We subscribed for a dedicated line between 2 datacenters, and when we were trying to copy some files over, it was really slow.
We were supposed to get few MB/s transfer rate, but were getting only 20KB/s which was unacceptable. We needed to make a clear case with the service provider to get their support on fixing this. Simple google searches showed me the tool called iperf, and it provided me what I wanted. It is a shame that I never knew this tool existed.
In the destination server, I ran the iperf server by below command:
.\iperf3.exe -s -p 136 (I only have few ports open in between, and that is not currently in use)
and in the source, I ran the iperf client:
.\iperf3.exe -c <Destination IP> -p 136
The results were enough to convince the service provider to fix their network.
This is probably the most basic test that can be done using the tool, but there are plenty of other options as documented here.