UDP provides multiplexing (the method for multiple signals to be transmitted concurrently into an input stream, across a single physical channe l) and demultiplexing (the actual separation of the streams that have been multiplexed into a common stream back into multiple output streams) between protocol and application software.
Multiplexing and demultiplexing, as they pertain to UDP, transpire through ports. Each station application must negotiate a port number before sending a UDP datagram. When UDP is on the receiving side of a datagram, it checks the header (destination port field) to determine whether it matches one of station’s ports currently in use. If the port is in use by a listening application, the transmission proceeds; if the port is not in use, an ICMP error message is generated, and the datagram is discarded.
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