> So, to traverse these multiple stateful firewalls, we need to share some information to get underway: the peers have to know in advance the ip:port their counterpart is using. > [...] To move beyond that, we built a coordination server to keep the ip:port information synchronized

This is where I wish SIP lived up to its name (Session Initiation Protocol, i.e. any session, such as a VPN one...) and wasn't such a complicated mess making it not worth the hassle. I mean it was made to be the communication side-channel used for establishing p2p rtp streams.