Internet Control Message Protocol
ICMP report TCP/IP issues during processing IP packet for detecting anomaly cases like
- Host is off
- Link is disconnected
출발지호스트에 이러한 사실을 알려야하지만, IP does not specify this issue reporting method.
traceroute use ICMP too
How ICMP works
ping is used for ICMP. There is no transport layer required.
ICMP message
type, code plus first 8 bytes of IP datagram causing error