We describe the design and implementation of a network layer interfacing scheme, which permits an OSI-8072 transport layer process to operate on a variety of network layer implementations. We describe a mapping function that would create a correspondence between the network services required by the correspondent transport process and the set of network services available. Further, we describe the design and implementation of an OSI-8072 Class-4 transport layer protocol. This transport layer service is implemented as a set of C library routines. The services are based on the networking interface scheme mentioned above. Using these services, users may design their own OSI compatible networking applications from the session layer upwards and run them on all network layer implementations supported by the UNIX networking environment.