1. Internet Control Message Protocol (ICMP)Chapter 7

2. ICMP • One of the fundamental protocols that is part of the TCP/IP suite • It is used to control and manage information that is transmitted using TCP/IP • It allows nodes to share status and error information • Helps to re-route messages when a route is busy or has failed

3. ICMP – Cont. • A very common use of ICMP is the passing of error information between host and router • ICMP helps to keep the network running when a router goes off line • All messages are carried as IP datagrams • Dynamic routers use ICMP messages to announce the contents of their routing tables • Hosts listen for these messages to acquire routing information • A router solicitation may be sent by a host when it starts up. This is a request for immediate ads

4. When are ICMP Messages Generated? • A packet cannot reach its destination-sent by router that cannot send the IP datagram any further • A packet’s Time to Live (TTL) expires-reaches its limit of hops before getting to destination-router discards it and sends ICMP messageto sending machine • An error is detected in IP header data • A router cannot buffer an incoming packer-buffer in router is full

5. ICMP Messages – Cont. • A router cannot keep up with an incoming stream of packets • A router sends a message to a host advising of a shorter route to a destination • A host sends an echo packet to determine if another host is alive (ping) • A host needs to determine the local network

6. ICMP – Cont. • ICMP doesn’t make IP reliable • It provides no facilities to provide retransmission of IP datagrams that encounter errors.

7. ICMP Echo/Echo Reply • One of the most heavily used ICMP features is the ability to send a simple message to an IP node and have the message echoed back to the sender. • This is handy for network troubleshooting and debugging. • Ping and Tracert both use Echo/Echo Reply • See p. 165 for message format

8. Ping Utility • Tests reachability, name resolution, and routing • -t :sends Echoes until interrupted • -w timeout :waits a specified amount of time, in milliseconds, for the Echo Reply. Default is 1000 • See p. 184 to see how ping works

9. Tracert Utility • Determines a path from a host to a destination node • The default action is to trace the route, report the round-trip time, and the near-side router IP address • See p. 186 for operation of tracert

10. ICMP Redirect • Host have minimal routing tables • The routers keep all other routing information in their tables • If there are multiple routers on a network, the possibility exists for non-optimal routing • See p. 176, Figure 7-8 • See p. 177, for details of redirect process