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Internet control message protocol.

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ICMP is an error reporting protocol that network devices like routers use to generate error messages

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to the source IP address when network problems prevent delivery of IP packets.

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ICMP messages are sent in several situations.

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For example, when a datagram cannot reach its destination.

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When the gateway does not have the buffering capacity to forward a datagram.

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And when the gateway 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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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 datagram will be delivered or a control message will be returned.

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Some datagrams 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.

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Procedures if reliable communication is required.

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So the ICMP header starts after the IPv4 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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While the last four bytes 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.

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So Ping is the most well known application 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.