Service data unit
In Open Systems Interconnection (OSI) terminology, a service data unit (SDU) is a unit of data that has been passed down from an OSI layer to a lower layer and that has not yet been encapsulated into a protocol data unit (PDU) by the lower layer. It is a set of data that is sent by a user of the services of a given layer, and is transmitted semantically unchanged to a peer service user.
It differs from a PDU in that the PDU specifies the data that will be sent to the peer protocol layer at the receiving end, as opposed to being sent to a lower layer.
The SDU at any given layer, layer 'n', is the PDU of the layer above, layer 'n+1'. In effect the SDU is the 'payload' of a given PDU. That is, the process of changing a SDU to a PDU, consists of an encapsulation process, performed by the lower layer. All the data contained in the SDU becomes encapsulated within the PDU. The layer n-1 adds headers or footers, or both, to the SDU, transforming it into a PDU of layer n-1. The added headers or footers are part of the process used to make it possible to get data from a source to a destination.