NAME

     cc_htcp — H-TCP Congestion Control Algorithm

DESCRIPTION

     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 below.

MIB Variables

     The algorithm exposes the following tunable variables in the net.inet.tcp.cc.htcp 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 (disabled).

     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).

SEE ALSO

     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, http://tools.ietf.org/id/draft-leith-tcp-htcp-06.txt.

     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.

ACKNOWLEDGEMENTS

     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.

HISTORY

     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:

     http://caia.swin.edu.au/urp/newtcp/

AUTHORS

     The cc_htcp congestion control module was written by James Healy <jimmy@deefa.com> and
     Lawrence Stewart <lstewart@FreeBSD.org>.

     This manual page was written by Lawrence Stewart <lstewart@FreeBSD.org> and David Hayes
     <david.hayes@ieee.org>.