IPv6 over IPv4 tunneling is done via the command sequence iptunnel mode sit under Linux. gif devices under Open/FreeBSD can handle arbitrary IPv4/IPv6 transport and carrier combinations. DVMRP erects IP-IP tunnel for multicast transport over the unicast Internet. Distance-Vector Multicast Routing Protocol (DVMRP) operation and termination affects manually configured IP-IP tunnels. For a more detailed discussion of multicasting concepts and DVMRP tunnels, see Chapter 14, "Multicast Architectures."