TCP/IP Sockets in C, 2nd Edition

Chapter 4 Using UDP Sockets

The User Datagram Protocol (UDP) provides a simpler end-to-end service than TCP provides. In fact, UDP performs only two functions: (1) It adds another layer of addressing (ports) to that of IP; and (2) it detects data corruption that may occur in transit and discards any corrupted datagrams. Because of this simplicity, UDP (datagram) sockets have some different characteristics from the TCP (stream) sockets we saw earlier.

For example, UDP sockets do not have to be connected before being used. Where TCP is analogous to telephone communication, UDP is analogous to communicating by mail: You do not have to “connect” before you send a package or letter, but you do have to specify the destination address for each one. ...