As shown in figure 
, UDP interacts with application programs
at the same relative layer as TCP.
However, there is no error correction or retransmission of misordered or
lost packets.
UDP is therefore not used for connection-oriented services that need a
virtual circuit.
It is used for services that are query-response oriented, such as NFS,
where the number of messages with regard to the exchange is small compared
to TELNET or FTP sessions.
Services that use UDP include RPC-based services such as NIS and NFS,
NTP (Network Time Protocol), and DNS (DNS also uses TCP).
It is easier to spoof UDP packets than TCP packets, since there is no initial connection setup (handshake) involved (since there is no virtual circuit between the two systems) [Ches94]. Thus, there is a higher risk associated with UDP-based services.