tcp congestion control

Question

Consider the effect of using slow start on a line with 10 msec round trip time. The receiver window and the size of congestion window are set to 38 KB and 36 KB respectively. Sender side threshold is set to 18 KB. After 8 transmission a time-out occurs, after time out, the time taken to send first full window of 18 KB is____________ (in msec). Assume window size at the start of slow start phase is 2 KB.

Comments

if we start with 2KB sender window size.and it is given that threshold value is 18 KB.

sender window size will increase as: 2KB,4KB,8KB ,16KB..
 
then we start additive increase phase after 16 KB( bcoz threshold value is 18 kB) or  after 32 KB ??

IF Additive increase will START after 16 KB then how transmitter will know threshold is reached??

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170msec?

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Same Question https://gateoverflow.in/154248/slow-start 

then we start additive increase phase after 16 KB( bcoz threshold value is 18 kB) or  after 32 KB ??

I have the same doubt. But seeing some answers it appears in such case sender window is not doubled but instead is set equal to the threshold value.