18.Suppose A is connected to B via an intermediate router R, as shown in the following figure. The A-R and R-B links each accept and transmit only one packet per second in each direction (so two packets take 2 seconds), and the two directions transmit independently. Assume A sends to B using the sliding window protocol with SWS = 4. (a) For Time 0,1,2,3,4,5, state what packets arrive at and leave each node, or label them on a timeline. (b) What happens if the links have a propagation delay of 1.0 second, but accept immediately as many packets as are offered (i.e., latency 1 second but bandwidth is infinite)