use a cubic function to replace the linear window growth to improve performance in high BDP network, also achieved RTT fairness.
tcp-cubic is succssor of tcp-bic, tcp-bic use binary search to find the avialble bandwidth when congestion happens, and MAY enter an lieaner increasing state (Additive Increase)if it dropped too low, if it passed the maximum window, it goes into max probing state and will do the reverse and grow slowly initially and switch to additive increase if it can't find new maximum window nearby.