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Verify that they have roughly the right RTT.
- RTTs of < 80ms from SLAC probably indicate the host is in North America.
- One can also use a ping server at a PingER, PerfSONAR or PlanetLab site (see http://www.slac.stanford.edu/comp/net/wan-mon/viper/tulipmap.html) that is supposedly near the target host and ensure that the RTT is suitably small.
Then you probably want to select hosts that are educational (e.g. have .edu or .ac. on the hostname) and representative of the country, e.g. in main cities, and do not overlap their location.
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