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Internet control message Protocol ICMP is an error reporting protocol that network devices like routers

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use to generate error messages to the source IP address when network problems prevent delivery of IP

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packets.

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ICMP messages are sent in several situations, for example, when a daily gram cannot reach its destination,

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when the gateway does not have the buffering capacity to forward a data gram, and when the gateway

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can direct the host to send traffic on a shorter route.

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The Internet protocol is not designed to be absolutely reliable.

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No, I'll explain why.

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Hold on.

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The purpose of these control messages is to provide feedback about problems in the communication environment,

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not to make IP reliable.

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There's still no guarantees that a data gram will be delivered or a control message will be returned.

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Some data grammes may still be undelivered without any report of their loss.

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The higher level protocols that use IP must implement their own reliability procedures if reliable communication

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is required.

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So the ICMP header starts after the IPV four header.

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All ICMP packets have an eight byte header and variable size data section.

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The first four bytes of the header have a fixed format.

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One of the last four bites depend on the type and code of that ICMP packet.

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ICMP type and code in the header identifies the control message.

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Some of the ICMP control messages are shown in this slide, so Ping is the most well known application

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of the ICMP.

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It operates by sending ICMP Echo request packets to the target host and then waiting for an ICMP echo

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reply.