Application-Layer Anycasting

Abstract

Server replication is a key approach for maintaining user-perceived quality of service within a geographically wide-spread network. The anycasting communication paradigm is designed to support server replication by allowing applications to easily select and communicate with the "best" server, according to some performance or policy criteria, in a group of content- equivalent servers. We examine the definition and support of the anycasting paradigm at the application layer, providing a service that maps anycast domain names into one or more IP addresses using anycast resolvers. In addition to being independent from network-layer support, our definition includes the notion of filters, functions that are applied to groups of addresses to affect the selection process. We consider both metric-based filters (e.g., server response time) and policy-based filters; we further allow filtering both at the anycast resolver and local to the anycast client. A key input to the filtering process is metric information describing the relative performance of replicated servers. We examine the use of various techniques for maintaining this information at anycast resolvers.