Protocol DATA unit
a Protocol DATA unit is in layer-based network concepts like the OSI model an entity, which is received with the transmitter by the upper layer, passed on with control information provided and to the lower layer. The control information is added in form of a header or a Trailers. Each PDU is packed on the underlying layers by adding new control information into a new PDU.
With the receiver the PDUs is from bottom to top through-handed and the respective control information are gradually again removed.