Testing Network Bandwidth with iperf3

This article provides a step-by-step guide to testing network bandwidth using iperf3, a widely-used tool for measuring network performance.

Prerequisites

• Two devices (one acting as the server, the other as the client) connected to the same network.

• iperf3 installed on both devices. You can install iperf3 on:

  • Ubuntu/Debian: sudo apt-get install iperf3

  • CentOS/RHEL: sudo yum install iperf3

  • macOS: brew install iperf3

  • Windows: Download the iperf3 binary from the official website or a trusted source.

• Administrative or sudo privileges for running iperf3 commands.

• Ensure firewall settings allow traffic on the default iperf3 port (5201) or a custom port if specified.

Steps to Test Bandwidth

Step 1: Start the iperf3 Server

1. On the device designated as the server, open a terminal or command prompt.

2. Run the following command to start the iperf3 server:

   iperf3 -s

   • -s indicates server mode.

   • Optionally, specify a port with -p <port_number> (e.g., iperf3 -s -p 5202).

3. The server will listen for incoming connections. Note the server's IP address (use ip addr or ifconfig on Linux, or ipconfig on Windows to find it).

Step 2: Run the iperf3 Client

1. On the client device, open a terminal or command prompt.

2. Run the following command to connect to the server and start the bandwidth test:

   iperf3 -c <server_ip>

   • Replace <server_ip> with the IP address of the server (e.g., iperf3 -c 192.168.1.100).

   • Optionally, use -t <seconds> to set the test duration (default is 10 seconds), e.g., iperf3 -c 192.168.1.100 -t 30.

   • To test bidirectional bandwidth, add the -R flag for reverse mode or use --bidir for simultaneous bidirectional testing.

Step 3: Interpret the Results

• After the test completes, iperf3 will display a summary on both the client and server terminals, including:

  • Bandwidth: The average data transfer rate (e.g., in Mbps or Gbps).

  • Data Transferred: Total amount of data sent during the test.

  • Test Duration: Time taken for the test.

• Example output:

  [ ID] Interval           Transfer     Bandwidth
  [  5]   0.00-10.00  sec  1.25 GBytes  1.07 Gbits/sec

  This indicates a bandwidth of approximately 1.07 Gbps over 10 seconds.

Step 4: Optional Configurations

• Test UDP instead of TCP: Use -u on the client (e.g., iperf3 -c <server_ip> -u). This measures UDP performance and jitter.

• Adjust parallel streams: Use -P <number> to run multiple parallel streams (e.g., iperf3 -c <server_ip> -P 4 for four streams).

• Save output to a file: Use --logfile <filename> (e.g., iperf3 -c <server_ip> --logfile results.txt).

• Test specific bandwidth: Limit the client's bandwidth with -b <rate> (e.g., iperf3 -c <server_ip> -u -b 10M for 10 Mbps UDP).

Step 5: Stop the Server

• On the server, press Ctrl+C to stop the iperf3 server process.

Troubleshooting

• Connection refused: Ensure the server is running, the correct IP and port are used, and firewall settings allow traffic on the specified port.

• Low performance: Check for network congestion, hardware limitations, or incorrect MTU settings.

• Command not found: Verify iperf3 is installed and added to your system's PATH.

Notes

• iperf3 is not backward compatible with iperf2. Ensure both devices use iperf3.

• For accurate results, run tests multiple times and avoid running other bandwidth-intensive applications during the test.

• For advanced configurations, refer to the iperf3 documentation (man iperf3 or online resources).