UDP
The User Datagran Protocol (UDP) is a transport protocol
on top of the IP protocol.
It adds the ability to distinguish
among multiple detinations withing a host by using the perception
of protocol ports.
A protocol port is identified as an integer number. Datagrams that arrive
at a computer will be handed over to a particular application
(e.g. ftp, telnet) according to the protocol number in the header
of the UDP message. UDP messages (User Datagrams) contain both
the port number of the destination program and the port number
of the source program, making it possible to deliver the
datagram to the correct recipient and for the recipient to send
a reply.
UDP datagrams are encapsulated into an IP datagram.
Although UPD is an unreliable connectionless service as the
underlying IP,
it is a useful protocol for those applications, that are not very
sensitive to reliable delivery and want to exchange messages, that fit
into a single datagram. E.g. RIP (Routing Information Protocol)
makes use of UDP on port 520.
RIP does not need the overhead of a reliable service like TCP,
since the amount of
data to be broadcasted is limited, while reliable delivery is not very
important, since routing information will be sent regularly again.