ICMP

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.

Links:

Using ICMP tunneling to steal Internet

Icmp.pdf

PingTunnel

related: TCP,IP,ping