Internet tunnels and Netsukuku
Internet tunnels can be used to link Netsukuku nodes.
Generally, it isn't a good idea because Netsukuku is meant to be a physical net, in fact, if the links are based on a lower network (the Internet), Netsukuku becomes dependent on it. If Netsukuku becomes dependent on another network, all the advantages of having a parallel net to the Internet are lost.
However the Internet tunnels can be useful.
Consider this scenario: there two large gnode in two near cities and it is difficult to link the cities, f.e. using a wireless hook. In this case they can be linked with the use of Internet tunnels.
Some of nodes with the best Internet connection will be linked to the nodes of the other city. In this way the links will be like real physical links, because they'll have the same topological meaning, and the delay between the two gnode won't be high (the cities are near). Moreover the ISPs will be used to connect two large gnode, therefore the Netsukuku net will be partially dependent on the Internet. These tunnels have to be created manually.
It is important to not create tunnels which undermine the topological meaning of physical links, thus you must not create a tunnel between Tokyo and Amsterdam.
If there is the danger that the ISPs used to create the tunnels will try to tap them, it is possible to create encrypted tunnels, or even better the tunnel can be encrypted and routed through different routes using Tor. In the latter case the link bandwidth will be heavily afflicted therefore this countermeasure must be used with caution.
For an example on how to link two nodes using a tunnel see Ntk_vpn_test.