Answer: SNMP
Step-by-step explanation:
SNMP (Simple Network Management Protocol) , which is a very used network monitoring protocol, uses normally UDP as a transport protocol.
It uses the port 161 for listening messages (events) from the network being monitored, as a "well known" port, like port 80 for HTTP and port 21 for FTP, and 162 for "traps" (interrupts when a abnormal condition is detected).