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:

> On Sat, Jun 02, 2001 at 05:28:52PM -0400, Timothy Brown wrote:
> > As an interesting aside to this discussion, Digital Island bills for 
> > total traffic transmitted per month (in GB increments).   Does anyone 
> > using them have any comments on this approach besides the obvious?  Does 
> > anyone else do a similar deal?
>
> This may be obvious, but billing by volume (bytes transferred) places far
> greater availability requirements on the measurement system than rate-based
> charging schemes.
>
> If I am charging by the byte, I have to count every packet. If my measurement
> system breaks, I lose money until it is fixed.
>
> If I am charging by the 95%tile of five-minute average throughput
> measurements obtained during a calendar month, I can make do with much
> more coarse-grained sampling. Measurement system breaks, I'm quite
> possibly going to bill the same amount as if it hadn't broken.

No, you are confused. A rate based billing system polls a byte counter on
a switch or router at set intervals (ex: every 5 mins), subtracts the
previously recorded value, and divides by the number of seconds in that
interval. If the polling system cannot reach the device it is monitoring,
samples can be missed, this is a very old problem of rate-based
monitoring. Every rate-based system of which I am aware orders these
"samples" to calculate 95th percentile, so a missed sample is equivilent
to a 0 sample. A rate can be interpolated for the missing time, but it is
pretty much guaranteed not to be accurate, and I'd suspect a case could be
made against a provider who "makes up numbers" because of a failure in
their billing system.

A volume based billing system on the other hand, could theoretically poll
only once a billing period. In reality it would probably poll more often,
both to keep the customer apprised of their currently used amount, and to
prevent the possibility of counter rollovers, but it would never "miss" a
billing sample.

-- 
Richard A Steenbergen <ras () e-gerbil net>       http://www.e-gerbil.net/ras
PGP Key ID: 0x138EA177  (67 29 D7 BC E8 18 3E DA  B2 46 B3 D8 14 36 FE B6)