Third party mediated hole punching really only works with UDP. TCP is stateful and thus negotiating a hole punch requires incredibly precise timing, which is ha

Third party mediated hole punching really only works with UDP. TCP is stateful and thus negotiating a hole punch requires incredibly precise timing, which is hard to achieve in practice. Without UDP we have a purely "cloud-to-ground" Internet unless we can convince router/NAT makers to always include and always turn on UPnP/NAT-PMP. Not likely, and those protocols suck anyway.