Obviously, writing code to deal with reassembling fragmented datagrams would make you think twice about how badly your application needs to communicate over a network. Fortunately, the Internet architects provided a protocol layer that sits on top of the Internet Protocol.
UDP uses the Internet Protocol to send datagrams from a source to a destination. When the datagram arrives at the destination, it hands the complete datagram packet to the client. If the datagram was fragmented along the way, it reassembles the fragments into a complete datagram beforehand.
Like the Internet Protocol it uses, UDP does not guarantee delivery of a datagram. Its purpose is simply to format a datagram with your data, send it via the Internet Protocol to a destination, and at the destination, deliver the complete datagram to a client.
One interesting aspect of the Internet Protocol is that datagrams can be delivered to a destination in a different sequence than the one in which they were sent. For example: your application sends two datagrams to another computer. The first datagram is routed along a longer path than the second datagram, and therefore arrives at the destination after the second datagram has arrived.
See Also
Transmission Control Protocol (TCP)
Simple Mail Transfer Protocol (SMTP)
An ECHO client and server using TCP