Network Throughput Testing with iPerf

iPerf is a command-line tool used in diagnosing network speed issues by measuring the maximum network throughput a server can handle. It is particularly useful when experiencing network speed issues, as you can use iPerf to determine which server is unable to reach maximum throughput.

• You will need root access to your Linode, or a user account with sudo privilege.
• Update your system.

The iperf package is included in most Linux distribution’s repositories.

apt-get install iperf

CentOS repositories do not have iPerf. Use the EPEL repository, which is a repository used to install third-party software packages on RedHat systems such as RHEL and CentOS:

yum install epel-release
yum update
yum install iperf

pacman -S iperf

zypper install iperf

emerge iperf

If you have not yet run emaint --sync you may need to do so before it will allow you to install the iPerf package. Additionally, by default you will need to substitute each iperf command with /usr/bin/iperf3. This path may differ dependent on your iPerf version.

iPerf must be installed on the computers at both ends of the connection you’re testing. If you are using a Unix or Linux-based operating system on your personal computer, you can install iPerf on your local machine.

If you are testing the throughput of your Linode, however, it’s better to use another server as the end point, as your local ISP may impose network restrictions that can affect the results of your test.

iPerf requires two systems because one system must act as a server, while the other acts as a client. The client connects to the server you’re testing the speed of.

1. On the Linode you plan to test, launch iPerf in server mode:

   iperf -s
   

   You should see output similar to:

    ------------------------------------------------------------
    Server listening on TCP port 5001
    TCP window size: 85.3 KByte (default)
    ------------------------------------------------------------

2. On your second Linode, connect to the first. Replace 198.51.100.5 with the first Linode’s IP address.

   iperf -c 198.51.100.5
   

   ------------------------------------------------------------
   Client connecting to 198.51.100.5, TCP port 5001
   TCP window size: 45.0 KByte (default)
   ------------------------------------------------------------
   [  3] local 198.51.100.6 port 50616 connected with 198.51.100.5 port 5001
   [ ID] Interval       Transfer     Bandwidth
   [  3]  0.0-10.1 sec  1.27 GBytes  1.08 Gbits/sec
     

3. You will also see the connection and results on your iPerf server:

           ------------------------------------------------------------
           Server listening on TCP port 5001
           TCP window size: 85.3 KByte (default)
           ------------------------------------------------------------
           [  4] local 198.51.100.5 port 5001 connected with 198.51.100.6 port 50616
           [ ID] Interval       Transfer     Bandwidth
           [  4]  0.0-10.1 sec  1.27 GBytes  1.08 Gbits/sec

4. To stop the iPerf server process, press CTRL + c.

Using iPerf, you can also test the maximum throughput achieved via UDP connections.

1. Start a UDP iPerf server:

   iperf -s -u
   

   ------------------------------------------------------------
   Server listening on UDP port 5001
   Receiving 1470 byte datagrams
   UDP buffer size:  208 KByte (default)
   ------------------------------------------------------------

2. Connect your client to your iPerf UDP server. Replace 198.51.100.5 with your IP address:

   iperf -c 198.51.100.5 -u
   

   ------------------------------------------------------------
   Client connecting to 198.51.100.5, UDP port 5001
   Sending 1470 byte datagrams
   UDP buffer size:  208 KByte (default)
   ------------------------------------------------------------
   [  3] local 198.51.100.6 port 58070 connected with 198.51.100.5 port 5001
   [ ID] Interval       Transfer     Bandwidth
   [  3]  0.0-10.0 sec  1.25 MBytes  1.05 Mbits/sec
   [  3] Sent 893 datagrams
   [  3] Server Report:
   [  3]  0.0-10.0 sec  1.25 MBytes  1.05 Mbits/sec   0.084 ms    0/  893 (0%)

   1.05 Mbits/sec is considerably less than what was observed on the TCP tests. It is also considerably less than the maximum outbound bandwidth cap provided by the 1GB Linode. This is because iPerf limits the bandwidth for UDP clients to 1 Mbit per second by default.

3. You can change this with the -b flag, replacing the number after with the maximum bandwidth rate you wish to test against. If you are testing for network speed, set this number above the maximum bandwidth cap provided by Linode:

   iperf -c 198.51.100.5 -u -b 1000m
   

   This tells the client that we want to achieve a maximum of 1000 Mbits per second if possible. The -b flag only works when using UDP connections, since iPerf does not set a bandwidth limit on the TCP clients.

     ------------------------------------------------------------
     Client connecting to 198.51.100.5, UDP port 5001
     Sending 1470 byte datagrams
     UDP buffer size:  208 KByte (default)
     ------------------------------------------------------------
     [  3] local 198.51.100.5 port 52308 connected with 198.51.100.5 port 5001
     [ ID] Interval       Transfer     Bandwidth
     [  3]  0.0-10.0 sec   966 MBytes   810 Mbits/sec
     [  3] Sent 688897 datagrams
     [  3] Server Report:
     [  3]  0.0-10.0 sec   966 MBytes   810 Mbits/sec   0.001 ms    0/688896 (0%)
     [  3]  0.0-10.0 sec  1 datagrams received out-of-order

   This time the results are considerably higher.

In some cases, you may want to test both servers for the maximum amount of throughput. This can easily be done using the built-in bidirectional testing feature iPerf offers.

to test both connections, run the following command from the client:

iperf -c 198.51.100.5 -d

The result is that iPerf will start a server and a client connection on the client server (198.51.100.6). Once this has been done, iPerf will connect the iPerf server to the client connection, which is now acting as both a server connection and a client connection.

------------------------------------------------------------
Server listening on TCP port 5001
TCP window size: 85.3 KByte (default)
------------------------------------------------------------
------------------------------------------------------------
Client connecting to 198.51.100.5, TCP port 5001
TCP window size:  351 KByte (default)
------------------------------------------------------------
[  3] local 198.51.100.6 port 50618 connected with 198.51.100.5 port 5001
[  5] local 198.51.100.6 port 5001 connected with 198.51.100.5 port 58650
[ ID] Interval       Transfer     Bandwidth
[  5]  0.0-10.1 sec  1.27 GBytes  1.08 Gbits/sec
[  3]  0.0-10.2 sec  1.28 GBytes  1.08 Gbits/sec

On the server, you will see:

------------------------------------------------------------
Client connecting to 198.51.100.6, TCP port 5001
TCP window size:  153 KByte (default)
------------------------------------------------------------
[  6] local 198.51.100.5 port 58650 connected with 198.51.100.6 port 5001
[  6]  0.0-10.1 sec  1.27 GBytes  1.08 Gbits/sec
[  5]  0.0-10.2 sec  1.28 GBytes  1.08 Gbits/sec