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