ICMP – protocol operation, analysis, and troubleshooting
Internet Control Message Protocol (ICMP) is a network layer protocol that serves the purpose of error reporting and network path diagnostic functions. The Ping and Traceroute utility tools leverage ICMP messages for fault detection and isolation. ICMP messages are sent using basic IP header. The protocol field in IP header will be set to ICMP followed by the ICMP payload. The ICMP packet has the following format:

ICMP header format
The ICMP message types used for network connectivity verification are echo request (Type = 8
) and echo reply (Type = 0
).
Getting ready
When there are any issues with an end application such as a web service or mail service, the first troubleshooting step using the bottom-up approach is to verify the data-link layer. Once we have verified the data-link layer using the steps defined in the previous section, the next step is to verify the network connectivity between the end points. Network connectivity between...