Skip site navigation (1)Skip section navigation (2)

FreeBSD Man Pages

Man Page or Keyword Search:
Man Architecture
Apropos Keyword Search (all sections) Output format
home | help
CC_HTCP(4)             FreeBSD Kernel Interfaces Manual             CC_HTCP(4)

     cc_htcp - 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 SEE ALSO section

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

     adaptive_backoff      Controls use of the adaptive backoff algorithm,
                           which is designed to keep network queues non-empty
                           during congestion recovery episodes.  Default is 0

     rtt_scaling           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).

     cc_chd(4), cc_cubic(4), cc_hd(4), cc_newreno(4), cc_vegas(4), mod_cc(4),
     tcp(4), mod_cc(9)

     D. Leith and R. Shorten, H-TCP: TCP Congestion Control for High
     Bandwidth-Delay Product Paths,

     D. Leith, R. Shorten, and T. Yee, "H-TCP: A framework for congestion
     control in high-speed and long-distance networks", Proc. PFLDnet, 2005.

     G. Armitage, L. Stewart, M. Welzl, and J. Healy, "An independent H-TCP
     implementation under FreeBSD 7.0: description and observed behaviour",
     SIGCOMM Comput. Commun. Rev., 3, 38, 27-38, July 2008.

     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 cc_htcp congestion control module first appeared in FreeBSD 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:

     The cc_htcp congestion control module was written by James Healy
     <> and Lawrence Stewart <>.

     This manual page was written by Lawrence Stewart <>
     and David Hayes <>.

FreeBSD 11.0-PRERELEASE       September 15, 2011       FreeBSD 11.0-PRERELEASE


Want to link to this manual page? Use this URL:

home | help