GSP
Quick Navigator

Search Site

Unix VPS
A - Starter
B - Basic
C - Preferred
D - Commercial
MPS - Dedicated
Previous VPSs
* Sign Up! *

Support
Contact Us
Online Help
Handbooks
Domain Status
Man Pages

FAQ
Virtual Servers
Pricing
Billing
Technical

Network
Facilities
Connectivity
Topology Map

Miscellaneous
Server Agreement
Year 2038
Credits
 

USA Flag

 

 

Man Pages


Manual Reference Pages  -  CC_VEGAS (4)

NAME

cc_vegas - Vegas Congestion Control Algorithm

CONTENTS

Description
Implementation Notes
MIB Variables
See Also
Acknowledgements
History
Authors

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)
.Rs TCP Vegas: end to end congestion avoidance on a global internet
.Re

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
.Fx 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


.An -nosplit The cc_vegas congestion control module and this manual page were written by
.An David Hayes Aq david.hayes@ieee.org .
Search for    or go to Top of page |  Section 4 |  Main Index


Powered by GSP Visit the GSP FreeBSD Man Page Interface.
Output converted with manServer 1.07.