Because UDP is connectionless, sessions are not established before communication takes place as they are with TCP. UDP is said to be transaction-based; that is, when an application has data to send, it simply sends the data.

Many applications that use UDP send small amounts of data that can fit in one segment. However, some applications send larger amounts of data that must be split into multiple segments. The UDP PDU is referred to as a datagram, although the terms segment and datagram are sometimes used interchangeably to describe a transport layer PDU.

When multiple datagrams are sent to a destination, they may take different paths and arrive in the wrong order. UDP does not track sequence numbers the way TCP does. UDP has no way to reorder the datagrams into their transmission order, as shown in the figure.

Therefore, UDP simply reassembles the data in the order that it was received and forwards it to the application. If the data sequence is important to the application, the application must identify the proper sequence and determine how the data should be processed.