protocol, or by some type of network-wide layer-element identifier. Such an identi- 2.2 Discrimination (Connectivity Layers Only) fier can comprise either a network address (NA) only, or an access-point identifier (APi) only. The Internet (another packet-switched network system), on the other hand, com- bines discrimination by NA and APi in the same connectivity layer (the IP layer). Furthermore, the Internet also supports discrimination by sockets (a kind of "con- nections" offered by TCP and UDP) by combining the NA of a lower layer (IP) with an APi of a higher layer (TCP or UDP). Other forms of discrimination exist as well, as described in the following sections. 2.2.2 Discrimination in the OSI RM In the OSI RM, the way the other layer element is identified in send and receive operations is used to characterize the class of service as either connection-less (CNL or "datagram") or connection-oriented (CON). The CON-type of service implies that before packets can be sent and received, the network must establish end-to-end "connection," which in a layer interface is identified by a CEi. Strictly speaking, the CON-CNL classification applies only to packet-switching networks. Although "circuits" in circuit-switched networks have similar properties and estab- lishment procedures as the OSI "connection," layer elements use no identifiers at all at sending and receiving. We will, in general, use the term CON both in the OSI RM sense and to characterize circuit switched connectivity services. The way OSI-layer elements that use a connectivity service can be structured internally depends on whether a CON or a CNL service is used (see Fig. 2.8). A single layer element type Three layer elements Layer- Layer- Layer Layer element element element element instance instance Higher layer protocol If CEi=x If CEi=y If APi=x If APi=y Discrimination Discrimination Transport protocol by CEi by APi {APi,..}} origNA,data CEi,data destNA,data OSI CON-network- {Ci,data} OSI CNL-network- {destNA,origNA,data} layer element layer element 