CC_VEGAS(4)	       FreeBSD Kernel Interfaces Manual 	   CC_VEGAS(4)


     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 con-
     gestion 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.


     The time from the transmission of a marked packet until the receipt of an
     acknowledgement for that packet is measured once per RTT.	This implemen-
     tation 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 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(4), cc_chd(4), cc_cubic(4), cc_hd(4), cc_htcp(4), cc_newreno(4),
     h_ertt(4), tcp(4), cc(9), khelp(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 Cen-
     tre for Advanced Internet Architectures, Melbourne, Australia.  More
     details are available at:


     The cc_vegas congestion control module and this manual page were written
     by David Hayes <>.

FreeBSD 9.0		       February 15, 2011		   FreeBSD 9.0

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