1.\" 2.\" Copyright (c) 2013 Swinburne University of Technology, Melbourne, Australia 3.\" All rights reserved. 4.\" 5.\" Redistribution and use in source and binary forms, with or without 6.\" modification, are permitted provided that the following conditions 7.\" are met: 8.\" 1. Redistributions of source code must retain the above copyright 9.\" notice, this list of conditions and the following disclaimer. 10.\" 2. Redistributions in binary form must reproduce the above copyright 11.\" notice, this list of conditions and the following disclaimer in the 12.\" documentation and/or other materials provided with the distribution. 13.\" 14.\" THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ``AS IS'' AND 15.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE 16.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE 17.\" ARE DISCLAIMED. IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE FOR 18.\" ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL 19.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS 20.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) 21.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT 22.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY 23.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF 24.\" SUCH DAMAGE. 25.\" 26.Dd May 13, 2021 27.Dt CC_CDG 4 28.Os 29.Sh NAME 30.Nm cc_cdg 31.Nd CDG Congestion Control Algorithm 32.Sh DESCRIPTION 33CAIA-Delay Gradient (CDG) is a hybrid congestion control algorithm which reacts 34to both packet loss and inferred queuing delay. 35It attempts to operate as a delay-based algorithm where possible, but utilises 36heuristics to detect loss-based TCP cross traffic and will compete effectively 37as required. 38CDG is therefore incrementally deployable and suitable for use on shared 39networks. 40.Pp 41During delay-based operation, CDG uses a delay-gradient based probabilistic 42backoff mechanism, and will also try to infer non congestion related 43packet losses and avoid backing off when they occur. 44During loss-based operation, CDG essentially reverts to 45.Xr cc_newreno 4 Ns - Ns like 46behaviour. 47.Pp 48CDG switches to loss-based operation when it detects that a configurable number 49of consecutive delay-based backoffs have had no measurable effect. 50It periodically attempts to return to delay-based operation, but will keep 51switching back to loss-based operation as required. 52.Sh MIB Variables 53The algorithm exposes the following variables in the 54.Va net.inet.tcp.cc.cdg 55branch of the 56.Xr sysctl 3 57MIB: 58.Bl -tag -width ".Va exp_backoff_scale" 59.It Va version 60Current algorithm/implementation version number. 61.It Va beta_delay 62Delay-based window decrease factor as a percentage (on delay-based backoff, w = 63w * beta_delay / 100). 64Default is 70. 65.It Va beta_loss 66Loss-based window decrease factor as a percentage (on loss-based backoff, w = 67w * beta_loss / 100). 68Default is 50. 69.It Va exp_backoff_scale 70Scaling parameter for the probabilistic exponential backoff. 71Default is 2. 72.It Va smoothing_factor 73Number of samples used for moving average smoothing (0 means no smoothing). 74Default is 8. 75.It Va loss_compete_consec_cong 76Number of consecutive delay-gradient based congestion episodes which will 77trigger loss-based CC compatibility. 78Default is 5. 79.It Va loss_compete_hold_backoff 80Number of consecutive delay-gradient based congestion episodes to hold the 81window backoff for loss-based CC compatibility. 82Default is 5. 83.It Va alpha_inc 84If non-zero, this enables an experimental mode where CDG's window increase 85factor (alpha) is increased by 1 MSS every 86.Va alpha_inc 87RTTs during congestion avoidance mode. 88(Setting 89.Va alpha_inc 90to 1 results in the most aggressive growth of the window increase factor over 91time. 92Use higher 93.Va alpha_inc 94values for slower growth.) 95Default is 0. 96.El 97.Sh SEE ALSO 98.Xr cc_chd 4 , 99.Xr cc_cubic 4 , 100.Xr cc_dctcp 4 , 101.Xr cc_hd 4 , 102.Xr cc_htcp 4 , 103.Xr cc_newreno 4 , 104.Xr cc_vegas 4 , 105.Xr h_ertt 4 , 106.Xr mod_cc 4 , 107.Xr tcp 4 , 108.Xr khelp 9 , 109.Xr mod_cc 9 110.Rs 111.%A "D. A. Hayes" 112.%A "G. Armitage" 113.%T "Revisiting TCP Congestion Control using Delay Gradients" 114.%J "Networking 2011 Proceedings, Part II" 115.%D "May 2011" 116.%P "328-341" 117.Re 118.Rs 119.%A "N. Khademi" 120.%A "G. Armitage" 121.%T "Minimising RTT across homogeneous 802.11 WLANs with CAIA Delay-Gradient TCP (v0.1)" 122.%R "CAIA Technical Report 121113A" 123.%D "November 2012" 124.%U "http://caia.swin.edu.au/reports/121113A/CAIA-TR-121113A.pdf" 125.Re 126.Sh ACKNOWLEDGEMENTS 127Development and testing of this software were made possible in part by grants 128from the FreeBSD Foundation and The Cisco University Research Program Fund, a 129corporate advised fund of Silicon Valley Community Foundation. 130.Sh HISTORY 131The 132.Nm 133congestion control module first appeared in 134.Fx 9.2 . 135.Pp 136The module was first released in 2011 by David Hayes whilst working on the 137NewTCP research project at Swinburne University of Technology's Centre for 138Advanced Internet Architectures, Melbourne, Australia. 139More details are available at: 140.Pp 141http://caia.swin.edu.au/urp/newtcp/ 142.Sh AUTHORS 143.An -nosplit 144The 145.Nm 146congestion control module was written by 147.An David Hayes Aq Mt david.hayes@ieee.org . 148This manual page was written by 149.An Lawrence Stewart Aq Mt lstewart@FreeBSD.org 150and 151.An Grenville Armitage Aq Mt garmitage@swin.edu.au . 152.Sh BUGS 153The underlying algorithm and parameter values are still a work in progress and 154may not be optimal for some network scenarios. 155