> A symmetric NAT is one where all requests from the same internal IP address and port, to a specific destination IP address and port, are mapped to the same external IP address and port.
My understanding is that it's not "required", but most of the CGNAT routers I've encountered do symmetric NAT, and they force-randomize the source port for each new connection, then keep it fixed for one external ip:port for some "session" duration, defeating traditional hole-punching.
When I've tried to build WebRTC P2P stuff I've experienced this making direct P2P WebRTC connections between CGNAT users nearly impossible, always requiring at least one node with a re-usable hole-punched public udp port or a relay server.
Such CGNAT should also be more likely to support PCP than a normal NAT. It does suck that, AFAIK, no browser has integrated support for this into their WebRTC stacks :/.
