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>.