This isn't entirely true these days - ICE (RFC5245) has been out for 6 years and defines a pretty robust, open and tractable solution for firewall traversal. Th

This isn't entirely true these days - ICE (RFC5245) has been out for 6 years and defines a pretty robust, open and tractable solution for firewall traversal. There are a bunch of libraries and apps out there that use it, including one built into most web browsers as part of WebRTC. If anything the exciting situation we're in currently is because the tech has caught up at last. Firewall traversal is no longer a black art; you just open a WebRTC data connection.