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CC_VEGAS(4)		     Kernel Interfaces Manual		     CC_VEGAS(4)

NAME
     cc_vegas -- Vegas Congestion Control Algorithm

DESCRIPTION
     The  Vegas  congestion control algorithm uses what the authors term the ac-
     tual and expected transmission rates to determine whether there is  conges-
     tion 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 implementa-
     tion does not implement Brakmo's and Peterson's original duplicate ACK pol-
     icy  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  re-
     covery 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 avoid-
     ance 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>.

FreeBSD 8.3		       September 15, 2011		     CC_VEGAS(4)
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