Sec.5.1 Introduction 365 Packet from user n 2 ④→包+回+ 回 Figure 5.2 Routing in a virtual circuit network.All packets of each virtual circuit use the same path.A routing decision is required only when a virtual circuit is set up. 5.1.1 Main Issues in Routing The two main functions performed by a routing algorithm are the selection of routes for various origin-destination pairs and the delivery of messages to their correct destination once the routes are selected.The second function is conceptually straightforward using a variety of protocols and data structures (known as routing tables),some of which will be described in the context of practical networks in Section 5.1.2.The focus will be on the first function (selection of routes)and how it affects network performance. There are two main performance measures that are substantially affected by the routing algorithm-throughput(quantity of service)and average packet delay (quality of service).Routing interacts with flow control in determining these performance measures Delay Offered load Throughput Flow control Routing Delay Rejected load Figure 5.3 Interaction of routing and flow control.As good routing keeps delay low. flow control allows more traffic into the network.Sec. 5.1 Introduction 365 : Packet from user n Figure 5.2 Routing in a virtual circuit network. All packets of each virtual circuit use the same path. A routing decision is required only when a virtual circuit is set up. 5.1.1 Main Issues in Routing The two main functions perfonned by a routing algorithm are the selection of routes for various origin-destination pairs and the delivery of messages to their correct destination once the routes are selected. The second function is conceptually straightforward using a variety of protocols and data structures (known as routing tables), some of which will be described in the context of practical networks in Section 5.1.2. The focus will be on the first function (selection of routes) and how it affects network perfonnance. There are two main perfonnance measures that are substantially affected by the routing algorithm-throughput (quantity of service) and average packet delay (quality of service). Routing interacts with flow control in detennining these perfonnance measures +e ay Offered load Flow control Throughput Routing I￾Rejected load r-----------------~ I D I : I I I _~ Delay Figure 5.3 Interaction of routing and flow control. As good routing keeps delay low, flow control allows more traffic into the network