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Table Of Contents
CEH v10 Certified Ethical Hacker Study Guide
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The ICMP is a special case when it comes to protocols, in that it doesn’t carry user data. Instead, it works with other protocols to provide error and control messaging. When something unexpected happens on the network, devices will generate ICMP messages to send back to the originating device to let them know that there was a problem. It does sit on top of IP, because it needs the addressing of IP, but it is considered to be part of the Internet layer as IP is. This also makes it a bit of an unusual protocol, because it sort of sits above the Internet layer but isn’t a Transport layer protocol.
ICMP is defined in RFC 792, which specifies a header of 8 bytes. This consists of the type and code fields, which convey the essential information for ICMP, a checksum field, and then 4 bytes that are labeled “rest of header.” The type and code are each a byte and the checksum is 2 bytes. The rest of the header field contains data...
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