in TCP congestion window changes dynamically depending on congestion window , advertised window by receiver , amount data unacknowledged .
Reason :
window size : amount of data we can transmit before acknowledgement received.
in sliding window protocol , until acknowledgment received, copy of transmitted data kept in buffer.
window size= min (congestion window , advertised window) = 18 KB
size of unacknowledged data =20480-8384 =12096 bytes = 11.8 KB
as this 11.8 KB is not acknowledged till now, this data is present in buffer , so (effective window size ) the amount of data that is possible to transmit before acknowledgement received is
current active window size = 18 KB - 11.8 KB = 6.2 KB