Layered transport service provider

One problem I had a couple of years ago was how to write an accurate internet simulator feature for RakNet. What I have in there now is what you find in your average games – a user can intentionally add latency to outgoing messages and can randomly drop packets to simulate packetloss. But this is not accurate and not very full-featured.

Here’s a better design:

Maximum outgoing bandwidth – Represents your network card’s ability to send data through the wire.
Maximum incoming bandwidth – As above, but for incoming data
One-way trip time – The minimum time a datagram would take to get to a remote system; the base speed of the network
Per-message additional trip time – On the internet, many people are sending messages, so there will be some delay between each of your messages as other messages goes out. So this number represents the rest of the internet using the router’s bandwidth too. The effect to the sender is that as you flood the connection, your ping goes up. This is what actually happens and is how flow control is accomplished in modern reliable packet implementations (the old way used sliding windows which controlled throughput based on how fast the other system acked your messages).
Maximum router queue – This represents how much average memory each router has. Go beyond this and you start getting packetloss.
Randomly dropped packets – Due to noise on the wire
Out of order packets – As above

The problem with hardcoding this in the user level network layer is that you have to implement a control interface (changable at runtime), you have to do it on both systems, and it can be complex to implement in every circumstance. So I’m writing a layered transport service provider. This is sort of like RakNet’s plugin system, but for Winsock, and it gives you the ability to override various network events with a procedure table. This is stored as a system DLL, and installed and uninstalled by an EXE. I will have this EXE double as a control panel and will use interprocess communication to have the EXE control the invoked DLL.

By the way, to say this is complicated is understating the issue. It’s about 1000 lines of code for the installer, 10,000 lines of code to properly create a plugin that does nothing but pass the data through. You have to understand overlapped events, IO completion ports, support for versions of winsock, an entirely different set of code to install the driver on Vista (plus permission sets), you have to use .def files for export, and more. The article that I’m going by is 8,500 words long (about 20 pages) and takes hours of study to understand well enough to do anything.

After going through all this I think Shunra is justified in charging $500 for their simulator. The solution is hard to write. However, Shunra doesn’t support loopback last I contacted them. My implementation does, so I think I can sell mine to gave developers and others that want a loopback solution. Plus I can put a nice GUI on it, so you can control what is going on at runtime.