Compare GBN, SR, and TCP (no delayed ACK). Assume that the timeout values for all three protocols are sufficiently long such that 5 consecutive data segments 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. In the end, all 5 data segments have been cor- rectly received by Host B.

a. GoBackN:

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.

Selective Repeat:

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, 2, 3, 4, 5 and later re-sent segments 2. B sends 5 ACKs. They are 4 ACKS with sequence number 2.  There is one ACK with sequence numbers 6. Note that TCP always send an ACK  with expected sequence number.  

TCP successfully delivers all five data segments in shortest  time interval. This is because TCP uses fast retransmit without waiting until time out.

Explanation: