iperf3 and throughput testing - measuring bandwidth

iperf3 is the standard tool for measuring network performance. It can measure TCP bandwidth, UDP throughput, latency, jitter, and packet loss. This lesson covers the common uses.

Install iperf3

sudo apt install iperf3
# or
sudo dnf install iperf3

TCP bandwidth test

# Server
iperf3 -s

# Client
iperf3 -c server

Output:

[ ID] Interval           Transfer     Bitrate         Retr
[  5]   0.00-10.00  sec  1.09 GBytes   935 Mbits/sec    0
[  5]   0.00-10.00  sec  1.09 GBytes   935 Mbits/sec    0

Reading:

• Transfer: 1.09 GB transferred.
• Bitrate: 935 Mbits/sec.
• Retr: 0 retransmits. Higher = more network loss.

For a 1 Gbps link, ~935 Mbps is normal. Lower indicates bottleneck.

UDP test

iperf3 -c server -u -b 1G

Output:

[ ID] Interval           Transfer     Bitrate         Jitter    Lost/Total Datagrams
[  5]   0.00-10.00  sec  1.19 GBytes  1.02 Gbits/sec  0.057 ms  0/858029 (0%)

• Jitter: variation in packet arrival time. Low is better for streaming.
• Lost/Total: packet loss. 0% is ideal; >0% indicates network loss or host saturation.

UDP tests at a target bitrate (-b 1G). The host sends 1 Gbps of UDP; the network may drop some. This tells you how much loss occurs at a given load.

Multiple parallel streams

iperf3 -c server -P 4

-P 4 runs 4 parallel streams. Some networks (Wi-Fi, multi-path TCP) perform better with multiple streams.

Reverse mode

iperf3 -c server -R

-R reverses the direction: server sends, client receives. Useful for testing download speeds.

UDP with bitrate limit

iperf3 -c server -u -b 100M    # 100 Mbps target
iperf3 -c server -u -b 1G      # 1 Gbps target

Use UDP to test how the network handles a specific load.

JSON output

iperf3 -c server -J    # JSON output

For machine-readable output, useful in CI or monitoring.

Test specific intervals

iperf3 -c server -t 30 -i 5    # 30s total, 5s intervals

Reports bitrate per interval. Useful for spotting variability.

Common patterns

Knowledge check