User Datagram Protocol: Is a connection-less protocol. Does not require a connection to be established. UDP is well suited for for protocols that rely on fast queries such as DNS, along with protocols that prioritize real-time communications such as video or audio services. This is because UDP "hopes" to establish a connection to the target port. This makes UDP scans slower and more difficult as UDP lacks acknowledgement (no response). When a packet is sent to an open UDP port there should be no response resulting in Nmap to label the port as "open|filtered".