Question

Consider a Go-Back-N protocol with a sender window size of 5 and a sequence number range of 1,024, and assume the receiver sends ACKs with the number of the next sequence number expected. Suppose that at time t, the next in-order packet that the receiver is expecting is a packet with the sequence number 8. Assume that the channel does not reorder packets.

(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

Given window size of the sender is = 5

we know in Go-Back-N protocol window size of the receiver is = 1

Given range of sequence numbers is = 1024 (0 to 1023)

In 1st window we will have sequence numbers from 0 to 4 (0 1 2 3 4)

in 2nd window we will have sequence numbers from 5 to 9 (5 6 7 8 9)

This is in normal case

a)

    But given that receiver is expecting is a packet with the sequence number 8

    Hence packets from 0 to 7 (0 1 2 3 4 5 6 7) are already acknowledged at sender side

    Now hence the sender window will slide and the new window is 8 9 10 11 12

    Hence the possible sets of sequence numbers inside the sender’s window at time t

    8 9 10 11 12.

b)

   If reciever is expecting packet with the sequence number 8 then he might

send the ACK for the packets below 8 in previous window (5 6 7 8 9)

Hence ACK 5 , ACK 6 , ACK 7 and ACK 8 will be sent