Why do you need icmp forwarding? :) You need protocol 41 forwarding. Protocol, not port. TCP would not be sane for tunnels, especially as then you will get TCP inside TCP, that gives a big mess. Which is why AYIYA, like Teredo and others, use UDP for tunneling. It is still in (currently closed) beta and we hope to release a production version soonish, but don't hold your breath. Complain to the company that makes that airport express device and tell them they need to support IPv6 or at least allow you to forward protocol 41 to a different box. Probably this will end you up in either turning off NAT on that box and doing the NAT on a linux/bsd box or replacing the box with one that does support it.