Describes the classes 0 to 3 of the OSI transport protocol, defined in ISO 8073 using the formal description technique LOTOS, which is defined in ISO 8807. Transport protocol class 4 and NMCS are not covered. In view of complexity and widespread use of OSI standards it is imperative to have precise and unambiguous definitions of these standards. Formal Description Techniques form an effective approach to providing such definitions. Annexes A and B give definitions relating to the Transport and Network Services, Annex C gives an interpretation of ISO 8073, Annexes E and F include indices of process and type definitions.