Figure 3.19 shows the sender’s view of the range of sequence numbers in a GBN
protocol. If we define base to be the sequence number of the oldest unacknowledged
packet and nextseqnum to be the smallest unused sequence number (that is, the
sequence number of the next packet to be sent), then four intervals in the range of
sequence numbers can be identified. Sequence numbers in the interval [0,base-1]
correspond to packets that have already been transmitted and acknowledged. The interval [base,nextseqnum-1] corresponds to packets that have been sent but not yet
acknowledged. Sequence numbers in the interval [nextseqnum,base+N-1] can