Simple minded ethernet-over-UDP tunnels

Simple minded ethernet-over-UDP tunnels

We provide very simple point-to-point UDP tunnels which openvswitch can
use as network interfaces. The implementation uses a pair of connected
UDP sockets for each tunnel. Each ethernet frame is sent as a single
UDP datagram.

Example usage:
The following commands create a tunnel between hosts 10.0.0.1 and
10.0.0.2 (port and bridge names are arbitrary).

 - On host 10.0.0.1:
ovs-vsctl add-br br0
ovs-vsctl add-port br0 tun0 -- set interface tun0 type=tunnel \
	options:remote_ip=10.0.0.2
ovs-appctl netdev-tunnel/get-lport tun0
<(1) the command replies with the local port number>

 - On host 10.0.0.2:
ovs-vsctl add-br br1
ovs-vsctl add-port br0 tun0 -- set interface tun1 type=tunnel \
	options:remote_ip=10.0.0.1
ovs-appctl netdev-tunnel/get-lport tun0
<(2) the command replies with the local port number>
ovs-appctl netdev-tunnel/set-rport tun0 <the port obtained in (1)>

 - Back on host 10.0.0.1:
ovs-appctl netdev-tunnel/set-rport tun0 <the port obtained in (2)>