Consider sending a large file from a host to another host using a reliable conne

Question

Consider sending a large file from a host to another host using a reliable connection such as TCP. Suppose the reliable connection uses additive-increase, multiplicative-decrease (AIMD) without slow start for congestion control. Assume a congestion window size of 1 MSS at the start of the transmission and an increase by 1 MSS every time a batch of ACKs is received within an RTT and a multiplicative decrease of 0.5 (half the size of a congestion window) when congestion occurs. Given these parameters, what is the average throughput (in terms of MSS and RTT) for this connection for 10 RTTs with congestion occurring immediately after the 5th RTT (so before the 6th RTT)? Report your answers in MSS/RTT. Come up with a systematic approach that calculates step-by-step the number of transmissions in terms of MSS after every RTT.

Explanation / Answer

a) It takes 1 RTT to increase CongWin to 6 MSS; 2 RTTs to increase to 7 MSS; 3 RTTs to increase to 8 MSS; 4 RTTs to increase to 9 MSS; 5 RTTs to increase to 10 MSS; 6 RTTs to increase to 11 MSS; and 7 RTTs to increase to 12MSS.

b) In the first RTT 5 MSS was sent; in the second RTT 6 MSS was sent; in the third RTT 7 MSS was sent; in the fourth RTT 8 MSS was sent; in the fifth RTT, 9 MSS was sent; and in the sixth
RTT, 10 MSS was sent. Thus, up to time 6 RTT, 5+6+7+8+9+10 = 45 MSS were sent (and acknowledged). Thus, we can say that the average throughput up to time 6 RTT was (45 MSS)/(6 RTT) = 7.5 MSS/RTT.

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