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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.

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‫For example, when a data gram cannot reach its destination, when the Gateway does not have the buffering

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‫capacity to forward a data gram.

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‫And when a 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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‫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 Graham will be delivered or a control message will be returned.

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‫Some two grams 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 headers start 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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‫While 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 a 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.