Latency Compensating Methods in Client/Server In-Game Protocol Design (2001)

Assuming full prediction1, the client will want to start with the latest data from the server, and then run the five user commands through “similar logic” to what the server uses for simulation of client movement. In Half-Life, minimizing discrepancies between client and server in the prediction logic is accomplished by sharing the identical movement code for players in both the server-side game code and the client-side game code. First, you’ll notice that, depending upon the client’s latency and how fast the client is generating user commands (i.e., the client’s framerate), the client will most often end up running the same commands over and over again until they are finally acknowledged by the server and dropped from the list (a sliding window in Half-Life’s case) of commands yet to be acknowledged.

Source: developer.valvesoftware.com