1.\" 2.\" Copyright (c) 2010-2011 The FreeBSD Foundation 3.\" All rights reserved. 4.\" 5.\" This documentation was written at the Centre for Advanced Internet 6.\" Architectures, Swinburne University, Melbourne, Australia by David Hayes 7.\" under sponsorship from the FreeBSD Foundation. 8.\" 9.\" Redistribution and use in source and binary forms, with or without 10.\" modification, are permitted provided that the following conditions 11.\" are met: 12.\" 1. Redistributions of source code must retain the above copyright 13.\" notice, this list of conditions and the following disclaimer. 14.\" 2. Redistributions in binary form must reproduce the above copyright 15.\" notice, this list of conditions and the following disclaimer in the 16.\" documentation and/or other materials provided with the distribution. 17.\" 18.\" THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ``AS IS'' AND 19.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE 20.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE 21.\" ARE DISCLAIMED. IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE FOR 22.\" ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL 23.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS 24.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) 25.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT 26.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY 27.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF 28.\" SUCH DAMAGE. 29.\" 30.\" $FreeBSD$ 31.\" 32.Dd February 15, 2011 33.Dt CC_VEGAS 4 34.Os 35.Sh NAME 36.Nm cc_vegas 37.Nd Vegas Congestion Control Algorithm 38.Sh DESCRIPTION 39The Vegas congestion control algorithm uses what the authors term the actual and 40expected transmission rates to determine whether there is congestion along the 41network path i.e. 42.Pp 43.Bl -item -offset indent 44.It 45actual rate = (total data sent in a RTT) / RTT 46.It 47expected rate = cwnd / RTTmin 48.It 49diff = expected - actual 50.El 51.Pp 52where RTT is the measured instantaneous round trip time and RTTmin is the 53smallest round trip time observed during the connection. 54.Pp 55The algorithm aims to keep diff between two parameters alpha and beta, such 56that: 57.Pp 58.Bl -item -offset indent 59.It 60alpha < diff < beta 61.El 62.Pp 63If diff > beta, congestion is inferred and cwnd is decremented by one packet (or 64the maximum TCP segment size). 65If diff < alpha, then cwnd is incremented by one packet. 66Alpha and beta govern the amount of buffering along the path. 67.Pp 68The implementation was done in a clean-room fashion, and is based on the 69paper referenced in the 70.Sx SEE ALSO 71section below. 72.Sh IMPLEMENTATION NOTES 73The time from the transmission of a marked packet until the receipt of an 74acknowledgement for that packet is measured once per RTT. 75This implementation does not implement Brakmo's and Peterson's original 76duplicate ACK policy since clock ticks in today's machines are not as coarse as 77they were (i.e. 500ms) when Vegas was originally designed. 78Note that modern TCP recovery processes such as fast retransmit and SACK are 79enabled by default in the TCP stack. 80.Sh MIB Variables 81The algorithm exposes the following tunable variables in the 82.Va net.inet.tcp.cc.vegas 83branch of the 84.Xr sysctl 3 85MIB: 86.Bl -tag -width ".Va alpha" 87.It Va alpha 88Query or set the Vegas alpha parameter as a number of buffers on the path. 89When setting alpha, the value must satisfy: 0 < alpha < beta. 90Default is 1. 91.It Va beta 92Query or set the Vegas beta parameter as a number of buffers on the path. 93When setting beta, the value must satisfy: 0 < alpha < beta. 94Default is 3. 95.El 96.Sh SEE ALSO 97.Xr cc 4 , 98.Xr cc_chd 4 , 99.Xr cc_cubic 4 , 100.Xr cc_hd 4 , 101.Xr cc_htcp 4 , 102.Xr cc_newreno 4 , 103.Xr h_ertt 4 , 104.Xr tcp 4 , 105.Xr cc 9 , 106.Xr khelp 9 107.Rs 108.%A "L. S. Brakmo" 109.%A "L. L. Peterson" 110.%T "TCP Vegas: end to end congestion avoidance on a global internet" 111.%J "IEEE J. Sel. Areas Commun." 112.%D "October 1995" 113.%V "13" 114.%N "8" 115.%P "1465-1480" 116.Re 117.Sh ACKNOWLEDGEMENTS 118Development and testing of this software were made possible in part by grants 119from the FreeBSD Foundation and Cisco University Research Program Fund at 120Community Foundation Silicon Valley. 121.Sh HISTORY 122The 123.Nm 124congestion control module first appeared in 125.Fx 9.0 . 126.Pp 127The module was first released in 2010 by David Hayes whilst working on the 128NewTCP research project at Swinburne University's Centre for Advanced Internet 129Architectures, Melbourne, Australia. 130More details are available at: 131.Pp 132http://caia.swin.edu.au/urp/newtcp/ 133.Sh AUTHORS 134.An -nosplit 135The 136.Nm 137congestion control module and this manual page were written by 138.An David Hayes Aq david.hayes@ieee.org . 139