The initial congestion window size over a TCP is 1. If slow strat algo...
For every packet sent the window size increases by 2. So, when
Window size [WS=1] initially
⇒ After 1 RTT, window size = 2 and 1 segment is sent
⇒ After 2 RTT window size = 4 and 3 segment sent in total
⇒ After 3 RTT, window size = 8 and 7 segment sent in total
⇒ After ‘x’ RTTS window size 2x and 2x−1 segment are sent
Now, 2x−1 = 3999
⇒ 2x = 4000
⇒ x = log2(4000)
⇒ x = 12 RTT's
Alternately we can say that,
1st RTT- 1 segment sent.
2nd RTT- 2 new segments sent.
3rd RTT- 4 new segment sent.
4th RTT- 8 new segments sent.
5th RTT- 16 new segments sent.
6th RTT- 32 new segments sent.
7th RTT- 64 new segments sent.
8th RTT- 128 new segments sent.
9th RTT- 256 new segments sent.
10th RTT- 512 new segments sent.
11th RTT- 1024 new segments sent.
12th RTT- 2048 new segments sent.
Total segments sent = 1 + 2 + 4 + 8 + 16 + 32 + 64 + 128 + 256 + 512 + 1024 + 2048
=4095 > 3999
Hence, the correct answer is 12.