ICMP Packet Format:

ICMP Packet Field Descriptions

• Standard IP header---with protocol set to 1.

• ICMP type---Indicates the type of ICMP message.

• ICMP code---Further qualifies the specific ICMP message.

• ICMP checksum---Checksum of the ICMP data beginning with the ICMP type field.

• ICMP message---Contains specific data based upon the type and code fields.

• ICMP data---If present, contains original IP header (20-60 octets) plus 8 octets of original IP data.
  UDP PDU