# hw6 - CS/ECE 438: Communication Networks for Computers...

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CS/ECE 438: Communication Networks for Computers Spring 2010 Problem Set 6 Internetworking and End-to-End Protocols Assigned reading: Peterson and Davie: Chapters 4, 5 and 9.1. All problems carry equal weight. For full credit, show all work. 1. TCP RTT Estimation One difficulty with the original TCP SRTT estimator is the choice of an initial value. In the absence of any special knowledge of network conditions, the typical approach is to pick an arbitrary value, such as 3 seconds, and hope this will converge quickly to an accurate value. If this estimate is too small, TCP will perform unnecessary retransmissions. If it is too large, TCP will wait a long time before retransmitting if the first segment is lost. Also, the convergence might be slow. a. Choose α = 0.85 and SRTT(0) = 3 seconds, and assume all measured RTT values = 1 second with no packet loss. What is SRTT(19)? Recall, SRTT(k + 1) = α * SRTT(k) + (1 - α ) * RTT(k + 1). b. Now let SRTT(0) = 1 second and assume RTT values = 3 seconds and no packet loss. What is SRTT(19)? c. Now consider α = 0.95. Repeat parts a. and b. and comment on the effect of a larger or smaller α . 2. TCP Slow Start Although slow start with congestion avoidance is an effective technique for coping with congestion, it can result in long recovery times in high-speed networks. a. Assume a roundtrip time delay of 125 ms (about what might occur across a continent) and a link with an available bandwidth of 1.2 Gbps and a segment size of 512 octets. Determine the window size needed to

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## hw6 - CS/ECE 438: Communication Networks for Computers...

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