5 5 units 5 units Origin Origin All tinks have a capacity of 10 units Destination Routing in Data Networks 5.1 INTRODUCTION We have frequently referred to the routing algorithm as the network layer protocol that guides packets through the communication subnet to their correct destination.The times at which routing decisions are made depend on whether the network uses datagrams or virtual circuits.In a datagram network,two successive packets of the same user pair may travel along different routes,and a routing decision is necessary for each individual packet (see Fig.5.1).In a virtual circuit network,a routing decision is made when each virtual circuit is set up.The routing algorithm is used to choose the communication path for the virtual circuit.All packets of the virtual circuit subsequently use this path up to the time that the virtual circuit is either terminated or rerouted for some reason(see Fig.5.2). Routing in a network typically involves a rather complex collection of algorithms that work more or less independently and yet support each other by exchanging services or information.The complexity is due to a number of reasons.First,routing requires coordination between all the nodes of the subnet rather than just a pair of modules as, for example,in data link and transport layer protocols.Second,the routing system must 3635 3 5 units 5 units All links have a capacity of 10 units Routing in Data Networks 5.1 INTRODUCTION We have frequently referred to the routing algorithm as the network layer protocol that guides packets through the communication subnet to their correct destination. The times at which routing decisions are made depend on whether the network uses datagrams or virtual circuits. In a datagram network, two successive packets of the same user pair may travel along different routes, and a routing decision is necessary for each individual packet (see Fig. 5.1). In a virtual circuit network, a routing decision is made when each virtual circuit is set up. The routing algorithm is used to choose the communication path for the virtual circuit. All packets of the virtual circuit subsequently use this path up to the time that the virtual circuit is either terminated or rerouted for some reason (see Fig. 5.2). Routing in a network typically involves a rather complex collection of algorithms that work more or less independently and yet support each other by exchanging services or information. The complexity is due to a number of reasons. First, routing requires coordination between all the nodes of the subnet rather than just a pair of modules as, for example, in data link and transport layer protocols. Second, the routing system must 363