NAME

       cc_hd — HD Congestion Control Algorithm

DESCRIPTION

       The  HD  congestion  control  algorithm  is  an  implementation  of  the Hamilton Institute's delay-based
       congestion  control  which  aims  to  keep  network  queuing  delays   below   a   particular   threshold
       (queue_threshold).

       HD  probabilistically  reduces  the congestion window (cwnd) based on its estimate of the network queuing
       delay.  The probability  of  reducing  cwnd  is  zero  at  hd_qmin  or  less,  rising  to  a  maximum  at
       queue_threshold, and then back to zero at the maximum queuing delay.

       Loss-based  congestion  control  algorithms  such as NewReno probe for network capacity by filling queues
       until there is a packet loss.  HD competes with loss-based congestion control algorithms by allowing  its
       probability  of reducing cwnd to drop from a maximum at queue_threshold to be zero at the maximum queuing
       delay.  This has been shown to work well when the bottleneck link is highly multiplexed.

MIB Variables

       The algorithm exposes the following tunable variables in the net.inet.tcp.cc.hd branch of  the  sysctl(3)
       MIB:

       queue_threshold  Queueing congestion threshold (qth) in ticks.  Default is 20.

       pmax             Per packet maximum backoff probability as a percentage.  Default is 5.

       qmin             Minimum queuing delay threshold (qmin) in ticks.  Default is 5.

SEE ALSO

       cc_chd(4),  cc_cubic(4),  cc_htcp(4), cc_newreno(4), cc_vegas(4), h_ertt(4), mod_cc(4), tcp(4), khelp(9),
       mod_cc(9)

       L. Budzisz, R. Stanojevic, R. Shorten, and F. Baker, “A strategy for fair coexistence of loss and  delay-
       based congestion control algorithms”, IEEE Commun. Lett., 7, 13, 555-557, Jul 2009.

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.

FUTURE WORK

       The  Hamilton  Institute have recently made some improvements to the algorithm implemented by this module
       and have called it  Coexistent-TCP  (C-TCP).   The  improvements  should  be  evaluated  and  potentially
       incorporated into this module.

HISTORY

       The cc_hd congestion control module first appeared in FreeBSD 9.0.

       The  module  was  first  released in 2010 by David Hayes whilst working on the NewTCP research project at
       Swinburne University of Technology's Centre for Advanced Internet  Architectures,  Melbourne,  Australia.
       More details are available at:

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

AUTHORS

       The   cc_hd   congestion   control   module   and   this   manual   page  were  written  by  David  Hayes
       <david.hayes@ieee.org>.

Debian                                         September 15, 2011                                       CC_HD(4)