Comment by lxgr
2 days ago
Port forwarding and hole punching have different objectives and outcomes, and I believe PCP only caters to the former.
While the outcomes might be similar (some inbound connections are possible), the scope (one specific external IP/port vs. everybody) and the semantics ("endorsement of public hosting" vs allowing P2P connections that are understood to require at least some third-party mediation) differ.
I also don't think that port forwarding is possible through multiple levels of firewalls (similar to "double NAT").
PCP has two operating modes, MAP and PEER. The latter should be similar to hole-punching.
And routers can forward PCP requests to their upstream routers. Some dualstack-lite routers do that and according to rumors (random internet forum comments) some CGNATs do support that.