In computer networking, particularly protocol layered models like OSI and/or TCP/IP, a Service Data Unit (SDU) is an unencapsulated unit of data received from an upper level.
This unit of data is readable by the receiving layer as a raw segment but not yet ready for transmission through lower network layers.
As an example; the network layer receives an encapsulated SDU from the transport layer directly above. This SDU is actually a transport layer PDU—meaning that the transport layer has encapsulated that segment.
When the SDU reaches the network layer below, it hasn’t yet been encapsulated for transmission to the link-layer below. In a sense, this means it is still “raw” SDU data from the perspective of the current layer.