Question

Consider the Go-back-N protocol with a sender window of size 4 and, for simplicity, assume infinite sequence numbers that never wrap. Suppose that at time T the next in-order packet that the receiver is expecting has a sequence number of K. Assume that the underlying channel may lose or corrupt packets but does not reorder them.

(a)What are the possible sets of sequence numbers inside the sender’s window at time T? Justify your answer.

(b) What are all possible values of the ACK field in all possible messages currently propagating back to the sender at time T? Justify your answer.

Answer 1

1.  [K-1, K-4]

• In Go back N protocol, the receiver window size is 1.
• It is given that receiver expects the packet having sequence number ‘K’.
• It means it has processed all the packets ranging from 0 to K-1.
• It is assumed that sender has received the acknowledgement for all these packets.
• So, outstanding packets in sender’s window waiting for the acknowledgement starts from K.
• Sender window size = 4.
• Therefore, last packet in sender’s window will have sequence number K+4-1.

2. [K, K-4]

• Acknowledgement number is the next expected sequence number by the receiver.
• Receiver expects the packet having sequence number ‘K’ at time ‘t’.
• It means it has received the packets ranging from 0 to K-1 whose acknowledgements are are on the way.
• For the (K-1)^th packet, acknowledgement number would be ‘K’.
• For the (K-2)^th packet, acknowledgement number would be ‘K-1’ and so on.

Now,

• At any time, maximum number of outstanding packets can be ‘4’.
• This is because sender’s window size is ‘4’.
• Therefore, the possible values of acknowledgement number ranges from [K-n+1, ……, K-3, K-2, K-1, K] (total 4 values)
• Here, we have assumed that the acknowledgement for all the packets are sent independently.