As with any type of packet, ICMP messages have special formats. Each ICMP message type shown in Figure has its own unique characteristics, but all ICMP message formats start with these same three fields:

• Type
• Code
• Checksum

The type field indicates the type of ICMP message being sent. The code field includes further information specific to the message type. The checksum field, as in other types of packets, is used to verify the integrity of the data.

Figure shows the message format for the ICMP echo request and echo reply messages. The relevant type and code numbers are shown for each message type. The identifier and sequence number fields are unique to the echo request and echo reply messages. The identifier and sequence fields are used to match the echo replies to the corresponding echo request. The data field contains additional information that may be a part of the echo reply or echo request message.

Interactive Media Activity Drag and Drop: ICMP ECHO Message Format After completing this activity, the student will be able to ICMP ECHO message format.