On the Asymmetry of Internet eXchange Points -Why Should IXPs and CDNs Care?

Internet eXchange Points (IXPs) provide an infrastructure where content providers and consumers can freely exchange network traffic. The main incentive for connecting to an IXP is to decrease costs and improve the user experience by having content closer to consumers. Despite these benefits, several small Content Delivery Networks (CDNs) avoid exchanging traffic on IXPs due to the poor routing quality via IXP paths. In this paper, we investigate how traffic asymmetry affects the quality of paths. IXP asymmetry occurs when traffic is sent (or received) via a direct IXP peering but received (or sent) on an alternative path outside the IXP. We employ a new method to quantify a symmetry rate for an IXP, which we evaluate on five IXPs. Our method covers three times more ASes than alternatives, such as using RIPE ATLAS. Our results show that IXPs have 15% asymmetric paths at a distance of one AS hop, i.e., when sending traffic to a given peer on the IXP, 15% of this traffic will be responded via a transit AS that does not use the IXP path. We also identify deaf neighbors, i.e., ASes that never return traffic to the IXP. We identify egress-only paths as a major cause of asymmetries and show that this occurs only for a small number of ASes. We also quantify the impact of traffic asymmetry at IXPs in terms of latency and show that traditional traffic engineering on IXP prefixes can actually make route quality worse.