Specifies classes of procedures when operating over the connection-mode network service for the connection-mode transfer of data and control information from one transport entity to a peer transport entity. These classes are class 0: simple, class 1: basic error recovery, class 2: multiplexing, class 3: error recovery and multiplexing, and class 4: error detection and recovery. A class 4 procedure is also specified when operating over a connectionless-mode network service. This Standard is identical with and has been reproduced from ISO/IEC 8073:1992 and ISO/IEC 8073:1992 Technical Corrigendum 1:1993.