ICMP is the Internet Control Message Protocol
You can read RFC 792 for a full explanation.
The well-known ping command is a type of ICMP request.
Packet Layout:
0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-------------------------------+-------------------------------+ | Type | Code | Checksum | +-------------------------------+-------------------------------+ | Identifier | Sequence Number | +-------------------------------+-------------------------------+ | Data ... +--------------------------------------------- Type 8 = echo request. Type 0 = echo reply.
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