Specifies procedures applicable to instances of communication between systems which support the Transport Layer of the OSI Reference Model and which wish to interconnect in an open systems environment. It defines five classes of procedures for the connection-oriented transfer of data and control information from one transport entity to a peer transport entity, the means of negotiating the class of procedures to be used by the transport entities and the structure and encoding of the transport protocol data units used for the transfer of data and control information. The procedures are defined in terms of the interactions between: peer transport entities; a transport entity and the transport-service used in the same system; and a transport entity and the network service provider. It also specifies conformance requirements for systems implementing these procedures. It does not contain tests which can be used to demonstrate this conformance. Identical with ISO 8073.