The basic tool for observing the messages exchanged between executing protocol entities is called a packet sniffer. As the name suggests, a packet sniffer captures $($sniffs$)$ messages being sent$/$received from$/$by your computer; it will also typically store and$/$or display the contents of the various protocol fields in these captured messages. A packet sniffer itself is passive. It observes messages being sent and received by applications and

protocols running on your computer, but never sends packets itself. Similarly, received

packets are never explicitly addressed to the packet sniffer. Instead, a packet sniffer

receives a copy of packets that are sent$/$received from$/$by application and protocols

executing on your machine.