how to collect information from users, which users using bandwidth the most.

I'm using Mikrotik RB2100 as a router. I need to collect report which user is using bandwidth the most, per 3 hours, per day, or per week. maybe like this as an example :

Rank 1 : User A : 2,313 MB

Rank 2 : user B : 1,677 MB

......

is there any free software or desktop built server to collect information like that?

thanks for reply.