Question

Hosts A and B are communicating over a TCP connection, and Host B has already received from A all bytes up through byte 100. Suppose Host A then sends three segments to Host B. The first, second, third segments contain 10, 20, and 30 bytes of data respectively. Host B sends an acknowledgment whenever it receives a segment from Host A.

1. What is the sequence number of the first, second, third segment A sends to B? (3 points)
2. Assume three segments arrive at B in order. When acknowledging the first, second, third segment, what is the ACK number, respectively? (3 points)
3. Assume B receives the second segment first, then the third segment, finally the first segment. When acknowledging these segments, what is the ACK number, respectively? (3 points)

Answer 1

A. Since 100 bytes has been send and considering initial sequence number to 0,

Sequence number of 1st segment = 100

Sequence number of 2nd segment = 100+10 = 110

Sequence number of 3rd segment = 110+20 = 130

B. Acknowledgement number specifies the next expected byte by the receiver.

So the first Acknowledgement number = 110 (Sequence number of 2nd segment)

Second acknowledgement number = 130

Third acknowledgement number = 160 ( sequence number of 4th segment which is 130+30 since 3rd segment contains 30B)

C. Here receiver was expecting 1st segment, but received it in the end.

So, all the 3 acknowledgement would be for the 1st segment.

Thus all the 3 Acknowledgement will contain ACK# = 100.

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