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>.