CUBIC is an implementation of TCP with an optimized congestion control algorithm for high bandwidth networks with high latency (LFN: Long Fat Networks).

It is a less aggressive and more systematic derivative of BIC TCP, in which the window is a cubic function of time since the last congestion event, with the inflection point set to the window prior to the event. Being a cubic function, there are two components to window growth. The first is a concave portion where the window quickly ramps up to the window size before the last congestion event. Next is the convex growth where CUBIC probes for more bandwidth, slowly at first then very rapidly. CUBIC spends a lot of time at a plateau between the concave and convex growth region which allows the network to stabilize before CUBIC begins looking for more bandwidth.

Another major difference between CUBIC and standard TCP flavors is that it does not rely on the receipt of ACKs to increase the window size. CUBIC's window size is dependent only on the last congestion event. With standard TCP, flows with very short RTTs will receive ACKs faster and therefore have their congestion windows grow faster than other flows with longer RTTs. CUBIC allows for more fairness between flows since the window growth is independent of RTT.

CUBIC TCP is implemented and used by default in Linux kernels 2.6.19 and above.

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