Content delivery networks (CDNs) use anycast routing — an algorithm that advertises unique IP addresses on multiple nodes — to rapidly deliver website content to users on a global scale.
A CDN is comprised of strategically located points of presence (PoPs) that advertise similar IP ranges. The most direct route to a given host is determined and maintained by border gateway protocol (BGP) pairing.
At its core, anycast is a network addressing and routing method in which data is transferred from a single sender to any one of several nodes, which are often selected based on proximity and server health.
Unicast and Multicast – The Anycast Alternatives
Unicast and multicast, while useful in some scenarios, have practical limitations in relation to anycast.
With unicast, individual IPs are assigned to a node; static routes are used in connecting senders and receivers. A given request is always routed along the same path irrespective of its origin. But this becomes problematic if the node becomes overburdened or goes down, as the route becomes unavailable.
Furthermore, large scale requests for bigger files or applications — such as videos and software — can become resource intensive with respect to the many connections between a host server and the many nodes.
Using the multicast method, a request is routed to an intermediary node. There it’s identified and routed to several recipients. Unlike unicast, multicast can scale; the source sends a packet for distribution to a group of users. Therefore, multicast is a good solution for bigger streams of data, e.g., games, videos and online stock exchanges.
But like unicast, multicast routing also has its drawbacks. A data stream must be redirected if a node is overloaded or suddenly goes offline; significant latency is likely the result. In addition, it’s far more expensive to operate multicast nodes.
When compared with unicast and multicast routing, anycast offers many advantages, including:
Speedier connectivity – Being more direct in reaching an intermediary node, anycast uses fewer internet hops. A faster request round-trip time (RTT) reduces latency and boosts the user experience.
Reduced setup complexity – With anycast, you only need one DNS server configuration; it’s distributed to every one of your network nodes.
DDoS protection – Anycast offers built-in DDoS mitigation; failover options exist if a node is targeted or taken offline.
High availability – Multiple nodes advertise a given IP address to offer redundancy. If a node is overloaded or fails, backup automatically kicks in.
The CDN POV
Along with HTTP request routing via anycast, CDNs also offer anycast DNS resolution. Here, a number of name servers perform low-latency name lookup to ISP-resolving host names. This results in quicker name lookups and file downloads.
Your local ISP can select from several access points to find the optimal traffic route, providing users with faster connection times to your web application. This is coupled with a higher level of security, as well as emergency failover if a server should become overloaded.