Question

Multiple choice questions: Circle the correct answer

7) Consider Go-Back-N protocol. Suppose that Host A sends 6 data segments to Host B, and the 4th segment is lost. In the end, all 6 data segments have been correctly received by Host B. Assume that the timeout values is sufficiently long such that 6 consecutive data segments and their corresponding ACKs can be received (if not lost in the channel) by Host B and Host A, respectively before the timer times out. If Go-Back-N protocol is used for retransmission, the total number of segments that Host A has sent to Host B is

(a) 7; (b) 8; (c) 9; (d) 10; (e) None of the above.

7.A) For 7.A), if Selective Repeat protocol is used for retransmission, the total number of segments that Host A has sent to Host B is

(a) 7; (b) 8; (c) 9; (d) 10; (e) None of the above.

Answer 1

1. In case of Go Back N, the receiver windo size is one, hence the receiver doesn't acknowledge the out of order packets and waits for the correct package, so if the 4th packet is lost, the other 5th and 6th are discarded and the sender again has to send the 4th, 5th and 6th data segment so total number of data segments sent = 6 + 3 = 9 so correct answer is (c) 9

2. In case of Selective repeat protocol, the receiver is expecting the same no. of data segments as the sender is sending which means all the segments except the one which is lost is received and the one that is lost, the acknowledgement for that is not recieved by the sender and then it selectively sends that data. So in our case, 4th data will be sent again so total data sent = 6+1 = 7. so correct answer is (a) 7

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