I AM NOT GETTING THE UNDERLINED PART ...........THAT IF ACKS ARE NOT RECEIVED THAN HOW DOES THE SENDER WINDOW SLIDE....
Consider the Go-Back-N (GBN) protocol with a sender window size of 3 and a sequence number range of 1024. Suppose that at time t, the next in-order packet that the receiver is expecting has a sequence number of k. Assume that the medium does not reorder messages. Answer the following questions: a. What are the possible sets of sequence numbers inside the sender’s window at time t? Justify your answer.
Here we have a window size of = 3. Suppose the receiver has received packet K− 1, and has ACKed that and all other preceding packets. If all of these ACK's have been received by sender, then sender's window is [K,K +N − 1].
Suppose next that none of the ACKs have been received at the sender. In this second case, the sender's window contains K− 1 and the packets up to N and including K− 1. The sender's window is thus [ K-N , K− 1]. By these arguments, the senders window is of size 3 and begins somewhere in the range [K −N ,1 ].