magic-wormhole (which croc was inspired by) also uses a public relay server: https://magic-wormhole.readthedocs.io/en/latest/welcome.html...

Any tool that wants to reliably connect two clients P2P is going to need something like a TURN server to traverse restrictive NATs. See for instance Tailscale's use of DERP servers: https://tailscale.com/blog/how-tailscale-works/#encrypted-tc...