network monitoring system with downtime reporting

[Mark Jayson Alvarez]

Hi,
  I'm looking for a network monitoring system with
downtime reporting. Currently, we are using nagios for
services monitoring and mrtg for traffic monitoring.
Do you know any tool which can report the downtimes of
a particular customer's internet link? We need it
because from time to time, customers are asking for
rebates because they say that their link went down a
couple of times, and mrtg cannot show this
accurately..

Thanks. 
-jay

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