Service data unit
Service Data Unit (SDU) is a set of data that is sent by a user of the services of a given layer, and is transmitted to a peer service user semantically unchanged.
Basically the SDU is the data that a certain layer will pass to the layer below.
It differs from the PDU in that the PDU specifies the data that will be sent to the peer protocol layer at the receiving end.
The PDU at one layer, 'n', is the SDU of the layer below, 'n-1'. In effect the SDU is the 'payload' of a given PDU.
Federal Standard 1037C http://www.tcpipguide.com/