KNOWN to party and somehow reachable