Emmanuel Duros presented the INRIA implementation of the tunneling protocol, which has been tested as part of the MERCI project. This uses FreeBSD based PCs as the routers on the uni-directional link, to perform the tunneling. This basically uses either IP-in-IP tunneling, or GRE. It seems that this implementation will move to GRE in future, since this is more flexible. At present the tunnel is configured manually, although work is ongoing to make this automatic.
The implementation must use a virtual device driver, which either sends over the uni-directional link, or over the tunnel. The presence of the uni-directional link is not visible to the rest of the system.
This doesn't quite work, since the return path is point-to-point, yet the forward path is point-to-multipoint. This may break some routing protocols, since although the sender can hear routing messages from all receivers (because each receiver has a tunnel back to the sender), the receivers cannot hear each other's routing messages. No specific examples of problems were given, but this is something to watch for in future...