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36 .Nd HD Congestion Control Algorithm
38 The HD congestion control algorithm is an implementation of the Hamilton
39 Institute's delay-based congestion control which aims to keep network queuing
40 delays below a particular threshold (queue_threshold).
42 HD probabilistically reduces the congestion window (cwnd) based on its estimate
43 of the network queuing delay.
44 The probability of reducing cwnd is zero at hd_qmin or less, rising to a maximum
45 at queue_threshold, and then back to zero at the maximum queuing delay.
47 Loss-based congestion control algorithms such as NewReno probe for network
48 capacity by filling queues until there is a packet loss.
49 HD competes with loss-based congestion control algorithms by allowing its
50 probability of reducing cwnd to drop from a maximum at queue_threshold to be
51 zero at the maximum queuing delay.
52 This has been shown to work well when the bottleneck link is highly multiplexed.
54 The algorithm exposes the following tunable variables in the
55 .Va net.inet.tcp.cc.hd
59 .Bl -tag -width ".Va queue_threshold"
60 .It Va queue_threshold
61 Queueing congestion threshold (qth) in ticks.
64 Per packet maximum backoff probability as a percentage.
67 Minimum queuing delay threshold (qmin) in ticks.
88 .%T "A strategy for fair coexistence of loss and delay-based congestion control algorithms"
89 .%J "IEEE Commun. Lett."
96 Development and testing of this software were made possible in part by grants
97 from the FreeBSD Foundation and Cisco University Research Program Fund at
98 Community Foundation Silicon Valley.
100 The Hamilton Institute have recently made some improvements to the algorithm
101 implemented by this module and have called it Coexistent-TCP (C-TCP).
102 The improvements should be evaluated and potentially incorporated into this
107 congestion control module first appeared in
110 The module was first released in 2010 by David Hayes whilst working on the
111 NewTCP research project at Swinburne University of Technology's Centre for
112 Advanced Internet Architectures, Melbourne, Australia.
113 More details are available at:
115 http://caia.swin.edu.au/urp/newtcp/
120 congestion control module and this manual page were written by
121 .An David Hayes Aq Mt david.hayes@ieee.org .