Re: 95th Percentile again (was RE: C&W Peering Problem?)

["Richard A. Steenbergen" <ras () e-gerbil net>]

On Sat, 2 Jun 2001, Joe Abley wrote:

> > No matter how you stack it, if you miss a rate sample there is no way to
> > go back and get the data again. You either discard it and lose the ability
> > to bill the customer for it (which demands high availability polling
> > systems), or you make up a number and hope the customer doesn't notice.
>
> No -- there is no need to do that. You don't need a sample for every
> single five-minute interval during the month to produce a meaningful
> 95%ile measurement for the month; you just need a representative
> sample population. You increase the chances of your sample population
> being representative if you consider lots of samples, but dropping one
> or two does not mean you lose revenue.

Actually you gain revenue if you drop samples below the 95th percentile
mark, since you are forcing the cutoff point higher by reducing the number
of samples.

I think your argument is in favor of 95th percentile vs an accurate
average, not rate vs amount samples. If for some reason you lose a sample
with an average system, your revenue goes down, whereas if you lose a
sample in 95th percentile you're more likely not to make it go down much.

But this is completely circumvented by polling the amount instead of
polling the rate. Measurements in amount are always better then
measurements by rate. If you have some horribly ghetto hack that makes you
count the packets yourself and you have the possibility of missing
samples, it may not be completely better then 95th percentile, but this is
a seperate issue.

> > Volume polling does not suffer from this problem.
>
> It does, if you don't have per-customer interface counters. You need
> to count every packet using some other method, and if you can't count
> packets, you can't bill for them.

I'd say the real problem is with the vendor. Fortunantly most people have
counters.

-- 
Richard A Steenbergen <ras () e-gerbil net>       http://www.e-gerbil.net/ras
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