RFC1933/mech-04 auto tunnel

IPv6 packets to ::10.1.1.1 will automatically be encapsulated into IPv4 packet

RFC1933 allows:
Native IPv6 packets with IPv4 compatible address
"Relays" that would translate native IPv6 packets into tunnelled ones

Putting non-unicast IPv4 compatible address into IPv6 destination
Force relays to genearate unwanted IPv4 traffic
Putting non-unicast IPv4 compatible address into IPv6 source
Anonymize even better

RFC1933 has no sanity checks against embedded address
mech-04 has some, but can never be complete
We cannot check for subnet broadcast in remote site