[rrd-users] DERIVE vs COUNTER

Hi.

I want to measure the amount of traffic going through my ADSL link.

The data will be fetched using snmp every minute.

One issue is that the snmp server on my ADSL modem only supports 32
bits counter. So it is possible for the counter to reset due to an
overflow.
Another issue is that if there's a connection drop-out (and this can
happen several times a day on my DSL connection) the modem when it
re-establishes the connection reset the counter I am reading.

In the past I used mrtg for measuring the bandwidth. Unfortunately,
often after a reset of the counter that wasn't due to an overflow,
mrtg recorded the value as if an overflow did happen and it would mess
up my graphs and the scale used (the remaining of the traffic would
look like a flat line close to 0).

My DSL modem is syncing at 12mbit/s, so an overflow can not happen
within a 1 or even 5 minutes interval.

The RRD documentation describes the difference between the type
COUNTER and DERIVE.

Which type of data should I use when using rrdtool create?
is there a way for rrd to always determine if a reset is a due to a
reset of the modem or due to an overflow?

thanks in advance
Jean-Yves

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