Assume that the timeout values for GBN, SR, and TCP protocols are sufficiently l
ID: 3861623 • Letter: A
Question
Assume that the timeout values for GBN, SR, and TCP protocols are sufficiently long such that 6 consecutive data segments (assume the payload is 1 byte) can be received (if not lost in the channel) by the receiving host (Host B) and their corresponding ACKs can received by the sending host (Host A). Suppose Host A sends 6 data segments to Host B, and the 4th segment (sent from A) is lost. In the end, all 6 data segments have been correctly received by Host B. For GBN, SR, and TCP (no delayed ACK and no fast retransmit) protocols how many segments has Host A sent in total and how many ACKs has Host B sent in total? What are their sequence numbers? Assume the sequence numbers start from 1.
Explanation / Answer
a. GoBackN:
A sends 9 segments in total. They are initially sent segments 1, 2, 3, 4, 5 and 6 and later
re-sent segments 4, 5 and 6.
B sends 8 ACKs. They are 5 ACKS with sequence number 1, and 3 ACKS with sequence numbers 4, 5 and 6.
b. Selective Repeat:
A sends 7 segments in total. They are initially sent segments 1, 2, 3, 4, 5 and 6 and later
re-sent segments 4.
B sends 6 ACKs. They are 5 ACKS with sequence number 1, 2, 3, 5, 6. And there is one ACK with sequence number 4.
c. TCP:
A sends 7 segments in total. They are initially sent segments 1, 2, 3, 4, 5, 6 and later
re-sent segments 4.
B sends 6 ACKs. They are 5 ACKS with sequence number 4.
There is one ACK with sequence numbers 7. Note that TCP always send an ACK with expected sequence number.