Anycast

Anycast is a network addressing and routing methodology in which the same IP address (or address block) is advertised from multiple physical locations, and the routing system delivers any given packet to whichever instance is topologically nearest to the sender. It contrasts with unicast, which addresses a single endpoint, and multicast, which addresses a group of subscribers in parallel. In practice anycast is implemented by having multiple sites announce the same prefix into the global routing table via Border Gateway Protocol (BGP). Each upstream router selects one path to the prefix using its normal best-path algorithm, which typically prefers the shortest AS path; the chosen path leads to the closest replica, but different routers in the internet will pick different replicas, so the same IP is effectively served by many distinct machines. IPv6 reserves specific anycast address ranges in its architecture, but the BGP-prefix technique used in IPv4 works for both protocols and is what large operators actually deploy. The technique was first documented in a 1989 IETF draft on load balancing and formalized in 1993 in RFC 1546. It reached critical-infrastructure scale in 2001 when the I-root DNS server began serving the root zone from multiple anycast sites; today most of the thirteen DNS root servers are anycast clusters, as are many recursive resolvers such as 1.1.1.1 and 8.8.8.8. Content Delivery Network (CDN) use anycast to steer users to the nearest PoP without DNS-based load balancing, and the same property makes anycast services naturally resistant to volumetric DDoS attacks, because attack traffic is split across every site that announces the prefix instead of converging on a single target.