cc_cubic(4) CUBIC Congestion Control Algorithm


The CUBIC congestion control algorithm was designed to provide increased throughput in fast and long-distance networks. It attempts to maintain fairness when competing with legacy NewReno TCP in lower speed scenarios where NewReno is able to operate adequately.

The congestion window is increased as a function of the time elapsed since the last congestion event. During regular operation, the window increase function follows a cubic function, with the inflection point set to be the congestion window value reached at the last congestion event. CUBIC also calculates an estimate of the congestion window that NewReno would have achieved at a given time after a congestion event. When updating the congestion window, the algorithm will choose the larger of the calculated CUBIC and estimated NewReno windows.

CUBIC also backs off less on congestion by changing the multiplicative decrease factor from 1/2 (used by standard NewReno TCP) to 4/5.

The implementation was done in a clean-room fashion, and is based on the Internet Draft and paper referenced in the Sx SEE ALSO section below.

MIB Variables

There are currently no tunable MIB variables.


Development and testing of this software were made possible in part by grants from the FreeBSD Foundation and Cisco University Research Program Fund at Community Foundation Silicon Valley.


The congestion control module first appeared in Fx 9.0 .

The module was first released in 2009 by Lawrence Stewart whilst studying at Swinburne University of Technology's Centre for Advanced Internet Architectures, Melbourne, Australia. More details are available at:


An -nosplit The congestion control module and this manual page were written by An Lawrence Stewart Aq [email protected] and An David Hayes Aq [email protected] .