Let's take a look at the Internet Control Message Protocol (ICMP). It is one of the most popular protocols, and is better known for being used in ping commands, which is where an ICMP echo request is sent to an IP address with some random data, and it then denotes whether the system is alive. A typical ICMP packet would look like this:
The ICMP has many messages, which are identified by the Type of Message field. The Code field indicates the type of message. The Identifier and Sequence Number can be used by the client to match the reply with the request that caused the reply.
The Data field may contain a random string or a timestamp to compute the round-trip time in a stateless manner. Let's ping https://www.google.com/ and analyze it in Wireshark:
We can see that we have four Echo request and four Echo reply packets. Let...
