# [mrtg] Re: 95th percentile?

George D. Nincehelser george at aaih.net
Fri Sep 24 19:59:31 MEST 1999

```The way it has been explained to me is:

1) Record the rate every 10 minutes around the clock for the billing period
2) Sort the recorded rates where highest is at the top, lowest at the bottom
3) Knock off the top 5 percent of the list
4) The resulting top reading is the rate upon which you are billed

So, if you had an abormally high transfer for a short period, it will most
likely be lopped off the top of the list and won't matter.

In a contract I'm not working on, we pay a base rate, if our 95th percentile
calculation exceeds this rate, then we pay more for that billing period
based on their pricing scale (their billing scale is in  0.5 Mbps
increments).

If anyone has any corrections to this, please let me know...I'm in the
process of negotiating this contract and I'm not yet sure I fully understand
how the formula works and its implications.  (I've yet to have anyone give
me the measurement process in writing, which really bugs me)

One thing I'm wondering about is their sampling rate.  How would the 95th
percentile change with different sampling rates and different traffic
patterns?  To take things to the extreme, suppose I sychronized myself to
their sampling schedule, loaded up the connection when they "weren't
looking", then choked back my transfer rate below my base rate during they
times they would be sampling.

Devious, I know.  I'd never actually *D0* anything like this, of
course...I'm just curious if it would work and if anyone as tried it. ;)

George

> Thanks for the excellent example. So I went back and took a look at
> above.net which supposedly had the 95th Percentile patched and found it
> actually listed on the Monthly graph, along with a Red Line across at
> this value. http://stats.sjc.above.net/traffic/
>
> So what I am to understand is that they take this number and charge a
> higher rate for all traffic above and beyond this value? So if I decide
> to copy the entire Windows NT i386 directory over the wire to another
> server at night, I am going to get charged an additional amount for this
> half-hour burst of abnormal traffic? Since it wasn't critical it would
> probably be cheaper to send the CD and have the local admin insert it
> into the CD-ROM drive instead?
>
> I don't really understand this, all that well. I've never had to
> provision major connections beyond a T-1 to an ISP that was X amount a
> month. No funny stuff in the contract that I saw.
>
> David C Prall, MCNE MCSE          DCP Technologies
> dcp at dcptech.com                       http://www.dcptech.com
> dcppage at dcptech.com
>
>
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