Allow the server accept past input buffers from client, make it send a warning that UDP packets were lost
The server should send a special type of packet to confirm that it misses the input. It'd make the client to include more past CMDs into their packets so their gameplay experience will be more smooth. AFAIK, the vanilla SRB2 has the functionality to send multiple input CMDs to clients with the command line variable -extratic (default is 1, it must actually be higher to play the game smoothly!)
The client MUST store their past input commands in the buffer. Netplus does this already.
Also we should somehow determine if the client uses NetPlus or not so we can use and send these special packets.