GATE | GATE CS 2020 | Question 65

Consider a TCP connection between a client and a server with the following specifications; the round trip time is 6 ms, the size of the receiver advertised window is 50 KB, slow-start threshold at the client is 32 KB, and the maximum segment size is 2 KB. The connection is established at time t=0. Assume that there are no timeouts and errors during transmission.

Then the size of the congestion window (in KB) at time t+60 ms after all acknowledgements are processed is _________ .

Note – This question was Numerical Type.
(A) 33
(B) 44
(C) 55
(D) 50


Answer: (B)

Explanation: Given,

Threshold = 32 Kb, 
MSS = 2KB, 
RTT = 6ms 

Here, t + 60 is nothing but at the 10 RTT (60/6 = 10).

Now,



1st transmission: 2 KB
2nd transmission: 4 KB
3rd transmission: 8 KB
4th transmission: 16 KB
5th transmission: 32 KB (Threshold reached)
6th transmission: 34 KB
7th transmission: 36 KB
8th transmission: 38 KB
9th transmission: 40 KB
10th transmission: 42 KB 

Now, after receiving acknowledgements of 10th transmission RTT = 10*6 = 60 ms.

And, after receiving acknowledgements of 10th transmission RTT (means at 11th transmission),
The congestion window size will be 44 KB.

Option (B) is correct.

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