Internet Control Message Protocol (ICMP) Chapter 7

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

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

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

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

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

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

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

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

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