8. TCP Fast Retransmit This problem demonstrates TCP's Fast Retransmit mechanism which allows for the quick retransmission of a single lost packet after 3 duplicate ACKs have been received. The current sender transmission window cwnd - 10 packets. The very first packet sent has SN -1 Assume cwnd stays constant. The second packet transmitted in Step 1 (SN-2) is LOST!! Sender Receiver Step 1 cwnd = ? Step 2 Step 3 Step 4 Step 5 Step 6 a. In Step 1, how many packets is the Sender allowed to send? How do you know? b. Write the Sequence Numbers of the packets transmitted during Step 1 What is the SN of the lost packet? d. c. Given the packets transmitted in Step 1 and the one lost packet, how many corresponding ACKs should be returned if there are no other losses (considering only the packets sent in Step 1)? e. In Step 2, write the value of each of the 4 ACKs that are returned by the Receiver f. After the ACKs are returned in Step 2, can any new packets be sent? If so, what SN? What are the type of ACKs in Step 27 g. h. At the beginning of Step 3, what does the Sender infer has happened? Why? i. What action does the Sender take in Step 3 (red arrow)? j. What is the SN of the packet transmitted in Step 3? k. Just before the packet is received in Step 4, what packets does the Receiver have in its buffer? 1. What is the value of the ACK returned by the Receiver in Step 4 (green arrow)? m. After the Sender receives the ACK in Step 5, what is the SN of the very next segment transmitted by the Sender in Step 6? (assume cwnd is constant) What does TCP Sender believe is the cause of the lost packet in Step 1? n. o. p. q. As a result, what should happen to the Sender's congestion window (cwnd) in Step 5? What situation did we avoid by using Fast Retransmit? Look over the problem again, and on the Client side, draw in the Receiver's Buffer with the SN of any out-of-order packets it needs to store When the retransmission is successfully received, what happens to the Buffer? r