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North American Network Operators Group

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

  • From: Alex Rubenstein
  • Date: Sun Jun 03 13:40:28 2001



> 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.

FYI:

2^64 is 1.844 * 10^19

at 2.5 gb/s (OC48 line speed) (2,500,000,000 bits/sec), you transfer
312,500,000 bytes/sec, or 298 megabytes/sec.

2^64 / 312,500,000 = 6.189 * 10^16 seconds per rollover.

or, 1.719 * 10^13 hours
or, 1,962,741,057 years.

My point: at least for the near future, 64 bit counters won't roll.


> (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. )

Of course; but, if using 64 bit counters, they should be damn near close.







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