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Re: What does 95th %tile mean?

  • From: Greg A. Woods
  • Date: Thu Apr 19 20:22:18 2001

[ On Friday, April 20, 2001 at 08:03:02 (+1000), Geoff Huston wrote: ]
> Subject: Re: What does 95th %tile mean?
>
> The 95% reading always struck me as a randomly generated number in any case.

Huh?  It's a simple and mathematically sound and highly repeatable and
auditable way of drawing a line on the usage graph that says something
like:  If you were to have had a fixed-rate connection this is the
bandwidth that you would have required over the previous billing period
in order to have obtained effectively the same level of performance as
you actually enjoyed over that period.  The only trick (from the
customer P.O.V.) is in understanding that this is what you're buying and
in realising that if you use it then you will pay for it.  It probably
works best for links that have aggregated traffic (eg. for 1st and 2nd
tier providers).

> Depending on the synchronization between the burst pattern and the sampling 
> system, and the sampling technique itself, the 95% reading can be zero, 
> half the line rate, or the line rate, and all answers are equally valid in 
> some sense.

Perhaps you need to learn that the "bit rate" values used in deriving an
N'th percentile value are first calculated by counting the number of
octets that crossed an interface since the last sample was taken and
dividing by the amount of time since that last sample was taken (and
then adjusting with a multiplier for different units, eg. octets vs.
bits or whatever).  In other words the bit rate values are taken as the
average rate over the specified sample time.  No data is thrown away or
ignored -- every single byte is counted and every count is critical to
finding the correct N'th percentile value.

There's absolutely nothing in the way of synchronisation required and
indeed there's no such thing as a "burst pattern" when you consider that
at any given instant in time an octet will cross a (to pick a specific
example) 10-mbit interface at ten megabits per second!  How else can you
imagine measuring the bit rate utilisation of a fixed-rate pipe?

The same N'th percentile measurement can always be calculated from
either end of a pipe so long as the sample interval is the same at both
ends, and so long as the pipe has no (measurable) loss.  If there's
measurable loss then you'd better measure it and take it into account or
else you will end up with unfair billing.

In fact the very same octet-count measurements are needed for any kind
of usage-based billing.  The only difference with N'th percentile
metering is that the sample time needs to be short enough to catch
user-noticable bursts (i.e. to avoid averaging out bursts that were they
to be flattened out to the average rate would be noticable to the user).
For most currently used IP services this might be somewhere between 5
seconds and 60 seconds.  For straight bulk throughput billing you only
need to sample often enough to aoivd missing counter roll-over or
counter reset events.

-- 
							Greg A. Woods

+1 416 218-0098      VE3TCP      <[email protected]>     <[email protected]>
Planix, Inc. <[email protected]>;   Secrets of the Weird <[email protected]>