Protocol Data Unit (PDU)

A protocol data unit is the block of information a protocol at one layer exchanges with the matching layer on another system. The OSI basic reference model (ISO/IEC 7498-1) describes it as the control information the current layer adds plus user data, typically the service data unit handed down from the layer above. Going down the stack, each layer wraps the unit it receives in its own header, and at the data link layer a trailer too; the receiver reverses the process. This wrapping is encapsulation.

Each layer’s PDU has a customary name, and questions tend to rely on them. At the transport layer the unit is a segment for TCP and a datagram for UDP; at the network layer it is a packet; at the data link layer it is a frame, addressed by MAC address; and at the physical layer it is bits. Above the transport layer it is usually just called data. Knowing the name places a device or an attack at the right layer of the OSI model: a switch forwards frames and a router forwards packets.

Exam relevance: a scenario may name a unit of data and ask which layer or device handles it. Candidates are expected to match frame, packet, segment and bits to the correct layers of the OSI and TCP/IP models.