Decoding ICMP headers

In the Decoding IP headers recipe of this chapter, we created a decodeIp() function that decoded the IP headers of a packet. If the protocol type was ICMP in that function, we called a decodeICMP() function. We will create the decodeICMP() function in this recipe.

Even though our layer diagram shows the ICMP packet as a part of the Internet layer, from the libpcap point of view, it is a part of the third layer (Protocol layer) of our header stack; this is because the IP header encapsulates the ICMP header.

The ICMP header looks like this:

The components are explained as follows:

• ICMP Type: This field identifies the ICMP message type. The Echo request message is type 8 and the Echo reply is type 0.

• Code: This field is not used in the Echo request or reply so this is set to 0.

• Checksum: This field contains the checksum for the ICMP header.

• Identifier: This is the identification field that can be used to match the Echo request with an Echo reply.

• Sequence Number: This field contains...