Why is the iperf client testing the outgoing speed?

Question

*nix-like systems

If you run iperf on the server in server mode and connect a client to it, then the client sends data to it (outgoing speed is tested), and in order to test the incoming speed on the client, you need to swap the client and server, but what if there is no white IP?
Task: check the speed on several UDP ports, to check the speed cutoff by the provider. Thank you!

Answer 1

# man iperf
...
CLIENT SPECIFIC OPTIONS
...
       -d, --dualtest
              Do a bidirectional test simultaneously
...
       -r, --tradeoff
              Do a bidirectional test individually
...
       -L, --listenport n
              port to receive bidirectional tests back on