[c-nsp] SNMP traffic monitoring / alerting

[Tony Tauber]

On Thu, Feb 14, 2008 at 07:13:43PM +0900, Aaron R wrote:
> Hi Howard,
> 
> You are absolutely correct. I am already graphing / totaling the 
> amount of traffic out the interface I just need a way to be 
> alerted when it exceeds a particular threshold and we have to 
> pay our ISP big bucks for going over our
> limit. Saves me from checking graphs each day!
> 
> Cheers,
> 
> Aaron.

It seems like you want the RATE, not the SNMP value since ISPs charge
based on the RATE (bits/sec) that you are transferring traffic across
their network.

Packages such as Cacti or MRTG are written to take this fact into
account (since this is what people generally care about and those
apps are written by people.)  Hence, they poll counters like
ifHCInOctets and ifHCOutOctets at a given interval (e.g., 5 min, 15 min)
and then take the difference between the two readings (e.g., 20 octets)
multiply by 8 bits/octet, then divide by the number of seconds in the
interval to produce a bits/second rate and then graph that rate 
against time of day.  I believe these packages generally have ways to
detect and account for counter-wrapping.

So what you want to track is threshold of the RATE bits are transiting
the interface, NOT the VALUE of the SNMP value itself.

Tony