cc_htcp(4) H-TCP Congestion Control Algorithm


The H-TCP 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. The window increase algorithm operates like NewReno for the first second after a congestion event, and then switches to a high-speed mode based on a quadratic increase function.

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

MIB Variables

The algorithm exposes the following tunable variables in the branch of the sysctl(3) MIB:

Controls use of the adaptive backoff algorithm, which is designed to keep network queues non-empty during congestion recovery episodes. Default is 0 (disabled).
Controls use of the RTT scaling algorithm, which is designed to make congestion window increase during congestion avoidance mode invariant with respect to RTT. Default is 0 (disabled).


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 2007 by James Healy and Lawrence Stewart whilst working on the NewTCP research project at Swinburne University of Technology's Centre for Advanced Internet Architectures, Melbourne, Australia, which was made possible in part by a grant from the Cisco University Research Program Fund at Community Foundation Silicon Valley. More details are available at:


An -nosplit The congestion control module was written by An James Healy Aq [email protected] and An Lawrence Stewart Aq [email protected] .

This manual page was written by An Lawrence Stewart Aq [email protected] and An David Hayes Aq [email protected] .