The slow start phase begins with 4 MSS .It is not saying that a maximum segment size(MSS) of 4kb.
AIIMD works with MSS ( 1mss 2mss 4mss 8mss exponentialy increase in SLOW START PHASE then linearly increse in CONGESTION AVOIDANCE PHASE then constant)
Since SLOW START is used, window size is increased by the number of segments successfuly sent. This happens until either threshold value is reached or time out occurs.
In both of the above situations AIIMD is used to avoid congestion. If threshold is reached, window size will be increased linearly. If there is timeout, window size will be reduced to half.
Window size for 1st transmission = 4 MSS
Window size for 2nd transmission = 8 MSS
Window size for 3rd transmission = 16 MSS
threshold reached, increase linearly (according to AIMD)
Window size for 4th transmission = 17 MSS
Window size for 5th transmission = 18 MSS
Window size for 6th transmission = 19 MSS
time out occurs,
Calculate new thresold= current window size/2 => 19/2=>9
resend 19th with window size starts with as slow start.
Window size for 7th transmission = 4 MSS
Window size for 8th transmission = 8 MSS
Window size for 9th transmission = 9 MSS
threshold reached, now increase linearly (according to AIMD)
Window size for 10th transmission = 10 MSS
Congestion window at 10th transmission =10
refrence:http://www.geeksforgeeks.org/computer-networks-set-2/