Question 1
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 ______.
       Computer-Networks       TCP-Congestion-Window       GATE 2020
Question 1 Explanation: 
Threshold = 32 Kb,  MSS = 2KB, RTT = 6ms
Here, t + 60 is nothing but at the 10 RTT (60/6 = 10), but here it’s asking after all acknowledgement are processed it means after the 10th RTT, .i.e at the 11RTT
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
At the completion of 10th transmission RTT = 10*6 = 60 ms
For the 11th transmission, The congestion window size  is 44 KB
Question 2

Consider the following statements regarding the slow start phase of the TCP congestion control algorithm. Note that cwnd stands for the TCP congestion window and MSS denotes the Maximum Segment Size.

    (i) The cwnd increase by 2 MSS on every successful acknowledgement.
    (ii) The cwnd approximately doubles on every successful acknowledgedment.
    (iii) The cwnd increase by 1 MSS every round trip time.
    (iv) The cwnd approximately doubles every round trip time.

Which one of the following is correct?

Only (ii) and (iii) are true
Only (i) and (iii) are true
Only (iv) is true
Only (i) and (iv) are true
       Computer-Networks       TCP-Congestion-Window       Gate 2018
Question 2 Explanation: 
In Slow-start, the value of the Congestion Window will be increased by 1 MSS with each acknowledgement (ACK) received, and effectively doubling the window size each round-trip time
Initially, TCP starts with cwnd of 1 MSS. On every ack, it increases cwnd by 1 MSS.
That is, cwnd doubles every RTT.
Initially sends 1 segment. On ack, sends 2 segments.
After these 2 acks come back, sends 4 segments etc.
TCP rate increases exponentially during slow start.
Slow start continues till cwnd reaches threshold.
After threshold is reached, cwnd increases more slowly, by one 1 MSS every RTT.
There are 2 questions to complete.
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