In traceroute, what value is the TTL in the IP header set to in the packet sent to make the fourth router from the source return an ICMP Time Exceeded message?

Answer: B. 4

Each router decreases the TTL by 1 when it forwards a packet, and when the TTL reaches 0 it discards the packet and returns an ICMP Time Exceeded message to the source. A packet sent with a TTL of 4 has 3 left after the first router, 2 after the second and 1 after the third, and reaches 0 at the fourth, so the fourth router returns the message. traceroute sends packets while raising the TTL one step at a time, 1, 2, 3 and so on, and from the source address and timing of the returned messages it shows the routers along the path and their delays in order. When packets reach the destination, it learns that it has finished from the destination's reply (an ICMP echo reply, or a message saying the destination port could not be reached).

• A. With a TTL of 3, it reaches 0 at the third router, and the third router returns the message.
• B. Correct. The TTL reaches 0 at the fourth router.
• C. With a TTL of 5, 1 is left at the fourth router, which forwards the packet, and the fifth router returns the message.