Monitor network traffic per user inside a Synology NAS

Hi, we have a Synology DS1512+ NAS server ([login to view URL]) that includes a software called DSM ([login to view URL]). It's a linux that can manage users with a lot of features like remote desktop, multimedia resources, VPN, Cloud files, Active Directory integration, VLAN, iSCSI, Mail server, DNS Server, etc. We just need someone to install any tool to monitor the traffic per user, so we can know what traffic used every user per day/month, etc. Maybe a cacti, vnstat or a netflow software. Take a look at this: [login to view URL] We want something simply but effective, iPKG is the main tool so install software. We can provide more info and also an ccount to a smaller synology box or demo account (DSM software is the same for all boxes). Please, only bid people withy a lot of experience in networking and system administration. Also provide the solution that you would provide so we can discuss the details. Thanks :)