Protocol data units or PDUs are the messages passed between entities at a given layer. Layer 2 PDUs are called LPDUs or frames; Layer 3 PDUs are called NPDUs or packets; Layer 4 PDUs are called TPDUs or segments.
In general, a PDU-regardless of the protocol layer-consists of header12 and data fields. The header field contains the information necessary to get the PDU to the peer entity and typically includes the source and destination addresses appropriate for that layer as well as error sequence and flow control information. The data field contains the information carried by the Layer (N) protocol in support of Layer (N+1); it is formally referred to as the Layer (N) service data unit or SDU.
Conceptually, when a Layer (N+1) PDU is passed via primitive to Layer (N) as a Layer (N) SDU, a Layer (N) header is prepended to create a Layer (N) PDU. Sometimes part of the "header" is actually appended at the end, usually for error correction purposes. The Layer (N) PDU is then passed via primitive to Layer (N-1) as a Layer (N-1) SDU where a Layer (N-1) header is added. This process continues as data units are passed down the OSI reference model "stack." This is depicted in Figure 0.9.
Similarly, when a Layer (N-1) SDU is passed up to Layer (N), the Layer (N-1) header is removed from the Layer (N-1) PDU. Likewise, the Layer (N) PDU header is stripped to provide the Layer (N) SDU for Layer (N+1). This process continues as data units are passed up from layer to layer in the OSI reference model. Eventually, as shown in Figure 0.9, the original data element has been recreated at the application layer of the destination.