North American Network Operators Group Date Prev | Date Next | Date Index | Thread Index | Author Index | Historical Re: 95th Percentile again (was RE: C&W Peering Problem?)
> 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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