xref: /freebsd/share/man/man4/cc_vegas.4 (revision fa9896e082a1046ff4fbc75fcba4d18d1f2efc19)

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.Dd May 13, 2021
.Dt CC_VEGAS 4
.Os
.Sh NAME
.Nm cc_vegas
.Nd Vegas Congestion Control Algorithm
.Sh DESCRIPTION
The Vegas congestion control algorithm uses what the authors term the actual and
expected transmission rates to determine whether there is congestion along the
network path i.e.
.Bl -item -offset indent
.It
actual rate = (total data sent in a RTT) / RTT
.It
expected rate = cwnd / RTTmin
.It
diff = expected - actual
.El
.Pp
where RTT is the measured instantaneous round trip time and RTTmin is the
smallest round trip time observed during the connection.
.Pp
The algorithm aims to keep diff between two parameters alpha and beta, such
that:
.Bl -item -offset indent
.It
alpha < diff < beta
.El
.Pp
If diff > beta, congestion is inferred and cwnd is decremented by one packet (or
the maximum TCP segment size).
If diff < alpha, then cwnd is incremented by one packet.
Alpha and beta govern the amount of buffering along the path.
.Pp
The implementation was done in a clean-room fashion, and is based on the
paper referenced in the
.Sx SEE ALSO
section below.
.Sh IMPLEMENTATION NOTES
The time from the transmission of a marked packet until the receipt of an
acknowledgement for that packet is measured once per RTT.
This implementation does not implement Brakmo's and Peterson's original
duplicate ACK policy since clock ticks in today's machines are not as coarse as
they were (i.e. 500ms) when Vegas was originally designed.
Note that modern TCP recovery processes such as fast retransmit and SACK are
enabled by default in the TCP stack.
.Sh MIB Variables
The algorithm exposes the following tunable variables in the
.Va net.inet.tcp.cc.vegas
branch of the
.Xr sysctl 3
MIB:
.Bl -tag -width ".Va alpha"
.It Va alpha
Query or set the Vegas alpha parameter as a number of buffers on the path.
When setting alpha, the value must satisfy: 0 < alpha < beta.
Default is 1.
.It Va beta
Query or set the Vegas beta parameter as a number of buffers on the path.
When setting beta, the value must satisfy: 0 < alpha < beta.
Default is 3.
.El
.Sh SEE ALSO
.Xr cc_cdg 4 ,
.Xr cc_chd 4 ,
.Xr cc_cubic 4 ,
.Xr cc_dctcp 4 ,
.Xr cc_hd 4 ,
.Xr cc_htcp 4 ,
.Xr cc_newreno 4 ,
.Xr h_ertt 4 ,
.Xr mod_cc 4 ,
.Xr tcp 4 ,
.Xr khelp 9 ,
.Xr mod_cc 9
.Rs
.%A "L. S. Brakmo"
.%A "L. L. Peterson"
.%T "TCP Vegas: end to end congestion avoidance on a global internet"
.%J "IEEE J. Sel. Areas Commun."
.%D "October 1995"
.%V "13"
.%N "8"
.%P "1465-1480"
.Re
.Sh ACKNOWLEDGEMENTS
Development and testing of this software were made possible in part by grants
from the FreeBSD Foundation and Cisco University Research Program Fund at
Community Foundation Silicon Valley.
.Sh HISTORY
The
.Nm
congestion control module first appeared in
.Fx 9.0 .
.Pp
The module was first released in 2010 by David Hayes whilst working on the
NewTCP research project at Swinburne University of Technology's Centre for
Advanced Internet Architectures, Melbourne, Australia.
More details are available at:
.Pp
http://caia.swin.edu.au/urp/newtcp/
.Sh AUTHORS
.An -nosplit
The
.Nm
congestion control module and this manual page were written by
.An David Hayes Aq Mt david.hayes@ieee.org .