The ns Manual (formerly ns Notes and Documentation)1 - NM Lab at ...

5.5 Routing Module and Classifier OrganizationAs we have seen, a ns node is essentially a collection of classifiers. The simplest node (unicast) contains only one addressclassifier and one port classifier, as shown in Figure 5.1. When one extends the functionality of the node, more classifiers areadded into the base node, for instance, the multicast node shown in Figure 5.2. As more function blocks is added, and each ofthese blocks requires its own classifier(s), it becomes important for the node to provide a uniform interface to organize theseclassifiers and to bridge these classifiers to the route computation blocks.The classical method to handle this case is through class inheritance. For instance, if one wants a node that supports hierarchicalrouting, one simply derive a Node/Hier from the base node and override the classifier setup methods to insert hierarchicalclassifiers. This method works well when the new function blocks are independent and cannot be “arbitrarily” mixed. Forinstance, both hierarchical routing and ad hoc routing use their own set of classifiers. Inheritance would require that we haveNode/Hier that supports the former, and Node/Mobile for the latter. This becomes slightly problematic when one wants an adhoc routing node that supports hierarchical routing. In this simple case one may use multiple inheritance to solve the problem,but this quickly becomes infeasible as the number of such function blocks increases.The only method to solve this problem is object composition. The base node needs to define a set of interfaces for classifieraccess and organization. These interfaces should• allow individual routing modules that implement their own classifiers to insert their classifiers into the node;• allow route computation blocks to populate routes to classifiers in all routing modules that need this information,• provide a single point of management for existing routing modules.In addition, we should also define a uniform interface for routing modules to connect to the node interfaces, so as to providea systematic approach to extending node functionality. In this section we will describe the design of routing modules as wellas that of the corresponding node interfaces.5.5.1 Routing ModuleIn general, every routing implementation in ns consists of three function blocks:• Routing agent exchanges routing packet with neighbors,• Route logic uses the information gathered by routing agents (or the global topology database in the case of static routing)to perform the actual route computation,• Classifiers sit inside a Node. They use the computed routing table to perform packet forwarding.Notice that when implementing a new routing protocol, one does not necessarily implement all of these three blocks. Forinstance, when one implements a link state routing protocol, one simply implement a routing agent that exchanges informationin the link state manner, and a route logic that does Dijkstra on the resulting topology database. It can then use the sameclassifiers as other unicast routing protocols.When a new routing protocol implementation includes more than one function blocks, especially when it contains its ownclassifier, it is desirable to have another object, which we call a routing module, that manages all these function blocks and tointerface with node to organize its classifiers. Figure 5.3 shows functional relation among these objects. Notice that routingmodules may have direct relationship with route computation blocks, i.e., route logic and/or routing agents. However, routecomputation MAY not install their routes directly through a routing module, because there may exists other modules that58