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

  • From: Geoff Huston
  • Date: Sun Jun 03 10:25:26 2001

At 6/3/01 11:58 AM, Joe Abley wrote:

This may be obvious, but billing by volume (bytes transferred) places far
greater availability requirements on the measurement system than rate-based
charging schemes.

No its not obvious. The SNMP byte counters are odometers - as long as you get two clean samples per counter wrap you can accurately count bytes. The trick is to ensure that you get a minimum of two clean samples of the odometer reading per counter wrap - for high speed interfaces that typically implies reading the MIB2 64 bit interface counters, or triggering an SNMP poll at relatively tight time intervals.

(My previous comments a month or so back about the inaccuracies inherant in 95% systems still apply - given a particular (extreme case) traffic load pattern it is possible for two measurement systems that are not phase locked, using precisely the same sampling technique and computation to deliver outcome values for the 95% point where one is up to twice the value of the other. )







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