We developed the conferencing system based on the conference service model which is defined in ITU-T T. 120 series standard. Our conferencing service system is operated over the local area network. The NetMeeting of Microsoft corporation is used as a terminal and the MCU is developed by ourselves. In this paper, we described the design and implementation method of transport protocol stack supporting Multipoint Communication Service (MCS). Also, we described the occurred problem during the interoperability test between MCU and NetMeeting. And then we suggested their solution.