What is this udp traffic ???

Hi to all,
I have a RouterOS that allow my lan to connect to Internet with a simple NAT masquerading…
The RouterOS have 2 interfaces : in (lan) and out (wan)
In the “in” interface with a torch I have always a traffic like this:

protocol UDP, src=255.255.255.255 dst=0.0.0.0, tx-rate variable from about 200 Kbps to 1.5 Mbps and rx-rate always at 0.
What is this traffic? there is a particular service on routeros that do this traffic?
Can you help me?

I checked also “out” interface and there isn’t this traffic …

Looks like dhcp at work.
http://en.wikipedia.org/wiki/Dynamic_Host_Configuration_Protocol