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IP v6 Networking - OSI CLNP
In the 1980s, the Open Systems Interconnection (OSI) protocol family was created by the International Organization for Standardization (ISO) and the International Telecommunication Union (ITU). The Connectionless Network Protocol (CLNP) provides a datagram service. In true OSI spirit, where even the most mundane details are carefully specified, there is a different name for the Connectionless Network Service (CLNS) and the actual CLNP protocol that is used to provide this service. However, this nuance is only appreciated by connoisseurs, so CLNP and CLNS are often used interchangeably. Some people even simply say “OSI,” disregarding the connection-oriented part of the protocol family (the X.25 protocol). CLNP addresses may vary in length, with a maximum of 160 bits, and include a system identifier that must be the same length for all systems inside a CLNP network. This means that in practice, the Ethernet MAC address is often used here, as shown in Figure 1-5. Unlike IPX, AppleTalk, IP, and IPv6, CLNP hosts don’t try to figure out which addresses are reachable locally without involving a router. A CLNP host simply sends the packet to a router. The router then may or may not send a redirect message to inform the host on which MAC address it can use to communicate with the other host.
OSI JARGON
For people who are used to IP and the terminology that comes with it, OSI jargon can be quite baffling. For instance, a host is called an End System (ES) and a router is an Intermediate System (IS). A MAC address is a Subnetwork Point of Attachment (SNPA), and even the word “address” is deemed no good, so Network Service Access Point (NSAP) is used instead. To confuse matters even more, routers are generally addressed with a Network Entity Title (NET) rather than an NSAP.
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