NAME

       cc_vegas — Vegas Congestion Control Algorithm

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.

             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:

             alpha < diff < beta

       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.

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.

MIB Variables

       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.

SEE ALSO

       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.

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.

HISTORY

       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/

AUTHORS

       The  cc_vegas  congestion  control  module  and  this  manual  page   were   written   by   David   Hayes
       <david.hayes@ieee.org>.

Debian                                         September 15, 2011                                    CC_VEGAS(4)