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3.2 Round Trip Times

Figure 2: A comparison of round-trip times shows that Azureus, typifying Internet end-users, spreads across a range one order-of-magnitude larger than MIT King, based on inter-DNS latencies. This larger spread tends to lead to lower accuracy embeddings.

In Figure 2, we illustrate the distribution of inter-node round trip times between nodes in the three data sets. The King measurements were limited to a maximum of 800ms. The data exhibit one important characteristic: spread. The application-level, Azureus round trip times spread across four orders-of-magnitude, while the inter-DNS, King data set spreads across three. In theory, this is not a harbinger of higher embedding error; in practice, however, as Hong et al. have shown, the error between nodes whose distance is near the middle of the latency distribution tends to be the lowest [33]: with longer tails to this distribution, there are more edges to be inaccurate. (We found ICMP measurements exhibit a similarly wide distribution; see Section 7.5.) This wide spread is a warning sign that Azureus will have higher error than a system with a narrower round trip time distribution.

   
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