A player opens the program, gets a square that they can control, and sees other player's squares move in real time. That's it.
Read more about it here and download the game here.
Created with the Windows API and SDL2. The server code runs on Linux. The code for data serialization and sending packets was created from scratch. I wanted the rendering code to be from scratch using D3D12. But all I really wanted to do was draw a few squares on the screen, which D3D12 turned out to be massive overkill for. You can see the remnants of that code in the abandoned win32_main.cpp. The main entry point for the finished version is sdl2_main.cpp.
The rendering is using SDL2's built in hardware accelerated renderer, which I think is just D3D9 in immediate mode.
At any rate, the server can handle up to 8 simultaneous connections, which is an arbitrary cap I put in place. Didn't want to overload my $5 DigitalOcean droplet. The server can handle disconnects from the client exiting either on purpose or not sending a message for 10 seconds.