Best way to simulate a WAN network

Question

Simplified, I have an application where data is intended to flow over the internet between two servers. Ideally, I'd like to test at what point the software ceases to function. At what lowerbound limit (bandwidth, latency, dropped packets) do things stop working to test the reliability of the software.

What I thought I would do was the following:

1. Setup up 3 machines (VMware instances)
2. Install the 2 applications on two of the servers.
3. Setup up the 3rd server to sit between the two machines by doing some sort of magic with Routing and Remote Access on Windows 2003
4. Install either Traffic Shaper XP or NetLimiter to limit the bandwidth
5. Run something like TMnetSim Network Simulator to simulate a bad connection.

Does this sound like a good idea or are there easier/better ways of doing this? I'm not that comfortable on Linux and my team mates are even less so.

Answer 1

FreeBSDs ipfw has provisions to simulate links with a given bandwith, latency or error rate. You could use that FreeBSD machine as your machine "in the middle" in your above setup.

You probably can also run at least one of the endpoints on the same machine if you want to reduce the amount of servers involved.

Answer 2

We had a similar requirement some ten years ago - I'll see if I can recall how we managed it.

If I remember, we wrote a socket proxy program which was controlled by inetd on a UNIX box. This socket would accept connections from a client and open equivalent sessions through to the server. It would then loop, passing messages in both directions.

The way we achieved WAN characteristics was to introduce random delays (with upper and lower limits) in both the connection establishment and the passing of data once the link was up.

It also had the feature to drop the link occasionally as WAN links were less reliable for us than local traffic.

I recall we had to make it threaded to stop the delays from affecting reverse traffic on the link.

Answer 3

Someone actually packaged up the settings and whatnot necessary for the FreeBSD solution to this problem and they call it DUMMYNET.

It simulates/enforces queue and bandwidth limitations, delays, packet losses, and multipath effects. It also implements a variant of Weighted Fair Queueing called WF2Q+. It can be used on user's workstations, or on FreeBSD machines acting as routers or bridges.

It can simulate exactly what you want, and its free and will boot onto commodity hardware. They even have a canned install of it that is small enough to put on a floppy disk (!) that you can download at that link.

Answer 4

WANem does exactly this. We have used it both in a virtual machine on the desktop and on a dedicated old pc and it worked great. It can simulate all sorts of broken connectivity.