UDP is a connectionless and lightweight transport layer protocol that has a four-field header. UDP doesn't have any handshake or connection process, ordering or reliability services, and there's no teardown. As a lightweight protocol, it's ideal where speed is an issue, and is used with time-sensitive applications such as Dynamic Host Configuration Protocol (DHCP), Routing Information Protocol (RIP), Voice over IP (VoIP), or Trivial File Transfer Protocol (TFTP).

Whether TCP or UDP is in use, the transport layer is a critical component of ensuring data transport. During the encapsulation process, the data begins to transform, and the PDU is now a segment. At this point, the transport layer requires a port number (or address), that is associated with an application or process that is in use, as discussed in the following section.