topology: caida's skitter track/depict topology cross-sections 22 monitors (inc. some root name servers) forward IP path and round-trip delay tens of thousands of dst (multiple lists) remove targets that complain architecture continuous, parallel 52-byte ICMP probes depending on dst list size, 0.3 to 200 probes/day/dst kernel time stamping correlate path perf. w events, e.g. BGP identify critical pieces of infrastructure case studies of relevant cross-sections