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>.

Debian                                         September 15, 2011                                      CC_CHD(4)