cc_vegas —
Vegas
Congestion Control Algorithm
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.
- actual rate = (total data sent in a RTT) / RTT
- expected rate = cwnd / RTTmin
- diff = expected - actual
where RTT is the measured instantaneous round trip time and RTTmin is the
smallest round trip time observed during the connection.
The algorithm aims to keep diff between two parameters alpha and beta, such
that:
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.
The implementation was done in a clean-room fashion, and is based on the paper
referenced in the
SEE ALSO
section below.
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.
The algorithm exposes the following tunable variables in the
net.inet.tcp.cc.vegas branch of the
sysctl(3) MIB:
- 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.
- 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.
cc_chd(4),
cc_cubic(4),
cc_hd(4),
cc_htcp(4),
cc_newreno(4),
h_ertt(4),
mod_cc(4),
tcp(4),
khelp(9),
mod_cc(9)
L. S. Brakmo and
L. L. Peterson, TCP Vegas: end to
end congestion avoidance on a global internet, IEEE J.
Sel. Areas Commun., 8,
13, 1465-1480,
October 1995.
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.
The
cc_vegas congestion control module first
appeared in
FreeBSD 9.0.
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:
http://caia.swin.edu.au/urp/newtcp/
The
cc_vegas congestion control module and this
manual page were written by
David Hayes
<
[email protected]>.