Indeed, Teredo assumes that when two clients exchange encapsulated IPv6 packets, the mapped/external UDP port numbers used will be the same as those that were used to contact the Teredo server (and building the Teredo IPv6 address). Without this assumption, it would not be possible to establish a direct communication between the two clients, and a costly relay would have to be used to perform triangle routing. A Teredo implementation tries to detect the type of NAT at startup, and will refuse to operate if the NAT appears to be symmetric. (This limitation can sometimes be worked around by manually configuring a port forwarding rule on the NAT box, which requires administrative access to the device).