Some of the most interesting components within your network go unchecked. Some of the well know information floating around the network comes from excellent sources, network devices. This protocol has been around a while, sense 1981, but is commonly overlooked. The information that ICMP provides is insightful and provides excellent insight into what's happening within your network.
1. Destination host unreachable
2. Destination network unreachable
3. Destination protocol unreachable
4. Destination port unreachable
5. Network administratively prohibited
6. Source quench
7. TTL expired
9. Time exceeded
10. Communication administratively prohibited
For example "destination unreachable" provides whether it was the end devices "host", network "router" or protocol "application" that is the issue. Congestion can be identified with "source quench". Understanding ICMP messages takes a few minutes but is well worth the effort, but monitoring ICMP Messages provides the magnification glass to identify key issues that are occuring in your network. It what we call forensic analysis.
We use this type of analysis along with others in Gearbit's GBX products.