The external interface makes it possible to connect the simulation with real
networks.
In this example the network consists of one client, one router, one internal
server and one external (real) server.

The client sends 100 MB of data to the external server using TCP for uplink traffic.
For downlink traffic the external server may send data to the internal server.

To run this example, some parameters have to be adjusted in the omnetpp.ini.

- Set **.ext[0].device to the device name of the interface of the computer
  running the simulation.
- Set IP address of the external server to: 192.168.0.111/24 and add a route
  to routers external interface 192.168.0.100/24 and set the gateway to the
  IP address of the computer, on which the simulation is running
  or adjust the omnetpp.ini and mrt files to your networks setup.
- For uplink traffic use e.g. FileZilla and setup your external server to
  listen on port 10021.
- For downlink traffic use e.g. Iperf with following command line:
  iperf -c 172.0.1.111 -P 1 -i 1 -m -p 10021 -M 1452.0B -l 100.0M -f m -t 0

Start the simulation with root privileges.

IP addresses and routing tables are set up by using mrt files.
