Introduction #

Throttle lets you simulate slow network connections on Linux and Mac OS X.

Throttle uses pfctl on Mac and tc on Linux (you also need ip and route for Throttle to work on Linux) to simulate different network speeds and is inspired by tylertreat/Comcast, the connectivity setting in the WPTAgent and sltc.

What is Throttle good for?

It is usually used for two different things:

  • You run it as a standalone tool setting simulate different connection speeds.
  • You integrate it in your (web performance) tool to simulate different connections.

You can set the download/upload speed and/or RTT. Upload/download is in kbit/s and RTT in ms.

Install #

npm install -g

On OSX, add these lines to /etc/pf.conf if they don’t exist, to prevent the pfctl: Syntax error in config file: pf rules not loaded error when you try to run throttle


Start simulate a slower network connection #

Here is an example for running with 3G connectivity. Remember: Throttle will use sudo so your user will need sudo rights.

throttle --up 330 --down 780 --rtt 200

Pre made profiles #

To make it easier we have pre made profiles, check them out by throttle –help:

--profile         Premade profiles, set to one of the following
                     3g: up:768 down:1600 rtt:150
                     3gfast: up:768 down:1600 rtt:75
                     3gslow: up:400 down:400 rtt:200
                     2g: up:256 down:280 rtt:400
                     cable: up:1000 down:5000 rtt:14
                     dsl: up:384 down:1500 rtt:14
                     3gem: up:400 down:400 rtt:200
                     4g: up:9000 down:9000 rtt:85
                     lte: up:12000 down:12000 rtt:35
                     edge: up:200 down:240 rtt:35
                     dial: up:30 down:49 rtt:60
                     fois: up:5000 down:20000 rtt:2

You can start throttle with one of the premade profiles:

throttle --profile 3gslow

or even simpler

throttle 3gslow

Stop simulate the network #

Stopping is as easy as giving the parameter stop to throttle.

throttle --stop


throttle stop

Add delay on your localhost #

This is useful if you run WebPageReplay and want to add some latency to your tests.

throttle --rtt 200 --localhost

Stop adding delay on localhost #

throttle --stop --localhost

Use directly in NodeJS #

import throttle from ''
// Returns a promise
throttle.start({up: 360, down: 780, rtt: 200}).then(() => ...


import throttle from ''
// Returns a promise
const options = {up: 360, down: 780, rtt: 200};
await throttle.start(options);
// Do your thing and then stop
await throttle.stop();

Log all commands #

You can log all the commands that sets up the throttling by setting LOG_THROTTLE=true.

LOG_THROTTLE=true throttle 3gslow

or use the CLI command --log:

throttle 3gslow --log

Run in Docker (on Linux) #

Make sure to run sudo modprobe ifb numifbs=1 before you start the container.

And then when you actually start your Docker container, give it the right privileges with --cap-add=NET_ADMIN

You can also use Docker networks to change connectivity when testing inside a container.