CS 656 Homework #3, Due April 28th

1. In this question, you use the early TCP equations to compute timeouts:

   RTT=RTT*0.9 + (NEW_RTT_SAMPLES)*0.1,
   Timeout = 2*RTT

   • (15pt) Starting from RTT = 100ms, calculate the Timeout value after processing 3 RTT samples: 120ms, 50ms, and 400ms.
   • (5pt) Notice that there probably is a developing congestion situation as implied in the 3rd sample. Your Timeout result will be used as the timeout interval for the next leaving packet. If the round-trip time of that packet is again 400ms, will it suffer retransmission ?

2. In this question, you use the refined equations to compute timeouts:

   DIFF=SAMPLE-RTT
   RTT = RTT+ DIFF/8
   DEV = DEV + (|DIFF| - DEV)/4
   Timeout = RTT + DEV * 4

   • (15pt) Starting from RTT = 100ms and DEV=50, calculate the Timeout value after processing 3 RTT samples: 120ms, 50ms, and 400ms.
   • (5pt) Notice that there probably is a developing congestion situation as implied in the 3rd sample. Your Timeout result will be used as the timeout interval for the next leaving packet. If the round-trip time of that packet is again 400ms, will it suffer retransmission ?

3. (10pt) Assume that a TCP receiver acknowledges every segment immediately and that it has received successfully everything up to byte 999 regarding a particular connection. Show the acknowledgments produced in response to the following segments of the connection, arriving in the order:

   Seg(1500,100), Seg(1000,300), Seg(1600,200), Seg(1800,200), Seg(1300,200),

   where Seg(x,y) denotes a segment with sequence number x and with y bytes of data. The SYN and FIN bits are OFF with all above segments. In your answer, use the notation Ack(x) to represent an acknowledgment with sequence number x.

4. (8pt) The TCP fast retransmission mechanism uses 3 duplicates of acknowledgments with sequence number x (that is, four Ack(x) in a row) as a indication of the loss of Segment(x). Explain the rational of this mechanism.