Thanks for your help Spike.
No Client Side prediction in quake2 ? Does it use Server side Lag compensation then ?
I get what CL_SetUpPlayerPrediction does, I just don't see the usefulness of running it with prediction set to false.
Regarding the sequence number, the client uses the incoming_sequenceACK: most prediction access are done via:
frame = &cl.frames[cl.parsecount&UPDATE_MASK];
where parsecount = cls.netchan.incoming_acknowledged.