Using NetEm to Emulate Networks

NetEm (Network Emulator) is an enhancement of the Linux traffic control facilities that allow adding delay, packet loss, duplication and other characteristics to packets outgoing from a selected network interface. NetEm uses the existing Quality Of Service (QOS) and Differentiated Services (diffserv) facilities in the Linux kernel.

It's a great tool to test SRT connections with network impairments like on real networks.

Installation

Linux traffic control (tc) and NetEm (netem) are available in most Linux distributions, typically as part of the iproute2 package.

On Ubuntu systems the iproute2 package (including netem) can be installed using following command:

sudo apt-get install iproute2

Installation on CentOS 8.2

On CentOS 8.2 the iproute-tc package (including netem) can be installed using following command:

sudo yum install iproute-tc

NOTE: RHEL-based systems like CentOS 8.2 might need an additional kernel module in order to run netem. This would be indicated if, in running netem, the look-up of the name for qdisc fails, like in example shown below:

sudo tc qdisc add dev enp7s0 root netem delay 10ms 10ms loss 2% rate 12mbit
Error: Specified qdisc not found.

In such a case, install the modules kernel-modules-extra and kernel-debug-modules-extra with following command:

sudo yum install kernel-debug-modules-extra kernel-modules-extra

After installation a reboot is needed to activate the kernel modules.

Getting Started with TC & NetEm

The following examples use the interface enp7s0. Modify to match your network adapter (e.g. eth0).

To be able to change settings, root privileges are required.

Adding Delay

NOTE: netem only adds delay to packets leaving the interface. If you want to simulate bi-directional delay, two instances of tc netem - one on each end - are required.

The following command adds 250 ms of delay to packets leaving interface enp7s0:

sudo tc qdisc add dev enp7s0 root netem delay 250ms

Adding Delay with Volatility

The following command sets the transmission of enp7s0 network interface to be delayed by 100ms + 10ms (any value between 90 and 110 ms):

sudp tc qdisc add dev enp7s0 root netem delay 100ms 10ms

The volatility (randomness) of such fluctuations can be further specified. The following command sets network interface enp7s0 transmission to 100 ms, while 30% of the packets selected at random are delayed by +10 ms:

sudo tc qdisc add dev enp7s0 root netem delay 100ms 10ms 30%

Adding Packet Loss

The following command sets the enp7s0 network interface transmission to randomly drop 1% of the packets:

sudo tc qdisc add dev enp7s0 root netem loss 1%

Adding Packet Loss with Success Rate

sudo tc qdisc add dev enp7s0 root netem loss 1% 30%

Duplicated Packets

sudo tc qdisc add dev enp7s0 root netem duplicate 1%

Corrupted Packets

For Linux kernel versions above 2.6.16, netem can also simulate damage to packets. The following command randomly damages 0.2% of the outgoing packets:

sudo tc qdisc add dev enp7s0 root netem corrupt 0.2%

Packet Reordering

The following command sets network interface enp7s0 to send out 25% of data packets (with 50% relevance) immediately and delay all other packets by 10 ms.

sudo tc qdisc add dev enp7s0 root netem delay 10ms reorder 25% 50%

View the Configured Network Conditions

sudo tc qdisc show dev enp7s0

Stopping Network Impairments

IMPORTANT: Please remember to disable the network impairments after tests are done by using the following command:

sudo tc qdisc del dev enp7s0 root netem

The above command will delete the root configuration that was used in the examples above.

NetEm Wrapper

We provide a netem wrapper written in Python to concatenate network conditions by means of simple configuration files.

Here is an example of a config file that can be created with it:

{
  "name": "example",
  "interface": "wlp3s0",
  "events": {
    "1": {
      "duration": 5000,
      "rules": [
        "clear"
      ]
    },
    "2": {
      "duration": 15000,
      "rules": [
        "delay 30ms 10ms distribution normal",
        "loss 0.1% 0.25%"
      ]
    },
    "3": {
      "duration": 10000,
      "rules": [
        "delay 50ms"
      ]
    },
    "4": {
      "duration": 5000,
      "rules": [
        "clear"
      ]
    }
  }
}

The wrapper has an optional parameter that outputs a json file with the timestamps of when the conditions were applied. It can be useful in case the time of changing the network impairments is required for further analysis.