Question

5. [11 points] Compare GBN, SR and TCP (no delayed ACK). Assume that timeout values for these protocols are sufficiently large such that 6 consecutive data segments and their corresponding ACKs can be received by the receiving Host B and the sending Host A, respectively. Suppose Host A sends 6 data segments to Host B, and the 3rd segment (sent from A) is lost. In the end, all 6 data segments have been correctly received by Host B. a. [(1+2) x 3 = 9 points) How many segments has Host A sent in total and how many ACKs has Host B sent in total by each protocol? What are their sequence and ACK numbers for each protocol? b. [2 points] If the timeout values for all three protocols are much longer than 6 RTT, then which protocol successfully delivers all six data segments in shortest time interval and why?

Answer 1

GBN:

A sends 10 segments in total. They are initially sent segments 1, 2, 3, 4, 5,6 and later re-sent segments 3, 4,5 and 6.

B sends 9 ACKs. They are 4 ACKS with sequence number 2, and 5 ACKS withsequence numbers 1, 3, 4,5 and 6.

B ACK: 1 2 2 2 2 3 4 5 6

SR:

A sends 7 segments in total. They are initially sent segments 1, 2, 3, 4, 5,6 and later re-sent segments 3.

B sends 6 ACKs. They are 5 ACKS with sequence number 1, 2, 4, 5,6. And there is oneACK with sequence number 3.

B ACK: 1 2 4 5 6 3

TCP:

A sends 7 segments in total. They are initially sent segments 1, 2, 3, 4, 5,6 and later re-sent segments 3.

B sends 5 ACKs. They are 4 ACKS with sequence number 3. There is one ACK withsequence numbers 7.

B ACK: 3 3 3 3 7.

(B).

If the timeout values for all three protocol are much longer than 6 RTT, then TCP protocol successfully delivers all five data segments in shortest time interval. The reason is that TCP uses fast retransmit without waiting time.