hw8sol - CSCI 690 Computer Networks Homework 7 Solution...

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Unformatted text preview: CSCI 690 Computer Networks Homework 7 Solution Saad Mneimneh Problem 1: A simple AIMD Consider a simple congestion control algorithm that uses linear increase and multiplicative decrease (no slow start). Assume the congestion window size is in units of packets rather than bytes, and it is one packet initially. (a) Give a detailed description of this algorithm. ANSWER : Initially, cwnd = 1 on ACK : cwnd = cwnd + 1 /cwnd on timeout : cwnd = min(1 ,cwnd/ 2) (b) Assume that for every group of packets sent, only one cumulative ACK is returned. Plot the congestion window size as a function of time (units of RTT) when the following packets are lost: 9, 25, 30, 38, and 50. For simplicity assume the timeout is equal to 1 RTT. RTT 1 2 3 4 sent 1 2-3 4-6 7-10 Packet 9 is lost. cwnd is reduced to 2. RTT 5 6 7 8 9 sent 9-10 11-13 14-17 18-22 23-28 Packet 25 is lost. cwnd is reduced to 3. RTT 10 11 sent 25-27 28-31 Packet 30 is lost. cwnd is reduced to 2. RTT 12 13 14 sent 30-31 32-34 35-38 Packet 38 is lost. cwnd is reduced to 2. RTT 15 16 17 18 sent 38-39 40-42 43-46 47-50 Packet 50 is lost. cwnd is reduced to 2. RTT 19 sent 50 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 1 2 3 4 5 6 7 (c) What is the eective throughput achieved for this connection if each packet is 1KB and the RTT is 100ms? ANSWER : It took 19 RTTs to send 50 packets, i.e. 50KB. The eective throughput is 50 1024 8 / 1 . 9 215 Kbps. (d) Explain why the changing cwnd each time an ACK arrives using the follow-ing formula cwnd = cwnd + MSS ( MSS/cwnd ) is not correct for such algorithm (or in general). ANSWER : The formula is accurate if each new ACK acknowledges one new MSS-sized segment. However, an ACK can acknowledge either small size pack-ets (smaller than MSS) or cumulatively acknowledge many MSSs worth of data....
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This note was uploaded on 03/27/2010 for the course CSCI 415 taught by Professor Saadmneimneh during the Spring '08 term at CUNY Hunter.

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hw8sol - CSCI 690 Computer Networks Homework 7 Solution...

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