known problems/assumptions:
 - does not handle protocols other than ICMP, UDP, TCP and GRE/ESP
 - assumes the handset has its own (routed) /64 ipv6 subnet
 - assumes the /128 ipv6 subnet it generates can use the nat64 gateway
 - assumes the nat64 gateway has the ipv4 address in the last 32 bits of the ipv6 address (that it uses a /96 plat subnet)