PDU and TPDU

In telecommunications, the term
protocol data unit (PDU) has the following
meanings:

Information that is delivered as a unit among
peer entities of a network and that may contain control
information, address information, or data.

In layered systems, a unit of data that is specified in a
protocol of a given layer and
that consists of protocol-control information of
the given layer and possibly user data of that layer. For
example: Bridge PDU or iSCSI
PDU

Source: from Federal Standard 1037C

PDUs are relevant in relation to one of the first 4 layers of the
OSI model as follows:

1. The Layer 1 PDU is the bit

2. The Layer 2 PDU is the frame

3. The Layer 3 PDU is the packet

4. The Layer 4 PDU is the segment (e.g. TCP segment)

An unformal acronym TPDU (Transport Protocol Data Unit) for messages
sent from transport entity to transport entity. Thus, TPDUs (exchanged by the transport layer) are
contained in packets (exchanged by the network layer). In turn,
packets are contained in frames (exchanged by the data link
layer).

Figure 6-3. Nesting of TPDUs, packets, and
frames.