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Consider an instance of TCP’s Additive Increase Multiplicative Decrease (AIMD) algorithm where the window size at the start of slow start phase is 2 KB and the threshold at the start of first transmission is 32 KB. Assume that 3 duplicate ACK are received during the 5th transmission and 1 duplicate ACK received during 6th transmission. what is the congestion window size at the end of 10th transmission?

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RTT

1

2

3

4

5

6

7

8

9

10

CW

2KB

4KB

8KB

16KB

32KB

2KB

4KB

8KB

16KB

18KB

TH

32KB

32KB

32KB

32KB

32KB

16KB

16KB

16KB

16KB

16KB

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Please correct me if I’m wrong-

Till 5th transmission, the CnWd=32KB and  SSthres=32KB

Here if the sender receives 3 Acknowledgements that means that congestion has occurred and the segments are getting lost. Hence as per Slow Start the CnWd=2KB and SSthres=16KB. In 6th transmission the duplicate acknowledgement is discarded and by 10th transmission the CnWd size is 18KB.

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just remember two points-

1- when time out occures (severe case) then senders renew the thresold window size and start from slow phase

2- when duplicate ack occures(light congestion) then sender renew the thresold window size and start congestion avoidance phase after from new thresold window

now answer – 1- 2 kb, 2 – 4kb, 3 - 8kb, 4 - 16kb, 5 - 32, time out now new thresold window set to 32/2 = 16kb 

and congestion avoidance phase will start from here that is increase gradually

6- 16kb , 7- 18kb, 8 – 20kb , 9 – 22kb , 10 – 24kb so at the end of 10th window size is 26 kb

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