 discriminating layer to send something. This is because the association between a layer element and its APi value is set in every node when the actual using layer is first installed in the net- work. Furthermore, the discrimination protocol need not identify sender's APi. In case of layer-element APi, only the range of the APi parameter is defined by the discriminating layer. The actual APi values are defined by a separate specifi- cation that belongs to the network system as a whole (e.g., for the Internet, the IANA publishes a separate list of "port numbers," which is the APi that TCP/ UDP uses for discrimination). This approach is used when there can be a large and variable number of using layers that are regarded as applications of a net- work system. Layer elements of the same using layer can be installed at different points in time, and can therefore be associated to any idle APi value. Examples of such discriminating layers are TCP and UDP. SCCP also allows this mode of operation. The layer-element APi approach is more complex than the layer APi method in that a sending using element must know the destination APi, and the discrimina- tion protocol must include both the originating and the destination APi. 2.2.3 Discrimination in the Internet The IP layer (the Internet "network service") offers a datagram service that per- forms discrimination according to the layer-APi method, based on an APi that is called "protocol number" (PRN). The IP layer can therefore, contrary to the OSI RM network layer, support multiple "transport layers." Most well-known are the TCP (a CON service) and UDP (a CNL service). Furthermore, the Internet layer structure supports discrimination by APi recur- sively since TCP/UDP also performs discrimination, according to the layer-ele- ment APi method, however. The APi is called "port number" (PON). Application layers (e.g., the "simple message transfer protocol" layer (SMTP)) can use TCP and UDP services by the same port numbers (see Fig. 2.10). 2 Layer Structures Other layers origSo