[LARTC] determine internet connection duration

[Ming-Ching Tiew]

Anyone has idea of what would be the best way to track 
connection time some a particular user to the internet ?

Imagine a wifi network where the users will connect to the
system via DHCP ( there is no PPPOE session involved ).
If there is a need to track internet usage based on connection
time ( to the internet ), what would be the best way to track it ?

Appreciate any input or ideas.