Consider the GBN protocol with a sender window size of 4 and a sequence number range of 1,024. Suppose that at time î, 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. 

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.