csDoom is a source port, originally based on ZDoom version 1.22, that was created in 2000 by Sergey "Fly" Makovkin. He intended to replace ZDoom's peer-to-peer multiplayer code with a client/server system using code from QuakeWorld. Makovkin achieved his goal, and by December 2000, csDoom already had a decently stable and well-functioning client/server multiplayer mode.