Question

Compare GBN, SR and TCP (no delayed ACK). Assume

that the timeout values

for all three protocols are sufficiently long such

that 7 consecutive data segments

and the 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 7 data segments to Host B, and the thi

rd segment (sent from A) is

lost. In the end, all 7 data segments have been cor

rectly received by Host B.

a. How many segments has Host A sent in total and h

ow many ACKs

has Host B sent in total? What are their sequence n

umbers? Answer

this question for all three protocols.

b. If the timeout values for all three protocols ar

e much longer than 7

RTT, then which protocol successfully delivers all

7 data segments in

the shortest time interval?

Answer 1

Answer:---------

a. GoBackN:
A sends 13 segments in total. They are initially sent segments 1, 2, 3, 4, 5, 6, 7 and later
re-sent segments 2, 3, 4, and 5, 6, 7.
B sends 12 ACKs. They are 6 ACKS with sequence number 1, and 6 ACKS with sequence numbers 2, 3, 4, 5, 6 and 7.

Selective Repeat:
A sends 8 segments in total. They are initially sent segments 1, 2, 3, 4, 5, 6, 7 and later
re-sent segments 2.
B sends 7 ACKs. They are 6 ACKS with sequence number 1, 3, 4, 5, 6, 7. And there is one ACK with sequence number 2.

TCP:
A sends 8 segments in total. They are initially sent segments 1, 2, 3, 4, 5, 6, 7 and later re-sent segments 2.
B sends 7 ACKs. They are 6 ACKS with sequence number 2.There is one ACK with sequence numbers 8.

Note that TCP always send an ACK with expected sequence number.

b. TCP. This is because TCP uses fast retransmit without waiting until time out.