Q. Tracert will return the IP addresses of the interfaces b, e, h, and k, even though there are also other interfaces on some of the traversed machines. Explain the reason that only b, e, h, k are reported, considering the order of the three things that a router performs on an arriving IP datagram?

A. TTL decrement is first executed on reception through the interfaces, before routing table lookup or checksum recomputation.

가정) TTL check comes before the header integrity check.

1. Decrement TTL
   1. TTL is 0, so discard the datagram
   2. Notify the source of the datagram that TTL has expired (ICMP Type: 11 Time Exceeded)
2. Compute IP header checksum and put it in the outgoing datagram
   1. IP header checksum is different from the computed value, so discard the datagram
3. Look up the routing table and find the outgoing interface and next hop IP address
   1. The link MTU is smaller than the datagram, and DF bit is set, so discard the datagram
   2. Notify the source that the datagram is too big for the next link (ICMP Type 3 Code 4 Fragmentation Required but DF bit is set)
4. Look up the ARP cache using the “gateway” value from the matching routing table entry as key
5. Call Ethernet driver with the outgoing datagram to send it to the destination link layer address