GO BACK N use cummulative ack. so , though it is not received ACK for packet 3 , ACK for packet 4 assures sender A that packet 3 successfully received by receiver . but due to some reason ack is lost . packet 5 itself is lost , so after that, all out of order packets are rejected by receiver and no ack is generated . so after timeout , a window of 7 packet is again transferred that starts from sequence no 5 .