unications between the router and the sending host computer as shown in Figure 6.45. ICMP must be implemented into every network element equipped with IP and it provides a means to communicate between the IP software of a host and a router. The value 1 (decimal) in the protocol field of the IP data- gram in Figure 6.41 identifies the ICMP. Communication problems may lead to discarding of the IP datagrams as shown in Figure 6.45 where router 3 is not able to transmit the datagram further. It then uses ICMP to inform the source host about the reason for the problem. The ICMP message contains a type field indicating the type of the problem and a set of parameters that may help the host to decide how to resolve the problem. Types of problems include, for example, the following: Introduction to Telecommunications Network Engineering Router 1 Router 2 Router 3 IP datagram IP datagram ICMP message ICMP message Figure 6.45 Operation of ICMP. Destination unreachable: The destination host is, for example, in the power-off condition. Time exceeded: Time-to-live field is subtracted to zero. Source quench: The source should reduce its transmission speed because the destination is not able to process incoming data or a router on the way does not have enough buffer space for IP packets. Redirect: A router finds that the source and next router are connected to the same network (according to the routing table) and requests the source to transmit the packet directly to the other router. For a more detailed description of ICMP the reader should refer to [4]. 6.6.8 Structure of Internet and IP Routing The Internet can be seen as a collection of subnetworks or autonomous sys- tems that are connected as shown in Figure 6.46. An IP Ethernet LAN of a company providing Web service and an ISP's network in Figure 6.46 are two examples of autonomous systems connected by regional and backbone net- works. There is no real fixed hierarchy but there are several backbones, which consist of high-data-rate lines and fast routers [3]. Regional netw