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UDP messages are called user datagrams. These datagrams are encapsulated in IP, including the UDP header and data, as it travels across the Internet. Basically, UDP adds a header to the data that a user sends, and passes it along to IP. The IP layer then adds a header to what it receives from UDP. Finally, the network interface layer inserts the datagram in a frame before sending it from one machine to another. As just mentioned, UDP messages contain smaller headers and consume fewer overheads than TCP. The UDP datagram format is shown in Figure 1.18, and its components are defined in the following list.

Source/Destination Port. A 16-bit UDP port number used for datagram processing. Message Length. Specifies the number of octets in the UDP datagram. Checksum. An optional field to verify datagram delivery. Data. The data handed down to the TCP protocol, including upperlayer headers.