iperf3 and iperf (v2) is supported for VOLIA subscribers only

$ iperf3 -c iperf3.volia.com -R -P 4

Please visit iperf3 homepage for details.

$ iperf -c iperf.volia.com -r -P 4

See iperf documentation for details.
Or visit its homepage.
Version of at least 2.0.5 is required for bidirectional (upload/download) test to work.

Love GUI? Windows user? Try jPerf.
You'll need Oracle/Sun Java Runtime Environment.