UDP, defined in [RFC 768], does just about as little as a transport protocol can. Aside from the multiplexing/demultiplexing function and some light error checking, it adds nothing to IP. In fact, if the application developer chooses UDP instead of TCP, then the application is talking almost directly with IP. UDP takes messages from application process, attaches source and destination port number fields for the multiplexing/demultiplexing service, adds two other fields of minor importance, and passes the resulting "segment" to the network layer. The network layer encapsulates the segment into an IP datagram and then makes a best-effort attempt to deliver the segment to the receiving host. If the segment arrives at the receiving host, UDP uses the port numbers and the IP source and destination addresses to deliver the data in the segment to the correct application process. Note that with UDP there is no handshaking between sending and receiving transport-layer entities before sending a segment. For this reason, UDP is said to be connectionless.

The UDP header consists of only 4 header fields of which two are optional. The source and destination port fields are 16-bit fields that identify the sending and receiving process. Since UDP is stateless and a UDP sender may not solicit replies, the source port is optional. If not used, the source port should be set to zero. The port fields are followed by a mandatory length field specified as bytes of the UDP datagram including the data. The minimum value of the length field is 8 (octets). The remaining header field is a 16-bit checksum field covering the header and data. The checksum is also optional, but almost always used in practice.