Thank you for your previous replies.
It really helped me.
I have a question about "--cport". I use this function by typing command like 'iperf3 -c 126.96.36.199 -p 12000 -u -B 188.8.131.52 --cport 11000'.
This command means that client use ip & port, 184.108.40.206 and 11000.
But I have checked packet in detail by using tcpdump,
It show that first packet from 220.127.116.11 client do not have port 11000, but randomly generated port.
After connection is accomplished, I think, 18.104.22.168 client generate packet having 11000.
Is it intended? I think that It is unnatural for client to use randomly port first, despite of specify port by using '--cport'.
-Byoung Uk Lee