Service data unit

Service data unit, also called service data unit (SDU), is the name used in the OSI model to describe a data unit once it is derived to the lower layer in the protocol stack and , once encapsulated by that layer, becomes the protocol data unit of the same.

Relationship with PDU

The difference with the PDU is that the PDU of a layer specifies the data set to be sent to the peer protocol located at the receiver, whereas the SDU is the set of data coming from the upper layer, not yet encapsulated . That is, the service data unit of a layer will correspond to the payload portion of the PDU of the same layer.

The N layer receives the SDU from the top layer, N + 1. Possibly the data received does not enter a minimum PDU of the N layer and for this it must perform a fragmentation. Then proceeds to add the header and possibly a terminator to each fragment. Each of the segments that are obtained is a PDU of the N layer, which will soon be an SDU of the N-1 layer.

wiki