Go-Back-N window size

Question

Why in TCP\'s Go-Back-N Algorithm window size(N) has to be smaller than the sequence number space(S): S>N? I tried figuring it out myself but don\'t quiet get it

Answer 1

Assume that sequence space was four (sequence numbers 0,1,2,3). Lets say window size was also 4. Sender sends 4 packets with sequence numbers (0,1,2,3). Receiver receives all four packets. So it sends 4 acknowledgements(0,1,2,3). Now assume all acknowledgements are lost. Sender will resend all four packets but receiver will assume they're the new ones. To avoid confusions arising from lost acknowledgements, we keep n < s