nds with a RARP response containing the IP address allocated to that host. 6.6.6 Routing Protocols As we can see in Figure 6.36, the network layer contains other protocols that control network layer routing. Interior Gateway Protocols (IGPs) are used Data Communications inside one autonomous system (AS) such as a LAN, and an Exterior Gateway Protocol (EGP) is used for communications between the exterior router and the other system [4]. One IGP protocol is OSPF, which is capable of dynamic updating of routing information. Routers exchange network topology and link status information with their neighbors and use that to derive the best, shortest path to the destinations [5]. Link metrics used in route computing include, for example, delay and throughput of the link. Another simple and popular IGP protocol is the Routing Information Protocol (RIP). A router running RIP broadcasts a routing update message every 30 seconds, which contains IP addresses and distances (hop counts) to those networks. All stations and routers running RIP update their tables accordingly [4]. One EGP protocol is the Border Gateway Protocol (BGP). The BGP router computes paths and tells its neighbors which routes it is using toward destinations [3]. Based on this information, neighbors select their own routes. For example, if router A tries to derive a path to a certain network C, and its neighbor B has told that it will use A as a second step toward network C, router A knows that it should use other neighbor routers instead of B. Exterior gateway routing protocols, such as BGP, are designed to allow many kinds of routing policies, which are considered in routing decisions. Policies are manually configured into each EGP router. Typical policies involve political, security, or economic considerations. For example, a tele- communications network operator is happy to act as carrier of the traffic from its own customers, but not the others. 6.6.7 The ICMP in Figure 6.36 is another protocol for network layer control. It is used especially for comm