1.\" 2.\" Copyright (c) 2010-2011 The FreeBSD Foundation 3.\" 4.\" This documentation was written at the Centre for Advanced Internet 5.\" Architectures, Swinburne University of Technology, Melbourne, Australia by 6.\" David Hayes under sponsorship from the FreeBSD Foundation. 7.\" 8.\" Redistribution and use in source and binary forms, with or without 9.\" modification, are permitted provided that the following conditions 10.\" are met: 11.\" 1. Redistributions of source code must retain the above copyright 12.\" notice, this list of conditions and the following disclaimer. 13.\" 2. Redistributions in binary form must reproduce the above copyright 14.\" notice, this list of conditions and the following disclaimer in the 15.\" documentation and/or other materials provided with the distribution. 16.\" 17.\" THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ``AS IS'' AND 18.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE 19.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE 20.\" ARE DISCLAIMED. IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE FOR 21.\" ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL 22.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS 23.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) 24.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT 25.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY 26.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF 27.\" SUCH DAMAGE. 28.\" 29.\" $FreeBSD$ 30.\" 31.Dd May 13, 2021 32.Dt CC_VEGAS 4 33.Os 34.Sh NAME 35.Nm cc_vegas 36.Nd Vegas Congestion Control Algorithm 37.Sh DESCRIPTION 38The Vegas congestion control algorithm uses what the authors term the actual and 39expected transmission rates to determine whether there is congestion along the 40network path i.e. 41.Bl -item -offset indent 42.It 43actual rate = (total data sent in a RTT) / RTT 44.It 45expected rate = cwnd / RTTmin 46.It 47diff = expected - actual 48.El 49.Pp 50where RTT is the measured instantaneous round trip time and RTTmin is the 51smallest round trip time observed during the connection. 52.Pp 53The algorithm aims to keep diff between two parameters alpha and beta, such 54that: 55.Bl -item -offset indent 56.It 57alpha < diff < beta 58.El 59.Pp 60If diff > beta, congestion is inferred and cwnd is decremented by one packet (or 61the maximum TCP segment size). 62If diff < alpha, then cwnd is incremented by one packet. 63Alpha and beta govern the amount of buffering along the path. 64.Pp 65The implementation was done in a clean-room fashion, and is based on the 66paper referenced in the 67.Sx SEE ALSO 68section below. 69.Sh IMPLEMENTATION NOTES 70The time from the transmission of a marked packet until the receipt of an 71acknowledgement for that packet is measured once per RTT. 72This implementation does not implement Brakmo's and Peterson's original 73duplicate ACK policy since clock ticks in today's machines are not as coarse as 74they were (i.e. 500ms) when Vegas was originally designed. 75Note that modern TCP recovery processes such as fast retransmit and SACK are 76enabled by default in the TCP stack. 77.Sh MIB Variables 78The algorithm exposes the following tunable variables in the 79.Va net.inet.tcp.cc.vegas 80branch of the 81.Xr sysctl 3 82MIB: 83.Bl -tag -width ".Va alpha" 84.It Va alpha 85Query or set the Vegas alpha parameter as a number of buffers on the path. 86When setting alpha, the value must satisfy: 0 < alpha < beta. 87Default is 1. 88.It Va beta 89Query or set the Vegas beta parameter as a number of buffers on the path. 90When setting beta, the value must satisfy: 0 < alpha < beta. 91Default is 3. 92.El 93.Sh SEE ALSO 94.Xr cc_cdg 4 , 95.Xr cc_chd 4 , 96.Xr cc_cubic 4 , 97.Xr cc_dctcp 4 , 98.Xr cc_hd 4 , 99.Xr cc_htcp 4 , 100.Xr cc_newreno 4 , 101.Xr h_ertt 4 , 102.Xr mod_cc 4 , 103.Xr tcp 4 , 104.Xr khelp 9 , 105.Xr mod_cc 9 106.Rs 107.%A "L. S. Brakmo" 108.%A "L. L. Peterson" 109.%T "TCP Vegas: end to end congestion avoidance on a global internet" 110.%J "IEEE J. Sel. Areas Commun." 111.%D "October 1995" 112.%V "13" 113.%N "8" 114.%P "1465-1480" 115.Re 116.Sh ACKNOWLEDGEMENTS 117Development and testing of this software were made possible in part by grants 118from the FreeBSD Foundation and Cisco University Research Program Fund at 119Community Foundation Silicon Valley. 120.Sh HISTORY 121The 122.Nm 123congestion control module first appeared in 124.Fx 9.0 . 125.Pp 126The module was first released in 2010 by David Hayes whilst working on the 127NewTCP research project at Swinburne University of Technology's Centre for 128Advanced Internet Architectures, Melbourne, Australia. 129More details are available at: 130.Pp 131http://caia.swin.edu.au/urp/newtcp/ 132.Sh AUTHORS 133.An -nosplit 134The 135.Nm 136congestion control module and this manual page were written by 137.An David Hayes Aq Mt david.hayes@ieee.org . 138