I would like to monitor server bandwidth usage.
The server OS is Windows Server 2012
This should give me as result the amount of data over a certain period of time and separate it by protocol and by remote IP.
Protocol to discriminate would be HTTP, FTP and RDP at least.
How can I do this?
I would recommend Nagios. It has a very intuitive interface and still meets any Enterprise needs. You can run it on upto 7 nodes for free.
https://www.nagios.com/downloads/nagios-xi/
Personally i haven't used it before but Microsoft has a network monitor too. I do not know if the reporting functionality meets your needs: http://www.microsoft.com/en-us/download/details.aspx?id=4865
I've found this on Spiceworks when I've had a similar need: http://www.paessler.com/prtg
It's free up to 100 sensors which is more than sufficient for monitoring a single server. It takes a bit of reading but works well.
You can mess around with the Resource Monitor as well but it has a bigger performance impact.
If you're on a corporate network your network guys can probably just give you that data though.