6.2. Testing the UDP throughput by using iperf3

Procedure

1. Optional: Display the maximum network speed of the network interface controller (NIC) on both the server and client:

   # ethtool enp1s0 | grep "Speed"
      Speed: 10000Mb/s

2. On the server:

   1. Display the maximum UDP socket read buffer size, and note the value:

      # sysctl net.core.rmem_max
      net.core.rmem_max = 16777216

      The displayed value is in bytes.

   2. Temporarily open the default iperf3 port 5201 in the firewalld service:

      # firewall-cmd --add-port=5201/tcp --add-port=5201/udp

      Note that, iperf3 opens only a TCP socket on the server. If a client wants to use UDP, it first connects to this TCP port, and then the server opens a UDP socket on the same port number for performing the UDP traffic throughput test. For this reason, you must open port 5201 for both the TCP and UDP protocol in the local firewall.

   3. Start iperf3 in server mode:

      # iperf3 --server

      The service now waits for incoming client connections.

3. On the client:

   1. Display the Maximum Transmission Unit (MTU) of the interface that the client will use for the connection to the server, and note the value:

      # ip link show enp1s0
      2: enp1s0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc fq_codel state UP mode DEFAULT group default qlen 1000
      ...

   2. Display the maximum UDP socket write buffer size, and note the value:

      # sysctl net.core.wmem_max
      net.core.wmem_max = 16777216

      The displayed value is in bytes.

   3. Start measuring the throughput:

      # iperf3 --udp --time 60 --window 16777216 --length 1472 --bitrate 2G --client 192.0.2.1

      • --udp: Use the UDP protocol for the test.
      • --time <seconds>: Defines the time in seconds when the client stops the transmission.
      • --window <size>: Sets the UDP socket buffer size. Ideally, the sizes are the same on both the client and server. In case that they are different, set this parameter to the value that is smaller: net.core.wmem_max on the client or net.core.rmem_max on the server.
      • --length <size>: Sets the length of the buffer to read and write. Set this option to the largest unfragmented payload. Calculate the ideal value as follows: MTU - IP header (20 bytes for IPv4 and 40 bytes for IPv6) - 8 bytes UDP header.

      • --bitrate <rate>: Limits the bit rate to the specified value in bits per second. You can specify units, such as 2G for 2 Gbps.

        Set this parameter to a value that you expect to work and increase it in later measurements. If the client sends packets at a faster rate than the devices on the transmit path or the server can process them, packets can be dropped.

      • --client <server>: Enables the client mode and sets the IP address or name of the server that runs the iperf3 server.

4. Wait until iperf3 completes the test. Both the server and the client display statistics every second and a summary at the end. For example, the following is a summary displayed on a client:

   [ ID] Interval       Transfer      Bitrate         Jitter    Lost/Total Datagrams
   [ 5] 0.00-60.00 sec  14.0 GBytes   2.00 Gbits/sec  0.000 ms  0/10190216 (0%) sender
   [ 5] 0.00-60.04 sec  14.0 GBytes   2.00 Gbits/sec  0.002 ms  0/10190216 (0%) receiver

   In this example, the average bit rate was 2 Gbps, and no packets were lost.

5. On the server:

   1. Press Ctrl+C to stop the iperf3 server.

   2. Close port 5201 in firewalld:

      # firewall-cmd --remove-port=5201/tcp --remove-port=5201/udp