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

Geoff Huston gih at telstra.net
Sun Jun 3 14:21:31 UTC 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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