[mrtg] Re: 95th percentile?

Martin Ansdell-Smith mas at ansdell.demon.co.uk
Sat Sep 25 13:31:00 MEST 1999

On Fri, 24 Sep 1999, Tim Kennedy wrote:

> there's a script around somewhere called "calculate_traffic.pl".
> It takes a begining date and an ending date, and tells you how much
> traffic passed in that time period, and you can do the math with your CIR
> or allowed rate, and see what percentile you are in for the month.

This highlights a point to consider when calculating percentiles: you need
to be using the same sampling period. If the provider is monitoring over a
ten-minute period then you need to do the same and synchronise the start
of each monitoring period. The calculation of the 95th percentile is
straightforward but you need to use the same sample interval and the same
number of samples to get the same answer. If you use the default 5-minute
load figures from mrtg and calculate the 95th percentile for the month
then what you have is the 95th percentile of the 5-minute loadings for the
month: this may be very different from the 95th percentile of 1-hour
loadings for the month or the 95th percentile of 1-second loadings for the

On a similar theme, CIR/EIR calculations are (usually?) done per second so
the 5-minute intervals shown (by default) in mrtg can only give you an
indication of what traffic might be in excess of the CIR if your traffic
profile is reasonably constant second by second over the whole 5-minute


Martin Ansdell-Smith
Network Analyst			http://www.ansdell.demon.co.uk/

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