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31 .Nd CDG Congestion Control Algorithm
33 CAIA-Delay Gradient (CDG) is a hybrid congestion control algorithm which reacts
34 to both packet loss and inferred queuing delay.
35 It attempts to operate as a delay-based algorithm where possible, but utilises
36 heuristics to detect loss-based TCP cross traffic and will compete effectively
38 CDG is therefore incrementally deployable and suitable for use on shared
41 During delay-based operation, CDG uses a delay-gradient based probabilistic
42 backoff mechanism, and will also try to infer non congestion related
43 packet losses and avoid backing off when they occur.
44 During loss-based operation, CDG essentially reverts to
45 .Xr cc_newreno 4 Ns - Ns like
48 CDG switches to loss-based operation when it detects that a configurable number
49 of consecutive delay-based backoffs have had no measurable effect.
50 It periodically attempts to return to delay-based operation, but will keep
51 switching back to loss-based operation as required.
53 The algorithm exposes the following variables in the
54 .Va net.inet.tcp.cc.cdg
58 .Bl -tag -width ".Va exp_backoff_scale"
60 Current algorithm/implementation version number.
62 Delay-based window decrease factor as a percentage (on delay-based backoff, w =
63 w * beta_delay / 100).
66 Loss-based window decrease factor as a percentage (on loss-based backoff, w =
69 .It Va exp_backoff_scale
70 Scaling parameter for the probabilistic exponential backoff.
72 .It Va smoothing_factor
73 Number of samples used for moving average smoothing (0 means no smoothing).
75 .It Va loss_compete_consec_cong
76 Number of consecutive delay-gradient based congestion episodes which will
77 trigger loss-based CC compatibility.
79 .It Va loss_compete_hold_backoff
80 Number of consecutive delay-gradient based congestion episodes to hold the
81 window backoff for loss-based CC compatibility.
84 If non-zero, this enables an experimental mode where CDG's window increase
85 factor (alpha) is increased by 1 MSS every
87 RTTs during congestion avoidance mode.
90 to 1 results in the most aggressive growth of the window increase factor over
94 values for slower growth.)
113 .%T "Revisiting TCP Congestion Control using Delay Gradients"
114 .%J "Networking 2011 Proceedings, Part II"
121 .%T "Minimising RTT across homogeneous 802.11 WLANs with CAIA Delay-Gradient TCP (v0.1)"
122 .%R "CAIA Technical Report 121113A"
124 .%U "http://caia.swin.edu.au/reports/121113A/CAIA-TR-121113A.pdf"
127 Development and testing of this software were made possible in part by grants
128 from the FreeBSD Foundation and The Cisco University Research Program Fund, a
129 corporate advised fund of Silicon Valley Community Foundation.
133 congestion control module first appeared in
136 The module was first released in 2011 by David Hayes whilst working on the
137 NewTCP research project at Swinburne University of Technology's Centre for
138 Advanced Internet Architectures, Melbourne, Australia.
139 More details are available at:
141 http://caia.swin.edu.au/urp/newtcp/
146 congestion control module was written by
147 .An David Hayes Aq Mt david.hayes@ieee.org .
148 This manual page was written by
149 .An Lawrence Stewart Aq Mt lstewart@FreeBSD.org
151 .An Grenville Armitage Aq Mt garmitage@swin.edu.au .
153 The underlying algorithm and parameter values are still a work in progress and
154 may not be optimal for some network scenarios.