Network topology


                      || external(tunnel)
                      || 
                    Router-1
                    |      |
         |---+------+-| |--+-----+----|
             |                   |
         Router-2            Router-3
           |                    |
     |-+---+-+--+++-|       |---++++++++---|
       |     |  |||             ||||||||
      DNS    |  end hosts      end hosts
      www   RR
      smtp  control