Answer to Question #247137 in Computer Networks for Ajith

Question

The diagram at left below shows the state of the sending side of a communication
session using a go-back-N protocol with cumulative acknowledgments, a window size
of 4, and a retransmission timeout of 25. The array represents the “re-send” buffer and
entry in the buffer represents a packet with its sequence number. As usual for
GoBack-N, we are using a single shared timer. Its expiration time is shown in the
diagram. Assume that the channel may lose packets but never re-orders them.
Suppose that at time 7, the application passes us a new payload to be sent and at time
12, we receive an ack with sequence number 3.
a. List all the packets sent by the sender between times 6 and 13, including
repeats (if any).
b. Show the state of the sender at time 13, in the right-hand diagram.
c. At time 13, how many additional packets can be sent before an ack is
received?

Answer 1

Given,

Window size = 4

Re-transmission time out = 25

Time to resend the payload = 7

The list of packets which is being sent between 6 and 13 is

$p_5,p_2,p_3,p_4,p_5$

At the given time there can be 2 packets.