FindMTU is a tool that performs IPv6 path MTU discovery. You can use it
to debug network problems and to detect IPv6-in-IPv4 tunnels in the path
to a destination.

FindMTU only performs IPv6 path MTU discovery. It does not know about
IPv4.

WWW: http://www.dia.uniroma3.it/~compunet/tunneldiscovery/