Consider the Selective Repeat protocol. Assume messages can be duplicated or lost, but the medium does not corrupt messages and does not re-order them (in both directions). Let the window size be 20. The sequence numbers range from 1 to 1024. Suppose that at time t the first packet in the receiver's window has a sequence number 100.

a. What are the various sequence number of messages that could be in transit from the sender to the receiver at time t?

b. What are all the possible ACK message number that could be currently propagating back from the receiver to the sender at time t (not necessarily together at the same scenario)?