In telecommunications , a protocol data unit (PDU) is a single unit of information transmitted among peer entities of a computer network . A PDU is composed of protocol-specific control information and user data . In the layered architectures of communication protocol stacks, each layer implements protocols tailored to the specific type or mode of data exchange.
For example, the Transmission Control Protocol (TCP) implements a connection-oriented transfer mode, and the PDU of this protocol is called a segment, while the User Datagram Protocol (UDP) uses datagrams as protocol units for connectionless communication . A layer lower in the Internet protocol suite , at the Internet layer , the PDU is called a packet , irrespective of its payload type.