The receiver’s actions in GBN are also simple. If a packet with sequence number n is received correctly and is in order (that is, the data last delivered to the upper
layer came from a packet with sequence number n – 1), the receiver sends an ACK
for packet n and delivers the data portion of the packet to the upper layer. In all other
cases, the receiver discards the packet and resends an ACK for the most recently
received in-order packet. Note that since packets are delivered one at a time to the
upper layer, if packet k has been received and delivered, then all packets with a
sequence number lower than k have also been delivered. Thus, the use of cumulative acknowledgments is a natural choice for GBN.