initially window size(WS) = 1, so we can transfer 1 packet.
after first RTT WS =2, so we can transfer 2 packet.
after second RTT WS = 4 , so in this we can transfer 4 packet.
after third RTT WS = 8, we can transfer 8 packet.
after fourth RTT WS = 16, transfer 16 packet
after 5th RTT WS = 32, transfer 32 packet.
after 6th RTT WS = 64, transfer 64 packet.
after 7th RTT WS = 128, transfer 128 packet.
after 8th RTT WS = 256, transfer 256 packet.
after 9th RTT WS will become= 512, so in 10th RTT we transfer 512 packet.
after the 10th RTT total packet send=1 + 2 + 4 + 8 + 16 + 32 + 64 + 128 + 256 + 512
= 1023
but we have to send 2000 packet.
no of packet still remaining=2000-1023=977
so after 10th RTT WS has become = 1024, so remaining 977 packet will be send in 11th RTT.
so RTT require = 11