In computer networking, the Protocol Data Unit (PDU) is a single unit of packaged data transferred between network end systems. In a layered network protocol stack, the PDU varies based on layer-specific constraints and convention.
For example, data packets are referred to as segments during TCP transmission on the transport layer, datagrams during UDP transmission, frames on the link layer, and again sometimes datagrams on the network layer irrespective of the upper-layer protocol.
A PDU represents data that has been received from an upper network layer and encapsulated with required lower layer header information. This data has been prepared for transmission to the next, lower, network layer. PDUs are comparable to service data units (SDUs) but SDUs have not yet been encapsulated for lower-layer transmission.