OSI Network Addresses

In the OSI architecture, the network layer provides services to a user in the layer above, such as the transport service element. If you designate the network layer by N, the network user is the N+1 service element (see Figure 4-2). An N+1 entity in Router A that wants to communicate with an N+1 peer in Router B must provide the peer's N service access point (NSAP) as the destination of the service request while providing its own NSAP as the source to facilitate bidirectional communication. In a router, the N+1 entity or the N service user is either a transport layer entity or the routing layer.

Figure 4-2. Peer-to-peer layer communications.

The NSAP address format has a component known as the NSAP selector ( NSEL ) for identifying the target network service user. The NSEL is further discussed later in this section. Other components of the NSAP are the System Identifier ( SysID ) and the Area Identifier ( Area ID). The SysID uniquely identifies a network node, whereas the Area ID specifies the home area of the node. IS-IS requires all systems to be associated with at least one area in the network domain. In summary, an NSAP address consists of the following three parts : the Area ID, SysID, and NSEL. When the routing layer is specified as the network service user, the value of the NSEL is set to zero, and the NSAP is called the network entity title ( NET ). In a pure ISO CLNS environment, ordinary data packets have source and destination NSAPs with non-zero NSEL values. Such packets are obviously not meant for the routing layer and might be directed at some transport services element in the router or possibly just traversing the router. On the other hand, in IP-only environments, such as the Internet, there are no CLNP applications, so the routing layer is the only user of CLNP network services. In this case, the only ISO packets in operation are ES-IS and IS-IS routing information and control packets.

The following material focuses only on the ISO CLNS architecture. Refer to Chapter 1 for additional information on IP addressing.