Imagine a TCP session over wireless where the congestion window is fixed at 5 segments (congestion control is turned off and no fast retransmits). Segments may get lost but are not reordered. The receiver has infinite buffer and it sends an acknowledgment as soon as it receives a segment, i. e., acknowledgments are not deferred. Similarly, sender transmits a segment as soon as it is allowed to. Each segment carries 1000 bytes and the time to transmit a segment is 2 ms. Assume that transmission of ACK takes negligible time. Note that the retransmission timer for a segment is started after the last bit of the segment is sent. Assume Go-Back-5, and accumulative ACM are used.

Answer and Explanation:

For RTT=100ms, Time out=152ms -> 10.

one lost data segment still resulted in 5 retransmitted segments. Total retransmissions for two losses=10.

ii. acknowledgements

For RTT=100ms ,Timeout=101ms -> 2

Timeout is very close to RTT.ACK for 3000 is lost, causing frame 3000 to be retransmitted. However subsequent ACKs arrived correctly to prevent further retransmission.so the loss of 1st acknowledgement only resulted in one transmission.similarly loss of ACK for 15000 also only resulted in one retransmission . The total retransmission is 2.

For RTT=100 ms , Timeout= 152 ms ->0

In this case,since timeout is much longer than RTT, the ACK for 4000 arrives before packet with 3000 timeout. hence no retransmission is necessary.similarly ACK for 16000 arrived before packet 15000 timeout. Total retransmission =0 .

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