NAME

     cc_chd — CHD Congestion Control Algorithm

DESCRIPTION

     CHD enhances the HD algorithm implemented in cc_hd(4).  It provides tolerance to non-
     congestion related packet loss and improvements to coexistence with traditional loss-based
     TCP flows, especially when the bottleneck link is lightly multiplexed.

     Like HD, the algorithm aims to keep network queuing delays below a particular threshold
     (queue_threshold) and decides to reduce the congestion window (cwnd) probabilistically based
     on its estimate of the network queuing delay.

     It differs from HD in three key aspects:

     •   The probability of cwnd reduction due to congestion is calculated once per round trip
         time instead of each time an acknowledgement is received as done by cc_hd(4).

     •   Packet losses that occur while the queuing delay is less than queue_threshold do not
         cause cwnd to be reduced.

     •   CHD uses a shadow window to help regain lost transmission opportunities when competing
         with loss-based TCP flows.

MIB Variables

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

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

     pmax             Per RTT maximum backoff probability as a percentage.  Default is 50.

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

     loss_fair        If 1, cwnd is adjusted using the shadow window when a congestion related
                      loss is detected.  Default is 1.

     use_max          If 1, the maximum RTT seen within the measurement period is used as the
                      basic delay measurement for the algorithm, otherwise a sampled RTT
                      measurement is used.  Default is 1.

SEE ALSO

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

     D. A. Hayes and G. Armitage, “Improved coexistence and loss tolerance for delay based TCP
     congestion control”, in 35th Annual IEEE Conference on Local Computer Networks, 24-31,
     October 2010.

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