NOFC-VRTT: a nonlinear AQM algorithm based on variable RTT. An active queue management algorithm is presented for network congestion, which is based on variable round trip time (RTT). A novel TCP window size observer is first proposed, which can prove that the observer state converges to the real state asymptotically, when packet dropping or marking ratio falls between 0 and 1. A nonlinear output feedback controller is designed based on the backstepping technique, and the value range of the parameters is also discussed. NS simulation results show that the proposed NOFC-VRTT presents high robust stabilization, high link utilization and low packet loss ratio, which even provides high fairness for the TCP sources.