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37 .Nd Vegas Congestion Control Algorithm
39 The Vegas congestion control algorithm uses what the authors term the actual and
40 expected transmission rates to determine whether there is congestion along the
43 .Bl -item -offset indent
45 actual rate = (total data sent in a RTT) / RTT
47 expected rate = cwnd / RTTmin
49 diff = expected - actual
52 where RTT is the measured instantaneous round trip time and RTTmin is the
53 smallest round trip time observed during the connection.
55 The algorithm aims to keep diff between two parameters alpha and beta, such
58 .Bl -item -offset indent
63 If diff > beta, congestion is inferred and cwnd is decremented by one packet (or
64 the maximum TCP segment size).
65 If diff < alpha, then cwnd is incremented by one packet.
66 Alpha and beta govern the amount of buffering along the path.
68 The implementation was done in a clean-room fashion, and is based on the
69 paper referenced in the
72 .Sh IMPLEMENTATION NOTES
73 The time from the transmission of a marked packet until the receipt of an
74 acknowledgement for that packet is measured once per RTT.
75 This implementation does not implement Brakmo's and Peterson's original
76 duplicate ACK policy since clock ticks in today's machines are not as coarse as
77 they were (i.e. 500ms) when Vegas was originally designed.
78 Note that modern TCP recovery processes such as fast retransmit and SACK are
79 enabled by default in the TCP stack.
81 The algorithm exposes the following tunable variables in the
82 .Va net.inet.tcp.cc.vegas
86 .Bl -tag -width ".Va alpha"
88 Query or set the Vegas alpha parameter as a number of buffers on the path.
89 When setting alpha, the value must satisfy: 0 < alpha < beta.
92 Query or set the Vegas beta parameter as a number of buffers on the path.
93 When setting beta, the value must satisfy: 0 < alpha < beta.
110 .%T "TCP Vegas: end to end congestion avoidance on a global internet"
111 .%J "IEEE J. Sel. Areas Commun."
118 Development and testing of this software were made possible in part by grants
119 from the FreeBSD Foundation and Cisco University Research Program Fund at
120 Community Foundation Silicon Valley.
124 congestion control module first appeared in
127 The module was first released in 2010 by David Hayes whilst working on the
128 NewTCP research project at Swinburne University's Centre for Advanced Internet
129 Architectures, Melbourne, Australia.
130 More details are available at:
132 http://caia.swin.edu.au/urp/newtcp/
137 congestion control module and this manual page were written by
138 .An David Hayes Aq david.hayes@ieee.org .