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

Sun Jun 3 03:30:24 UTC 2001

On Sat, Jun 02, 2001 at 11:17:48PM -0400, Richard A. Steenbergen wrote:
> There are only two ways you can poll the rate, either you poll a "rate"
> value maintained on the device, or you poll a difference in bytes divide
> by the length of time between samples to calculate a rate. Either way, if
> the device does not support polling of the "interface" in question you are
> pretty screwed.

Not necessarily; you just have to find other ways of measuring, as we did,
to good effect.

> 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.

> 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.

Joe