A common issue in application development nowadays is that people do not always test their application against devices and networks that have very low performance. This is somewhat understandable, especially when not everyone has the resources to setup a big testing rig testing high end, mid tier, and low end devices on a variety of different networks. This is not to mention that even when there are the means to test, people probably skip it as it is a giant hassle to do so.

During Shopify's Hack Days, the lovely @amecila and I worked on a project to simplify testing against slow performing networks. The basic idea is to be able to control the bandwidth, latency, and packet loss for individual devices connected to a special WiFi network. This system was named "Traffic Cop".

Traffic Cop is an application running on a router with OpenWRT that allows you to emulate different networks such as 3G, EDGE (2G) for individual devices connected to this router.

All a client has to do is connect to this network, go to the router's IP address (at port 8080 by default), and select a network profile to simulate. You can then watch the client's ping latency, bandwidth, and packet loss rate match the ones indicated for that profile.

The whole project is open source here: https://github.com/shuhaowu/trafficcop.

Spoiler! The finished product looks like the following:

Note: the UI text has been slightly updated in the current version, with "None" changed to "No profile" and the caption set to "Best available connection".

This already exists. Why?

This is probably the first thing that popped up in your mind after reading the above and you are right, several tools already exists for this purpose.

One thing about these tools is that they are all locked down to a particular platform. As an example, Network Link Conditioner is tied down to iOS and OS X only. Android does not have a network emulation tool on physical devices. This only exists in the emulator.

So you are right: the idea exists out there. However, I am not aware of any attempts to put this directly on a router.

Running this on a router has the advantage of being "cross platform", as long as the device you are testing your application on has some sort of networking capability (WiFi/Ethernet/Other) and has a browser to connect to the page to change your profile. This is much easier than downloading, installing, and then configuring an application to do this for your particular setup.

Furthermore, if your company has permanent test stations, you can just have a router nearby with this software running and all your devices connected. Since Traffic Cop supports per device network settings, all your devices can be using different settings and people coming over to test can set it themselves.

How does it work?

Traffic Cop runs on a router running OpenWRT, which you can think of as a Linux distribution built for routers. OpenWRT allow you to run a wide variety of software by installing them via opkg, a lightweight package manager. The availability of software makes it a perfect target environment for what we are trying to do.

On a normal router, if you visited the router's IP address in your browser, you would likely get to a login page to the router's administration panel. On a Traffic Cop enabled router, you get select an internet profile for the device you used to connect to the page1.

This page is simply a static HTML + CSS + JS page that makes certain calls to a CGI backend written in shell (Ash, to be specific). In these scripts, a few common network profiles (3G, 2G) are defined. When you select a different profile, an AJAX call to the server executes these scripts.

These scripts then call out to a tool known as tc. This tool is a front end to the bandwidth management features of the Linux kernel, which is very advanced and a topic of its own. Using tc, we can then limit the bandwidth, increase the latency, and randomly drop packets for clients that chooses a degraded network profile.

1 Some additional steps are required to enable this. By default, Traffic Cop runs on port 8080,

How does it really work? I mean, how do you use tc and stuff?

This is actually a somewhat complicated topic. It took us about a whole morning of reading to understand enough to implement the backend. The LARCT documentation is very good at explaining all of this so I'll just give a brief explanation that may not be easily understood without reading the linked documentations. Feel free to skip the section if you have a lot of difficulty reading it or otherwise don't care.

In OpenWRT, the local network is typically under the interface br-lan. Using tc, Traffic Cop creates a htb qdisc2 to limit the bandwidth, a netem qdisc to delay packets and drop them randomly, and a filter to filter out particular IPs.

The tc filter requires an id to send traffic to for throttling, delaying, and/or dropping. Hence, the htb qdisc for a particular client gets assigned an id which comes from the last group of the IPv4 address. Since ids must be unique, the netem qdisc gets assigned an id of the last group of the IPv4 address multiplied by 100 plus 66 (to avoid collision of .2 and .200). The netem qdisc also is a child of htb, allowing the filter to simply send the traffic to the correct htb qdisc, which will pass it to netem.

Any traffic from clients without network profiles will be tossed into id 265, which has no limit and never collides with the control chain id of another client. This is created during ifup br-lan.

You can see the code for individual clients here and the code for boot up here.

2 qdisc is an algorithm that manages the queue of a device, either incoming (ingress) or outgoing (egress). [Credit: LARCT]

I want to run this myself, how?

During the two days of development, the code was built straight into an image that could be ran on our test device. Afterwards, I spent a little bit more time packaging it into an opkg package, which will work as soon as you install it. You can download the ipk file from https://github.com/shuhaowu/trafficcop/releases and install it with opkg on your OpenWRT router. You will need a couple hundred kilobytes of space for this.

Sample commands:

root@OpenWRT:~# cd /tmp
root@OpenWRT:/tmp# wget https://github.com/shuhaowu/trafficcop/releases/download/....
root@OpenWRT:/tmp# opkg update && opkg install trafficcop*.ipk

After installing, you should be able to choose a network profile for your device by using that device and navigating to http://{your-router-ip}:8080.

As a note, with this setup, you can even have LuCI installed as well as Traffic Cop :)

Closing Notes

This project taught me a great deal about Linux networking. Writing a web app in shell was also a great surprise (:troll:). Hopefully, this will be useful for people writing apps for different platforms.

For future work, some aspects of the opkg package can be improved in order to submit this into the OpenWRT repository. Also, it would be nice to be able to specify your own network profiles, as well as simulate jitter and other more network issues.