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Consider a TCP connection between two machines (A and B) in an environment with 0% packet loss. Assume the round trip time (RTT) between the two machines is 4 [seconds], and the segment size is 3 [Kbytes]. The bandwidth of the connection is 500 [kbps]. What is the minimum possible TCP window size for which there will be no stalling?(We say a TCP connection experiences no stalling if the acknowledgments arrive back to the sender before the sliding window over the send buffer close to zero. I.e., TCP packets are continuously, back-to-back, sent out of the sender before the first ack arrives back.)
in Computer Networks by Loyal (7.6k points) | 68 views

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There will be no stalling if
time-to-send-entire-window ≤ time-for-first-ack-to-arrive-back
That is:
W*S / B ≤ RTT + S / B
Hence: W ≤ RTT * B / S + 1
In this particular case: W ≤ 4 sec * 500 kbps / 24 kbits + 1 = 83.3 + 1 = 84.3

(3Kbytes = 24 Kbits)
That is: W ≤ 84

So the smallest TCP window size is 84.
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