RFC 792 / START HERE
ICMPv4, before the details
ICMP: Internet Control Message Protocol
ICMP carries control and error information about IPv4 traffic. It helps devices report problems that ordinary application data cannot explain.
The idea in one picture
A packet encounters a problem
ICMP reports information
The sender can react
A simplified picture of the idea; the chapters explain the actual mechanisms.
Why it exists
A sender needs feedback when a destination cannot be reached or a packet cannot proceed.
A familiar example
The ping utility uses ICMP echo messages to check whether another device responds.
Helpful background
You can start here without reading another RFC. If these ideas are unfamiliar, these short introductions may help.
What you’ll learn next
Begin with chapter one, or choose a topic.
- Report an IPv4 delivery problemICMP returns control information to the source of an IPv4 datagram.→
- Match an echo replyAn Echo Reply returns the request data along with its identifier and sequence number.→
- Explain an unreachable destinationType 3 reports reasons such as unreachable networks, hosts, protocols, or ports.→
- Observe a lifetime expiryTime Exceeded distinguishes transit TTL expiry from fragment-reassembly timeout.→
- Recognize retired Source Quench behaviorSource Quench was an early congestion signal and is now deprecated.→
This introduction explains RFC 792 (1981). Read the original RFC ↗