Networking & Internet
ICMP
Definition
The Internet Control Message Protocol (ICMP) is a supporting protocol in the Internet protocol suite. It is used by network devices, including routers, to send error messages and operational information indicating, for example, that a requested service is not available or that a host or router could not be reached.
Why It Matters
ICMP is the "health and status" protocol of the internet. It provides feedback about problems in the network, which is crucial for troubleshooting.
Contextual Example
The `ping` command uses ICMP. It sends an "Echo Request" message to a target host. If the host is reachable, it replies with an "Echo Reply" message. If a router along the way cannot find the destination, it might send back an ICMP "Destination Unreachable" message.
Common Misunderstandings
- ICMP is not used for transporting application data like TCP or UDP are.
- Firewalls often block some types of ICMP messages for security reasons, which can sometimes interfere with network diagnostics.