A TCP entity opens a connection and uses slow start. Approximately how many round-trip times are required before TCP can send N segments

Round trip times required =  log2N

Explanation:

The round-trip times required before TCP can send N segments using a slow start is log2N. we can arrive at this by looking at the mode of operation of TCP which is  at the 1st time of using a TCP it starts the congestion window as 1 then it  sends an initial segment. When the acknowledgement of the initial segment arrives, TCP increases the congestion window to 2 and then sends 2 segments, When the 2 acknowledgements of the segments sent out arrives, they each increase the congestion window by one, thereby increasing the congestion window to 4 . therefore it takes   log2N round trips before TCP can send N segments