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/share/man/man4/cc_chd.4

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  2.\" Copyright (c) 2010-2011 The FreeBSD Foundation
  3.\" All rights reserved.
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  5.\" This documentation was written at the Centre for Advanced Internet
  6.\" Architectures, Swinburne University of Technology, Melbourne, Australia by
  7.\" David Hayes under sponsorship from the FreeBSD Foundation.
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 30.\" $FreeBSD$
 31.\"
 32.Dd September 15, 2011
 33.Dt CC_CHD 4
 34.Os
 35.Sh NAME
 36.Nm cc_chd
 37.Nd CHD Congestion Control Algorithm
 38.Sh DESCRIPTION
 39CHD enhances the HD algorithm implemented in
 40.Xr cc_hd 4 .
 41It provides tolerance to non-congestion related packet loss and improvements to
 42coexistence with traditional loss-based TCP flows, especially when the
 43bottleneck link is lightly multiplexed.
 44.Pp
 45Like HD, the algorithm aims to keep network queuing delays below a particular
 46threshold (queue_threshold) and decides to reduce the congestion window (cwnd)
 47probabilistically based on its estimate of the network queuing delay.
 48.Pp
 49It differs from HD in three key aspects:
 50.Bl -bullet
 51.It
 52The probability of cwnd reduction due to congestion is calculated once per round
 53trip time instead of each time an acknowledgement is received as done by
 54.Xr cc_hd 4 .
 55.It
 56Packet losses that occur while the queuing delay is less than queue_threshold
 57do not cause cwnd to be reduced.
 58.It
 59CHD uses a shadow window to help regain lost transmission opportunities when
 60competing with loss-based TCP flows.
 61.El
 62.Sh MIB Variables
 63The algorithm exposes the following tunable variables in the
 64.Va net.inet.tcp.cc.chd
 65branch of the
 66.Xr sysctl 3
 67MIB:
 68.Bl -tag -width ".Va queue_threshold"
 69.It Va queue_threshold
 70Queueing congestion threshold (qth) in ticks.
 71Default is 20.
 72.It Va pmax
 73Per RTT maximum backoff probability as a percentage.
 74Default is 50.
 75.It Va qmin
 76Minimum queuing delay threshold (qmin) in ticks.
 77Default is 5.
 78.It Va loss_fair
 79If 1, cwnd is adjusted using the shadow window when a congestion
 80related loss is detected.
 81Default is 1.
 82.It Va use_max
 83If 1, the maximum RTT seen within the measurement period is used as the basic
 84delay measurement for the algorithm, otherwise a sampled RTT measurement
 85is used.
 86Default is 1.
 87.El
 88.Sh SEE ALSO
 89.Xr cc_cubic 4 ,
 90.Xr cc_hd 4 ,
 91.Xr cc_htcp 4 ,
 92.Xr cc_newreno 4 ,
 93.Xr cc_vegas 4 ,
 94.Xr h_ertt 4 ,
 95.Xr mod_cc 4 ,
 96.Xr tcp 4 ,
 97.Xr khelp 9 ,
 98.Xr mod_cc 9
 99.Rs
100.%A "D. A. Hayes"
101.%A "G. Armitage"
102.%T "Improved coexistence and loss tolerance for delay based TCP congestion control"
103.%J "in 35th Annual IEEE Conference on Local Computer Networks"
104.%D "October 2010"
105.%P "24-31"
106.Re
107.Sh ACKNOWLEDGEMENTS
108Development and testing of this software were made possible in part by grants
109from the FreeBSD Foundation and Cisco University Research Program Fund at
110Community Foundation Silicon Valley.
111.Sh HISTORY
112The
113.Nm
114congestion control module first appeared in
115.Fx 9.0 .
116.Pp
117The module was first released in 2010 by David Hayes whilst working on the
118NewTCP research project at Swinburne University of Technology's Centre for
119Advanced Internet Architectures, Melbourne, Australia.
120More details are available at:
121.Pp
122http://caia.swin.edu.au/urp/newtcp/
123.Sh AUTHORS
124.An -nosplit
125The
126.Nm
127congestion control module and this manual page were written by
128.An David Hayes Aq david.hayes@ieee.org .