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Correlations between RTT and Distance

Filtering Landmarks

If we are not interested in using delay to distance mapping for landmarks too far away from the target to be usable in the geolocation algorithm, we can neglect large values of RTT between landmarks and targets. But sometimes it's possible that our target is lying in such a geographical location where there are no active landmark nearby. In such a case probably some distantly located landmarks would be candidates for the final selection, so the threshold maximum RTT beyond which we do not use the landmarks measurement may depend on the landmark and target regions involved. If we want to avoid making such useless measurements then we need to identify the region of the target early on in the process.

Other metrics that can be useful for selecting tier1 landmarks are:

• The landmark should be reliable, i.e. up and responding correctly most of the time.
• The landmark should be able to tell whether a target is in the same region or not (see Tier1 Landmark Selection).
• The landmark should have a strong correlation between RTT and distance for targets in its region.
• THe landmark should be able to ping the largest possible number of targets.