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>.