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.Dd May 13, 2021 30.Dt CC_VEGAS 4 31.Os 32.Sh NAME 33.Nm cc_vegas 34.Nd Vegas Congestion Control Algorithm 35.Sh DESCRIPTION 36The Vegas congestion control algorithm uses what the authors term the actual and 37expected transmission rates to determine whether there is congestion along the 38network path i.e. 39.Bl -item -offset indent 40.It 41actual rate = (total data sent in a RTT) / RTT 42.It 43expected rate = cwnd / RTTmin 44.It 45diff = expected - actual 46.El 47.Pp 48where RTT is the measured instantaneous round trip time and RTTmin is the 49smallest round trip time observed during the connection. 50.Pp 51The algorithm aims to keep diff between two parameters alpha and beta, such 52that: 53.Bl -item -offset indent 54.It 55alpha < diff < beta 56.El 57.Pp 58If diff > beta, congestion is inferred and cwnd is decremented by one packet (or 59the maximum TCP segment size). 60If diff < alpha, then cwnd is incremented by one packet. 61Alpha and beta govern the amount of buffering along the path. 62.Pp 63The implementation was done in a clean-room fashion, and is based on the 64paper referenced in the 65.Sx SEE ALSO 66section below. 67.Sh IMPLEMENTATION NOTES 68The time from the transmission of a marked packet until the receipt of an 69acknowledgement for that packet is measured once per RTT. 70This implementation does not implement Brakmo's and Peterson's original 71duplicate ACK policy since clock ticks in today's machines are not as coarse as 72they were (i.e. 500ms) when Vegas was originally designed. 73Note that modern TCP recovery processes such as fast retransmit and SACK are 74enabled by default in the TCP stack. 75.Sh MIB Variables 76The algorithm exposes the following tunable variables in the 77.Va net.inet.tcp.cc.vegas 78branch of the 79.Xr sysctl 3 80MIB: 81.Bl -tag -width ".Va alpha" 82.It Va alpha 83Query or set the Vegas alpha parameter as a number of buffers on the path. 84When setting alpha, the value must satisfy: 0 < alpha < beta. 85Default is 1. 86.It Va beta 87Query or set the Vegas beta parameter as a number of buffers on the path. 88When setting beta, the value must satisfy: 0 < alpha < beta. 89Default is 3. 90.El 91.Sh SEE ALSO 92.Xr cc_cdg 4 , 93.Xr cc_chd 4 , 94.Xr cc_cubic 4 , 95.Xr cc_dctcp 4 , 96.Xr cc_hd 4 , 97.Xr cc_htcp 4 , 98.Xr cc_newreno 4 , 99.Xr h_ertt 4 , 100.Xr mod_cc 4 , 101.Xr tcp 4 , 102.Xr khelp 9 , 103.Xr mod_cc 9 104.Rs 105.%A "L. S. Brakmo" 106.%A "L. L. Peterson" 107.%T "TCP Vegas: end to end congestion avoidance on a global internet" 108.%J "IEEE J. Sel. Areas Commun." 109.%D "October 1995" 110.%V "13" 111.%N "8" 112.%P "1465-1480" 113.Re 114.Sh ACKNOWLEDGEMENTS 115Development and testing of this software were made possible in part by grants 116from the FreeBSD Foundation and Cisco University Research Program Fund at 117Community Foundation Silicon Valley. 118.Sh HISTORY 119The 120.Nm 121congestion control module first appeared in 122.Fx 9.0 . 123.Pp 124The module was first released in 2010 by David Hayes whilst working on the 125NewTCP research project at Swinburne University of Technology's Centre for 126Advanced Internet Architectures, Melbourne, Australia. 127More details are available at: 128.Pp 129http://caia.swin.edu.au/urp/newtcp/ 130.Sh AUTHORS 131.An -nosplit 132The 133.Nm 134congestion control module and this manual page were written by 135.An David Hayes Aq Mt david.hayes@ieee.org . 136