Message-ID: <BF9651D8732ED311A61D00105A9CA3150446DA09@berkeley.gci.com>
From: Leif Sawyer <lsawyer@gci.com>
To: Ian Stirling <root@mauve.demon.co.uk>, linux-kernel@vger.kernel.org
Subject: RE: IP Acounting Idea for 2.5
Date: Mon, 16 Apr 2001 15:52:39 -0800
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> From: Ian Stirling [mailto:root@mauve.demon.co.uk]
> > Manfred Bartz responded to
> > > Russell King <rmk@arm.linux.org.uk> who writes:
> <snip>
> > > You just illustrated my point.  While there is a reset capability
> > > people will use it and accounting/logging programs will get wrong
> > > data.  Resetable counters might be a minor convenience 
> when debugging
> > > but the price is unreliable programs and the loss of the 
> ability of
> > > several programs to use the same counters.
> > 
> > You of course, are commenting from the fact that your 
> applications are
> > stupid, written poorly, and cannot handle 'wrapped' data.  Take MRTG
> <snip>
> > Similarly, if my InPackets are at 102345 at one read, and 
> 2345 the next
> > read,
> > and I know that my counter is 32 bits, then I know i've 
> wrapped and can do
> 
> I think the point being made is that if InPackets are at 
> 102345 at one read,
> and 2345 the next, and you know it's a 32 bit counter, it's completely
> unreliable to assume that you have in fact recieved 4294867295
> packets, if the counter can be zeroed.
> You can say nothing other than at least 2345 packets, at most 
> 2345+n*2^32 have been got since you last checked.

Ah, yes.. I seem to have misplaced a bit of text in my reply.

The continuation of thought:

How the application derives the status of a wrapped counter or
a zero'ed counter is dependant on the device being monitored.

Yes, you have to know what your interface is capable of (maxbytes/sec)
so that you can do a simple calculation where:

maximum_throughput = maxbytes_sec * (time_now - time_last_read)

and if your previous good counter + the maximum throughput wraps the
counter, you have a good chance that you've simply wrapped.

If not, then you can assume that your counters were cleared at some point,
log the data you've got, and keep moving forward.

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