What does the TTL expired message indicate in ICMP?

The TTL (Time to Live) expired message in ICMP (Internet Control Message Protocol) indicates that a packet has traveled through the network and exceeded its lifespan before reaching its destination. Each packet has a TTL value set by the sender, which decrements by one each time it passes through a router. When the TTL reaches zero, the packet is discarded, and an ICMP message is sent back to the source, indicating that the packet could not be delivered because it timed out.

This mechanism helps prevent packets from circulating indefinitely on the network, which can lead to congestion and wasted resources. Therefore, the TTL expired message specifically signifies that the packet did not arrive at its destination within the allotted TTL, resulting in a timeout and packet delivery failure. Understanding this helps network professionals diagnose issues related to routing and connectivity in a network environment.