Step # 1 Download and install IPerf3 (latest version) from many of the Free download Internet sites on two of your laptops (Computers) if you’re using it for both the client and server side.

Step # 2 Attach the two laptops to the ends of the circuit (Between two sites). Make sure both laptops have valid and reachable IP addresses.

Step # 3 Activate the server mode using the command iperf3 -s

A server mode is activated and shows you the port that it is listening on

Step # 4 To run/initiate an Iperf test with 20 simultaneous streams from server side of IPerf you will need the following command. It is recommended to run multiple parallel streams (TCP/UDP) to replicate a real user scenario. Use parameter -P X, where X represents the number of parallel flows as follows:

iperf3 -P 20 -c $192.168.2.1 (IP address of the client on the other end)

I have used this tool extensively for determining the best TCP window size which is the amount of data that a receiver can accept at one time.

If the TCP Window size is too small, we notice that that the sender will be idle with poor throughput and performance. The formula you can use to calculate the theoretical value (also called the bandwidth delay product) of the TCP window size is:

bottleneck bandwidth * round trip time

So for example, if I have a 100Mbit link with 32 ms round trip ping time connecting two sites

= 100 Mbit/sec * 32 ms

= (100^e6) * (32^e-3)

= 3200000 bits/8 = 400 Kbyte

Once we have this theoretical value figured out, it gives us the starting point to find the best window size. We can now run tests by tweaking (increasing or decreasing)to determine the optimum window size. Here is how you can set the window size during a test in IPerf:

Laptop1> iperf -s -w 400k