Assume Selective Repeat with window size 5 and sequence numbers 0,1,2,3,4,5,6,7,0,1,2,3,4,5,6,7,0,1,2,3,4,5,6,7 ... etc.The current sender's window is 4,5,6,7,0. The receiver's window is also 4,5,6,7,0. Now the receiver correctly receives packets 4,5,6,7,0, sends ACKs for each packet, and advances its window to 1,2,3,4,5. Now all the ACKs are lost or corrupted, packet 4 times out at the sender, and the sender retransmit packet 4. What is the problem ?