In the slow start phase, the congestion window size (cwnd) is increased exponentially. The formula for the slow start phase is given by:
cwnd=cwnd+MSScwnd=cwnd+MSS
where MSS is the Maximum Segment Size. Given that the current cwnd is 16 MSS and the sender receives 4 ACKs, the cwnd after the 4 ACKs would be:
cwnd=16+(4×MSS)cwnd=16+(4×MSS)
You mentioned that the maximum segment size (MSS) used is 4 KB. So,
cwnd=16+(4×4)cwnd=16+(4×4)
cwnd=16+16cwnd=16+16
cwnd=32cwnd=32
Now, let's convert the cwnd from segments to kilobytes. If MSS is 4 KB, then 32 segments would be 32×432×4 KB:
cwnd in KB=32×4cwnd in KB=32×4
cwnd in KB=128cwnd in KB=128
Therefore, the value of the congestion window after receiving 4 ACKs in the slow start phase would be 128 KB.