Anycast is a network addressing and routing method where incoming requests can be routed to different locations or nodes. In a content delivery network (CDN), anycast typically routes incoming traffic to the nearest data center that can process the request efficiently. Selective routing allows an anycast network to stand high traffic volume, network congestion, and even distributed denial-of-service (DDoS) attacks.

You can compare anycast with a telephone network switch that directs a customer call to an available customer service representative (CSR).

Read More about Anycast

Anycast is one of the four major types of Internet Protocol (IP) addressing methods or how traffic or requests are directed to servers or computers online. The following distinguishes one method from another:

  • Unicast: Sends a message to a specific node using a one-to-one association between a sender and a destination.
  • Broadcast: Sends a message to all nodes in a network using a one-to-all association.
  • Multicast: Sends a message to a group of nodes that expressed interest in receiving it using a one-to-many-of-many or many-to-many-of-many association.
  • Anycast: Sends a message to any one out of a group of nodes, almost always the one nearest the source, using a one-to-one-of-many association.

To better understand how the IP addressing methods work, look at the diagram below.

Anycast and other IP addressing methods

When Was Anycast First Used?

Anycast was first used for topological load-balancing of Internet-connected services in 1989. But its use was only formally documented by the Internet Engineering Task Force (IETF) four years later in RFC 1546. It was then first applied to critical infrastructure in 2001.

How Does Anycast Work?

Anycast network routing can route incoming connection requests across multiple data centers. When requests come to a single IP address associated with an anycast network, it distributes the data based on a prioritization methodology. The selection process for data centers is typically optimized to reduce latency by selecting one with the shortest distance from the requester.

What Are the Uses of Anycast?

Anycast grew in popularity due to the increased demand for high-availability network services. It is handy for:

  • Domain Name System (DNS): All Internet root name servers are clusters of hosts that use anycast addressing. Many commercial DNS providers switched to IP anycast to increase query performance and redundancy and implement load balancing.
  • IPv6 transition: Anycast addressing makes IPv6 addresses compatible with IPv4 hosts. That lets multiple providers implement IPv6 addresses without changing their IPv4 hosts.
  • CDNs: CDNs may use anycast for actual HyperText Transfer Protocol (HTTP) connections to their distribution centers or DNS.
  • Connectivity between anycast and multicast networks: Since anycast has redundancy and load-sharing capabilities, multiple anycast rendezvous points can route IP addresses automatically by selecting the closest rendezvous point for each source and receiver. Anycast thus makes a multicast network fault-tolerant.
  • Mitigation of DDoS attacks: After other DDoS mitigation tools filter out some attack traffic, anycast can distribute the remaining traffic across multiple data centers, preventing any one location from getting overwhelmed with requests. So long as the capacity of the anycast network is greater than the attack traffic, the attack is effectively mitigated.
  • Prioritizing local over global nodes: You can configure an anycast network to distinguish between local and global nodes and prioritize requests coming from local nodes.

Why Use an Anycast Network?

A server that receives several requests at once from the same origin server can get overwhelmed with traffic and become unable to respond efficiently to additional incoming requests. You can spread the load across other available data centers in an anycast network instead of making one origin server take the brunt of the traffic. This routing method can prevent an origin server from extending its capacity and thus avoid service interruptions.

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Anycast is, as you’ve learned, useful in mitigating DDoS attacks, which is helpful for any network that needs to stay up no matter what happens.

Key Takeaways

  • Anycast is a network addressing and routing method where incoming requests can be routed to different locations or nodes. In a CDN, anycast typically routes incoming traffic to the nearest data center that can process the request efficiently. Selective routing allows an anycast network to stand high traffic volume, congestion, and even DDoS attacks.
  • Anycast is one of the four major IP addressing methods, including unicast, broadcast, and multicast.
  • Anycast was first used for topological load-balancing of Internet-connected services in 1989. But IETF only formally documented its use four years later in RFC 1546. It was first applied to critical infrastructure in 2001.
  • Anycast makes the DNS work, eases IPv4 to IPv6 transition, makes CDNs work, provides connectivity between anycast and multicast networks, mitigates DDoS attacks, and can prioritize local over global nodes in a network.