Simulating terrible network conditions

If you’re an app developer reading this, can you tell me, off the top of your head, how your app behaves on a link with 40 kbps available bandwidth, 1,000 ms latency, occasional jitter of up to 2,000 ms, packet loss of 10%, and a complete 15-second connectivity dropout every few minutes?

— https://brr.fyi/posts/engineering-for-slow-internet

https://chatgpt.com/share/13e444bc-f16e-4ce7-841f-e8362f366da4

macOS 🔗

1. Create a configuration file for pfctl 🔗

sudo nano /etc/pf.conf:
dummynet-anchor "shaping"
anchor "shaping"

2. Create the dummynet rules 🔗

sudo nano /etc/pf.anchors/shaping:
dummynet out proto tcp from any to any pipe 1
dummynet out proto udp from any to any pipe 1

# Define the pipe with bandwidth, delay, and packet loss
pipe 1 config bw 40Kbit/s delay 1000ms plr 0.1 queue 50

3. Enable pfctl and load the configuration 🔗

sudo pfctl -e
sudo pfctl -f /etc/pf.conf
sudo dnctl -f flush
sudo dnctl pipe 1 config bw 40Kbit/s delay 1000ms plr 0.1

4. Simulate jitter and periodic connectivity dropouts 🔗

To simulate jitter and dropouts, you can use a looped script that periodically changes the conditions. Create a script network_simulation.sh:

#!/usr/bin/env bash

while true; do
  # Simulate jitter
  sudo dnctl pipe 1 config bw 40Kbit/s delay $((1000 + RANDOM % 2000))ms plr 0.1
  sleep 30

  # Simulate connectivity dropout
  sudo dnctl pipe 1 config bw 40Kbit/s delay 1000ms plr 1
  sleep 15

  # Restore normal conditions
  sudo dnctl pipe 1 config bw 40Kbit/s delay 1000ms plr 0.1
  sleep $(( RANDOM % 120 + 60 ))
done

5. Make the script executable and run it 🔗

chmod +x network_simulation.sh
./network_simulation.sh

This setup will simulate the specified network conditions, including bandwidth limitations, latency, jitter, packet loss, and periodic connectivity dropouts.

6. Restoring network conditions 🔗

#!/usr/bin/env bash

# Flush all dummynet configurations
sudo dnctl -f flush

# Disable pfctl
sudo pfctl -d

echo "Network conditions restored to normal."

Linux 🔗

To achieve the same network conditions on Linux, you can use tc (Traffic Control) along with netem for network emulation. Here’s how to set it up:

1. Install necessary tools 🔗

sudo apt-get update
sudo apt-get install iproute2

2. Set up the basic network conditions 🔗

sudo tc qdisc add dev eth0 root handle 1: htb default 30
sudo tc class add dev eth0 parent 1: classid 1:1 htb rate 40kbps
sudo tc qdisc add dev eth0 parent 1:1 handle 10: netem delay 1000ms loss 10%
Replace eth0 with your network interface name, which you can find using ifconfig or ip a.

3. Simulate jitter and periodic connectivity dropouts 🔗

Create a script network_simulation.sh:

#!/usr/bin/env bash

while true; do
  # Simulate jitter
  sudo tc qdisc change dev eth0 root handle 1: htb default 30
  sudo tc class change dev eth0 parent 1: classid 1:1 htb rate 40kbps
  sudo tc qdisc change dev eth0 parent 1:1 handle 10: netem delay $((1000 + RANDOM % 2000))ms loss 10%
  sleep 30

  # Simulate connectivity dropout
  sudo tc qdisc change dev eth0 root handle 1: htb default 30
  sudo tc class change dev eth0 parent 1:1 htb rate 40kbps
  sudo tc qdisc change dev eth0 parent 1:1 handle 10: netem delay 1000ms loss 100%
  sleep 15

  # Restore normal conditions
  sudo tc qdisc change dev eth0 root handle 1: htb default 30
  sudo tc class change dev eth0 parent 1:1 htb rate 40kbps
  sudo tc qdisc change dev eth0 parent 1:1 handle 10: netem delay 1000ms loss 10%
  sleep $(( RANDOM % 120 + 60 ))
done

Replace eth0 with your network interface name, which you can find using ifconfig or ip a.

4. Make the script executable and run it 🔗

chmod +x network_simulation.sh
./network_simulation.sh

This script periodically changes the network conditions to include jitter and simulate complete connectivity dropouts as specified.

5. Restoring network conditions 🔗

#!/usr/bin/env bash

# Delete all tc configurations
sudo tc qdisc del dev eth0 root

echo "Network conditions restored to normal."

Replace eth0 with your network interface name, which you can find using ifconfig or ip a.