In overlay networks, both local and long-distance links appear as a single hop to a routing protocol, and traditional routing metrics (based on hop count or packet loss) fail to take the differences between such links into account. In this paper, we describe the design and implementation of a metric based on packet delay that is designed to improve routing in overlay networks.Using delay naively leads to persistent routing oscillations, but the Babel routing protocol within which our metric is implemented employs a number of features to bound the frequency of oscillations and limit their impact by ensuring consistency even during reconvergence. We show experimental data that indicate that the protocol causes no oscillations in real-world situations, and has oscillations with a period on the order of minutes in artificially constructed topologies.
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