More about CAIDA data collection
The IP record-route option allows some accuracy for the first 9 hops of any path. Nevertheless, this method is unreliable.
Use a method similar to traceroute, probing each hop along the path by incrementing time-to-live in the IP header.
Method for determining how skitter labels IP hops: the N-th hop in an IP path is the device that sends an ICMP TIMEXCEED message in response to a packet with a TTL of N.