r. userData IP-PDU BCA broadcast address Fig. 3.80 Handling of ICMP messages in IP networks These message are packed in the IP-SDU of IP-PDUs as any user data, see Fig. 3.80. However, the destination address that is used is a special broadcast address. As a matter of fact, there are two variants of addresses, depending on if the mes- sage concerns only routers and hosts to which the detecting router has routes, or if it must be broadcast outside that context. When the receiver reads the broadcast address, it reads and acts on the ICMP message, e.g. by updating information in the tables of RA and AT. We can model this communication as a part of the IP process control system, each router and host having a separate IPU (an ICMP handler) that handles IP-IDUs with broadcast addresses. 3.4 Modelling Telecom Resource Systems Each router in the IP network monitors the reception of IP-PDUs with respect to errors of different kinds, and analyses problems that are encountered by its RA and AT (e.g. network and host numbers that do not exist in the tables). The scenario in Fig. 3.80 assumes that the IP-UDT to the right has detected some serious problems with a received IP-PDU from the router to the left (parameter problems, dropped packages etc.). 1. The IP-UDT informs the ICMP handler in its own router about the problem, also providing the route identifier of the route of concern. 2. The ICMP handler analysis the problem. It concludes that the route to the send- ing router should not be used as a first-hand route. It orders its own AT to modify its tables. It then sends an ICMP message to all other routers (in this model only one) over the ATM AAL3/4-CSP, telling them which type of problem it has detected. The ICMP handler in a receiving router analyses the ICMP message and takes its own decisions. It might also update its tables, depending on the type of problem that was reported. The ICMP message is sent as userData in an IP-PDU as usual. The difference is the destination address (called BCA in the model). The final issue