Final answer:
During a traceroute, a router decrements the value in the Time To Live (TTL) field by 1 before forwarding the packet to the next hop. When the TTL value becomes 0, the router discards the packet and sends an ICMP Time Exceeded message back to the sender.
Step-by-step explanation:
During a traceroute, a router decrements the value in the Time To Live (TTL) field by 1 before forwarding the packet to the next hop. The TTL field is initially set by the sender to a specific value, such as 64 or 128, in order to limit the lifespan of the packet.
Each time the packet is forwarded by a router, the TTL value is decremented until it reaches 0. When the TTL value becomes 0, the router discards the packet and sends an ICMP Time Exceeded message back to the sender, indicating that the packet was dropped due to the TTL expiration.