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. (a) Suppose two data segments with byte sequence numbers 3000 and 15000 are lost once during the transmission. How many segments get retransmitted under each of the following conditions? A. Round trip time 100 ms, Timeout = 101 ms -> 10 B. Round trip time = 100 ms, Timeout = 152 ms -> 10 If they assume selective repeat, accept 2 retransmissions as answers for A & B
(b) Suppose acknowledgments corresponding to the above data segments are lost instead of the data segments. How many segments get retransmitted under the above conditions? A. Round trip time = 100 ms, Timeout = 101 ms -> 2 B. Round trip time = 100 ms, Timeout 152 ms -> 0 Remember that ACK is cumulative. Once the ACK for next segment arrives at the sender, the sender can stop retransmitting all the unnecessary packets.