Defines in an abstract way the externally visible service provided by the (Open Systems Interconnection) Transport Layer in terms of the primitive actions and events of the service, the parameter data associated with each primitive action and event and the relationship between, and the valid sequences of the actions and events. The service is provided by all OSI Transport Protocols (in conjunction with the Network Service) and may be used by any OSI Session Protocol. The Standard does not specify individual implementation or products, nor does it contain the implementation of entities and interfaces within a computer system. There is, therefore, no conformance to this Standard. Identical with ISO 8072:1986.