Of course NAT allows application layer protocols layered on TCP or UDP to pass through without the NAT understanding the application layer – otherwise, NATted n

Of course NAT allows application layer protocols layered on TCP or UDP to pass through without the NAT understanding the application layer – otherwise, NATted networks would be entirely broken. The end-to-end principle at the IP layer (i.e. having the IP forwarding layer be agnostic to the transport layer protocols above it) is still violated.