John Heidemann

Anycast Latency: How Many Sites Are Enough?

TitleAnycast Latency: How Many Sites Are Enough?
Publication TypeConference Paper
Year of Publication2017
AuthorsR. O. de Schmidt, J. Heidemann, and J. H. Kuipers
Conference Nameproc # caida-pam
Date Publishedmar
Conference LocationSydney, Australia

Anycast is widely used today to provide important services such as DNS and Content Delivery Networks (CDNs). An anycast service uses multiple \emphsites to provide high availability, capacity and redundancy. BGP routing associates users to sites, defining the \emphcatchment that each site serves. Although prior work has studied how users associate with anycast services informally, in this paper we examine the key question \emphhow many anycast sites are needed to provide good latency, and the worst case latencies that specific deployments see. To answer this question, we first define the \emphoptimal performance that is possible, then explore how routing, specific anycast policies, and site location affect performance. We develop a new method capable of determining optimal performance and use it to study four real-world anycast services operated by different organizations: C-, F-, K-, and L-Root, each part of the Root DNS service. We measure their performance from more than 7,900 vantage points (VPs) worldwide using RIPE Atlas. (Given the VPs uneven geographic distribution, we evaluate and control for potential bias.) Our key results show that a few sites can provide performance nearly as good as many, and that geographic location and good connectivity have a far stronger effect on latency than having many sites. We show how often users see the closest anycast site, and how strongly routing policy affects site selection.