CC_HTCP(4)	       FreeBSD Kernel Interfaces Manual 	    CC_HTCP(4)

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 con-
		       gestion avoidance mode invariant with respect to RTT.
		       Default is 0 (disabled).

SEE ALSO

     cc(4), cc_chd(4), cc_cubic(4), cc_hd(4), cc_newreno(4), cc_vegas(4),
     tcp(4), 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, Aus-
     tralia, which was made possible in part by a grant from the Cisco Univer-
     sity 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>.

FreeBSD 9.0		       February 15, 2011		   FreeBSD 9.0