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32 .Dd September 15, 2011
37 .Nd CHD Congestion Control Algorithm
39 CHD enhances the HD algorithm implemented in
41 It provides tolerance to non-congestion related packet loss and improvements to
42 coexistence with traditional loss-based TCP flows, especially when the
43 bottleneck link is lightly multiplexed.
45 Like HD, the algorithm aims to keep network queuing delays below a particular
46 threshold (queue_threshold) and decides to reduce the congestion window (cwnd)
47 probabilistically based on its estimate of the network queuing delay.
49 It differs from HD in three key aspects:
52 The probability of cwnd reduction due to congestion is calculated once per round
53 trip time instead of each time an acknowledgement is received as done by
56 Packet losses that occur while the queuing delay is less than queue_threshold
57 do not cause cwnd to be reduced.
59 CHD uses a shadow window to help regain lost transmission opportunities when
60 competing with loss-based TCP flows.
63 The algorithm exposes the following tunable variables in the
64 .Va net.inet.tcp.cc.chd
68 .Bl -tag -width ".Va queue_threshold"
69 .It Va queue_threshold
70 Queueing congestion threshold (qth) in ticks.
73 Per RTT maximum backoff probability as a percentage.
76 Minimum queuing delay threshold (qmin) in ticks.
79 If 1, cwnd is adjusted using the shadow window when a congestion
80 related loss is detected.
83 If 1, the maximum RTT seen within the measurement period is used as the basic
84 delay measurement for the algorithm, otherwise a sampled RTT measurement
102 .%T "Improved coexistence and loss tolerance for delay based TCP congestion control"
103 .%J "in 35th Annual IEEE Conference on Local Computer Networks"
108 Development and testing of this software were made possible in part by grants
109 from the FreeBSD Foundation and Cisco University Research Program Fund at
110 Community Foundation Silicon Valley.
114 congestion control module first appeared in
117 The module was first released in 2010 by David Hayes whilst working on the
118 NewTCP research project at Swinburne University of Technology's Centre for
119 Advanced Internet Architectures, Melbourne, Australia.
120 More details are available at:
122 http://caia.swin.edu.au/urp/newtcp/
127 congestion control module and this manual page were written by
128 .An David Hayes Aq david.hayes@ieee.org .