SPDU

SPDU stands for Session Protocol Data Unit. It is a data unit used by the Session Layer in the OSI reference model to manage dialog control between communicating applications. In the ISO/IEC 8328 standard, SPDU messages are exchanged between Session Protocol Entities and carry the control information required to establish, maintain, and terminate a session, as well as to transfer user data in a coordinated manner.

Structure and function: An SPDU typically consists of a header and an optional data field. The header carries a SPDU type indicator, a length field, and may include references to related SPDU instances or parameters negotiated for the session. The interpretation of header fields and the meaning of any included data depend on the specific SPDU type.

Typical SPDU types: The messages are categorized by function, including those for connection establishment (setting up a session or association), data transfer (carrying user data with sequencing and flow-control information), and session termination or abort. Additional SPDU types handle error reporting, synchronization, and negotiation of session parameters.

Context and usage: SPDU concepts arise from the OSI model and its Session Layer, and are defined in the ISO/IEC 8328 family of standards. While the OSI protocol suite saw limited deployment in practice, SPDU provides a formal framework for what the Session Layer is intended to manage: dialog control, synchronization, and orderly data exchange between peers.

See also: OSI model, Session Layer, ISO/IEC 8328, SPDU types.