A Practical Architecture for an Anycast CDN

Abstract

IP Anycast has many attractive features for any service that involve the replication of multiple instances across the Internet. IP Anycast allows multiple instances of the same service to be “naturally” discovered, and requests for this service to be delivered to the closest instance. However, while briefly considered as an enabler for content delivery networks (CDNs) when they first emerged, IP Anycast was deemed infeasible in that environment. The main reasons for this decision were the lack of load awareness of IP Anycast and unwanted side effects of Internet routing changes on the IP Anycast mechanism. In this article we re-evaluate IP Anycast for CDNs by proposing a load-aware IP Anycast CDN architecture. Our architecture is prompted by recent developments in route control technology, as well as better understanding of the behavior of IP Anycast in operational settings. Our architecture makes use of route control mechanisms to take server and network load into account to realize load-aware Anycast. We show that the resulting redirection requirements can be formulated as a Generalized Assignment Problem and present practical algorithms that address these requirements while at the same time limiting connection disruptions that plague regular IP Anycast. We evaluate our algorithms through trace based simulation using traces obtained from a production CDN network.