Command Line format:

relayer <port1> <IP2> <port2> <dropping_rate> <dropping_mode>

<port1>: the port that the relayer is listening to.
<IP2>:<port2> The address that the incoming packets are sent to.
<dropping_rate>: packet drop rate. It should be larger or equal to 1. Approximately, the relayer will drop 1 packet out of <dropping_rate> packets. 
<dropping_mode>: the mode that the packet is dropped. It can be 1, 2 or 3:
1: Random dropping: Drop packet randomly
2: Interval dropping: Drop 1 packet for every <dropping_rate> packets
3: Continuous dropping: Drop first <dropping_rate> packets for every <dropping_rate>*<dropping_rate> packets.


A possible use of this relayer in Lab3:

Assume you are using <IP1>:<port1> for your sender, <IP2>:<port2> for your receiver.
Now, you run this relayer in <IP3>:<port3>.

Let the sender send packets to <IP3>:<port3>
Let the relayer send packets to <IP2>:<port2>
Let the receiver send feedback to <IP1>:<port1>

So, you have a loop with loss in the forwarding path.

If you want to test the scenario that there is loss in the feedback, you can:
Let the sender send packets to <IP2>:<port2>
Let the relayer send packets to <IP1>:<port1>
Let the receiver send feedback to <IP3>:<port3>

If you want to test the scenario that there is loss in feedback path and also in forwarding path, you can run one more relayer on <IP4>:<port4> and:
Let the sender send packets to <IP3>:<port3>
Let the relayer (forwarding) send packets to <IP2>:<port2>
Let the receiver send feedback to <IP4>:<port4>
Let the relayer (feedback) send packets to <IP1>:<port1>

Two relayers can run on the same machine, with different ports. So, you don't need to have multiple machines actually. 

For Windows/MAC OS/FreeBSD/Solaris users, you can still run this program on login.cs or other Linux machines to test.