how to keep track of data usage (mobile broadband)

[Michael]

Hi.

Currently I'm planning to go with mobile broadband on my laptop. I got 
my card working fine but the remaining problem is - there is a limit of 
5GB per month and I really don't want to go over it.

How can I keep track of data usage? It's a laptop so these counters 
needs to be reboot persistent. I'm running 8.1-R with ue0 interface.

Any hints are welcome!
Michael