traceroute typically uses Internet Control Message Protocol (ICMP) echo requests or User Datagram Protocol (UDP) packets. The specific protocol can vary based on the implementation (e.g., Windows uses ICMP, while Unix-like systems often use UDP by default). TTL is a field in the IP header that indicates the maximum number of hops a packet is allowed to traverse before being discarded. Each router that forwards the packet decrements the TTL by one. If the TTL reaches zero, the packet is discarded and an ICMP "Time Exceeded" message is sent back to the sender.