Internet anycast: Performance, problems, & potential

Abstract

Internet anycast depends on inter-domain routing to direct clients to their “closest” sites. Using data collected from a root DNS server for over a year (400M+ queries/day from 100+ sites), we characterize the load balancing and latency performance of global anycast. Our analysis shows that site loads are often unbalanced, and that most queries travel longer than necessary, many by over 5000 km. Investigating the root causes of these inefficiencies, we can attribute path inflation to two causes. Like unicast, anycast routes are subject to inter-domain routing topology and policies that can increase path length compared to theoretical shortest (e.g., great-circle distance). Unlike unicast, anycast routes are also affected by poor route selection when paths to multiple sites are available, subjecting anycast routes to an additional, unnecessary, penalty. Unfortunately, BGP provides no information about the number or goodness of reachable anycast sites. We propose an additional hint in BGP advertisements for anycast routes that can enable ISPs to make better choices when multiple “equally good” routes are available. Our results show that use of such routing hints can eliminate much of the anycast path inflation, enabling anycast to approach the performance of unicast routing.