TPDU

TPDU stands for Transport Protocol Data Unit, the unit of data exchanged by the Transport Layer in the OSI model. It is defined in ISO/IEC standards for the OSI transport protocols (notably the Transport Protocol Class 0/1/2/3/4) and in ITU-T recommendations X.225 and X.226. The TPDU is the fundamental message unit used by transport entities to establish, manage, and carry user data across a network. Depending on the TP class, a TPDU may be a control TPDU (for connection setup, maintenance, and teardown) or a data TPDU (which carries user data).

As a data structure, a TPDU consists of a header and, for many types, a payload. The header indicates the TPDU type and length and may include fields such as source and destination references, sequence numbers, and window information used for flow control. The content of the payload varies by type: data TPDUs carry application data; control TPDUs carry signaling information to establish, acknowledge, or terminate a transport connection. In TP4, the transport service provides reliability through sequencing, acknowledgments, and retransmission, implemented within the TPDU exchange.

Although OSI transport protocols influenced many concepts, TPDU-based communication is largely historical; modern networks primarily use TCP/UDP. Nevertheless, the TPDU concept remains a foundational part of OSI transport theory and is discussed in standards documents as part of the transport layer design.

See also: OSI model, Transport Protocol Class 4 (TP4), ISO/IEC 8073.