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137 lines
4.6 KiB
Groff
137 lines
4.6 KiB
Groff
.\"
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.\" Copyright (c) 2010-2011 The FreeBSD Foundation
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.\" All rights reserved.
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.\"
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.\" This documentation was written at the Centre for Advanced Internet
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.\" Architectures, Swinburne University of Technology, Melbourne, Australia by
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.\" David Hayes under sponsorship from the FreeBSD Foundation.
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.\"
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.\" Redistribution and use in source and binary forms, with or without
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.\" modification, are permitted provided that the following conditions
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.\" are met:
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.\" 1. Redistributions of source code must retain the above copyright
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.\" notice, this list of conditions and the following disclaimer.
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.\" 2. Redistributions in binary form must reproduce the above copyright
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.\" notice, this list of conditions and the following disclaimer in the
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.\" documentation and/or other materials provided with the distribution.
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.\"
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.\" THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ``AS IS'' AND
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.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
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.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
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.\" ARE DISCLAIMED. IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE FOR
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.\" ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
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.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
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.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
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.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
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.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
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.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
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.\" SUCH DAMAGE.
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.\"
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.\" $FreeBSD$
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.\"
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.Dd September 15, 2011
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.Dt CC_VEGAS 4
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.Os
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.Sh NAME
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.Nm cc_vegas
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.Nd Vegas Congestion Control Algorithm
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.Sh DESCRIPTION
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The Vegas congestion control algorithm uses what the authors term the actual and
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expected transmission rates to determine whether there is congestion along the
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network path i.e.
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.Bl -item -offset indent
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.It
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actual rate = (total data sent in a RTT) / RTT
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.It
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expected rate = cwnd / RTTmin
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.It
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diff = expected - actual
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.El
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.Pp
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where RTT is the measured instantaneous round trip time and RTTmin is the
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smallest round trip time observed during the connection.
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.Pp
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The algorithm aims to keep diff between two parameters alpha and beta, such
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that:
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.Bl -item -offset indent
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.It
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alpha < diff < beta
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.El
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.Pp
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If diff > beta, congestion is inferred and cwnd is decremented by one packet (or
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the maximum TCP segment size).
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If diff < alpha, then cwnd is incremented by one packet.
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Alpha and beta govern the amount of buffering along the path.
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.Pp
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The implementation was done in a clean-room fashion, and is based on the
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paper referenced in the
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.Sx SEE ALSO
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section below.
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.Sh IMPLEMENTATION NOTES
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The time from the transmission of a marked packet until the receipt of an
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acknowledgement for that packet is measured once per RTT.
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This implementation does not implement Brakmo's and Peterson's original
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duplicate ACK policy since clock ticks in today's machines are not as coarse as
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they were (i.e. 500ms) when Vegas was originally designed.
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Note that modern TCP recovery processes such as fast retransmit and SACK are
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enabled by default in the TCP stack.
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.Sh MIB Variables
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The algorithm exposes the following tunable variables in the
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.Va net.inet.tcp.cc.vegas
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branch of the
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.Xr sysctl 3
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MIB:
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.Bl -tag -width ".Va alpha"
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.It Va alpha
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Query or set the Vegas alpha parameter as a number of buffers on the path.
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When setting alpha, the value must satisfy: 0 < alpha < beta.
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Default is 1.
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.It Va beta
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Query or set the Vegas beta parameter as a number of buffers on the path.
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When setting beta, the value must satisfy: 0 < alpha < beta.
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Default is 3.
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.El
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.Sh SEE ALSO
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.Xr cc_chd 4 ,
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.Xr cc_cubic 4 ,
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.Xr cc_hd 4 ,
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.Xr cc_htcp 4 ,
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.Xr cc_newreno 4 ,
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.Xr h_ertt 4 ,
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.Xr mod_cc 4 ,
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.Xr tcp 4 ,
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.Xr khelp 9 ,
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.Xr mod_cc 9
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.Rs
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.%A "L. S. Brakmo"
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.%A "L. L. Peterson"
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.%T "TCP Vegas: end to end congestion avoidance on a global internet"
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.%J "IEEE J. Sel. Areas Commun."
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.%D "October 1995"
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.%V "13"
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.%N "8"
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.%P "1465-1480"
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.Re
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.Sh ACKNOWLEDGEMENTS
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Development and testing of this software were made possible in part by grants
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from the FreeBSD Foundation and Cisco University Research Program Fund at
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Community Foundation Silicon Valley.
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.Sh HISTORY
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The
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.Nm
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congestion control module first appeared in
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.Fx 9.0 .
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.Pp
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The module was first released in 2010 by David Hayes whilst working on the
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NewTCP research project at Swinburne University of Technology's Centre for
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Advanced Internet Architectures, Melbourne, Australia.
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More details are available at:
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.Pp
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http://caia.swin.edu.au/urp/newtcp/
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.Sh AUTHORS
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.An -nosplit
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The
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.Nm
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congestion control module and this manual page were written by
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.An David Hayes Aq Mt david.hayes@ieee.org .
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