What Is Anycasting?

Date First Published: 8th June 2022

Topic: Computer Networking

Subtopic: Data Transmission Technologies

Article Type: Computer Terms & Definitions

Difficulty Level: 6/10

Learn more about what anycasting is in this article.

Anycasting is a network addressing and routing method where a group of nodes share the same IP addresses and messages can be sent to a range of different nodes. As suggested in the term, anycasting allows messages to be sent to any node in the group, depending on its distance and its capability to process the request. Anycasting is commonly used in CDN to route incoming traffic to the nearest server, reducing the distance between the users and the data centres, speeding up the delivery of content and reducing latency. Anycast routing was first used in 1989 and four years later, it was defined in RFC 1546.

The steps of anycasting are:

• The user makes the request.
• The anycast router automatically determines the best destination, based on a variety of factors.
• The user receives the same data, regardless of destination.

IPv4 and IPv6

In IPv4, anycasting can be implemented through the use of the Border Gateway Protocol (BGP). Multiple hosts are given the same unicast IP address and different routes to the address are determined by BGP. Routers determine the best route based on a variety of factors.

In IPv6, it is supported in RFC 4291. This covers IPv6 addressing architecture and reserves Interface Identifier 0 within an IPv6 subnet as the ‘Subnet Router’ anycast address.

Why Are Anycast Networks Used?

The main reason why anycast networks are used is to prevent one server from becoming overwhelmed with traffic and extending its capacity, causing service interruptions for users. With an anycast network, instead of one server handling all of the traffic, several different servers are used to handle and process each request. The load is spread across different data centres.

In addition, if a CDN used a unicast network, where traffic is routed to one specific node, it would be vulnerable to downtime when it becomes overwhelmed with traffic, such as in a DDoS attack. The use of anycasting results in a resilient network, since traffic will automatically find the best path. If a data centre was taken offline, traffic would automatically flow to the next data centre, based on distance and its capability to handle and process requests.