Question

Compare GBN, SR, TCP (no delayed ACK, no SACK) – Assume that the timeout values for all three protocols are sufficiently long such that 5 consecutive data segments (each containing only one byte) and their corresponding ACKs can be received (if not lost in the channel) by the receiving host (Host B) and the sending host (Host A), respectively. Suppose Host A sends 5 data segments to Host B, and the 2nd segment (sent from A) is lost – this is the only data segment or ACK loss. In the end, all 5 data segments have been correctly received by Host B. How many segments has Host A sent in total and how many ACKs has host B sent in total? What are data segment and ACK sequence numbers? Answer these two questions for all three protocols.

Answer 1

Solution. GBN: A sends 9 segments in total. They are initially sent segments 1, 2, 3, 4, 5 and later re-sent segments 2, 3, 4 and 5 B sends 8 ACKs. They are 4 ACKs with sequence number 1 and 4 ACKs with sequence numbers 2, 3, 4 and 5. SR: A sends 6 segments in total. They are initially sent segments 1, 2, 3, 4, 5 and later re-sent segments 2. B sends 5 ACKs. They are 4 ACKs with sequence number 1, 3, 4, 5 and there is one ACK with sequence number 2. TCP: A sends 6 segments in total. They are initially sent segments 1 5 ACKs. They , 2, 3, 4, and later re-sent segments 2. B sends are 4 ACKs with sequence number 2 and there is one ACK with sequence numbers 6