[38313] in North American Network Operators' Group
Re: 95th Percentile again (was RE: C&W Peering Problem?)
daemon@ATHENA.MIT.EDU (Geoff Huston)
Sun Jun 3 10:23:10 2001
Message-Id: <4.3.2.7.2.20010604001355.00b64da0@localhost>
Date: Mon, 04 Jun 2001 00:21:31 +1000
To: Joe Abley <jabley@automagic.org>
From: Geoff Huston <gih@telstra.net>
Cc: nanog@merit.edu
In-Reply-To: <20010602215849.J21377@buddha.home.automagic.org>
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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. )
