2 .\" Copyright (c) 2010-2011 The FreeBSD Foundation
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.
8 .\" Redistribution and use in source and binary forms, with or without
9 .\" modification, are permitted provided that the following conditions
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.
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
34 .Nd Vegas Congestion Control Algorithm
36 The Vegas congestion control algorithm uses what the authors term the actual and
37 expected transmission rates to determine whether there is congestion along the
39 .Bl -item -offset indent
41 actual rate = (total data sent in a RTT) / RTT
43 expected rate = cwnd / RTTmin
45 diff = expected - actual
48 where RTT is the measured instantaneous round trip time and RTTmin is the
49 smallest round trip time observed during the connection.
51 The algorithm aims to keep diff between two parameters alpha and beta, such
53 .Bl -item -offset indent
58 If diff > beta, congestion is inferred and cwnd is decremented by one packet (or
59 the maximum TCP segment size).
60 If diff < alpha, then cwnd is incremented by one packet.
61 Alpha and beta govern the amount of buffering along the path.
63 The implementation was done in a clean-room fashion, and is based on the
64 paper referenced in the
67 .Sh IMPLEMENTATION NOTES
68 The time from the transmission of a marked packet until the receipt of an
69 acknowledgement for that packet is measured once per RTT.
70 This implementation does not implement Brakmo's and Peterson's original
71 duplicate ACK policy since clock ticks in today's machines are not as coarse as
72 they were (i.e. 500ms) when Vegas was originally designed.
73 Note that modern TCP recovery processes such as fast retransmit and SACK are
74 enabled by default in the TCP stack.
76 The algorithm exposes the following tunable variables in the
77 .Va net.inet.tcp.cc.vegas
81 .Bl -tag -width ".Va alpha"
83 Query or set the Vegas alpha parameter as a number of buffers on the path.
84 When setting alpha, the value must satisfy: 0 < alpha < beta.
87 Query or set the Vegas beta parameter as a number of buffers on the path.
88 When setting beta, the value must satisfy: 0 < alpha < beta.
107 .%T "TCP Vegas: end to end congestion avoidance on a global internet"
108 .%J "IEEE J. Sel. Areas Commun."
115 Development and testing of this software were made possible in part by grants
116 from the FreeBSD Foundation and Cisco University Research Program Fund at
117 Community Foundation Silicon Valley.
121 congestion control module first appeared in
124 The module was first released in 2010 by David Hayes whilst working on the
125 NewTCP research project at Swinburne University of Technology's Centre for
126 Advanced Internet Architectures, Melbourne, Australia.
127 More details are available at:
129 http://caia.swin.edu.au/urp/newtcp/
134 congestion control module and this manual page were written by
135 .An David Hayes Aq Mt david.hayes@ieee.org .