Question

Consider sending a large file from a host to another over a TCPconnection that has no loss.

a. Suppose TCP uses AIMD for its congestion control without a slowstart. Assuming CongWin increases by 1 MSS every time a batch ofACKs is received and assuming approximately constant round triptimes, how long does it take for CongWin to increase from 1 MSS to6 MSS (assuming no loss events)
b. What is the average throughput (in terms of RSS and RTT) for this connection up though time = 5RTT??

Answer 1

In AIMD congestion control without slow start, it takes 5 round trip times for CongWin to increase from 1 MSS to 6 MSS. The average throughput can be calculated by dividing the number of bytes transmitted by the total round trip time.

Step-by-step explanation:

In AIMD (Additive Increase Multiplicative Decrease) congestion control algorithm, the CongWin (Congestion Window) of TCP increases by 1 MSS (Maximum Segment Size) for every batch of ACKs received. Since there is no loss in the connection, the CongWin would keep increasing until it reaches the desired value. In this case, we want to increase CongWin from 1 MSS to 6 MSS. So, CongWin needs to increase by 5 MSS.

Assuming the round trip times are approximately constant, we can assume that it takes 1 round trip time for CongWin to increase by 1 MSS. Therefore, it would take 5 round trip times for CongWin to increase by 5 MSS and reach the desired value of 6 MSS.

The average throughput of a connection up to a certain time can be calculated by dividing the number of bytes transmitted up to that time by the total round trip time up to that time.