Talking tech words with Neil Francis.....

Posted in: Networks

This month's buzzword comes from the networking team. Neil Francis talks about:



Neil says,

'Anycast is networking technique where the same IP prefix is advertised from multiple locations. The network then decides which location to route a user request to, based on routing protocol costs and possibly the 'health' of the advertising servers."Is it meaningful? Probably not unless you are a networking person. However there are several benefits to anycast which is why we use it at the University for some of our services.

Most people are probably familiar with the concept of a broadcast. In any information environment such as a network the source simply distributes content for anybody to receive. An analogy would be a radio or TV broadcast. The polar opposite of a broadcast is a unicast. This is communication between a single sender and a single receiver. A telephone call would be a typical example.

In IP networking both broadcast and unicast are used widely. Multicast is another methodology in which a single sender transmits information to multiple receivers who have expressed an interest in receiving that information. A common use of multicast is for IP streaming services for audio and video.

So where does anycast fit in? In an anycast addressing system IP packets are routed to any single member of a group of potential receivers which are all identified by the same IP address. This is the potentially confusing aspect of anycast. Most people are aware that anything on the Internet needs a unique IP address - similar to having a globally unique telephone number - or how would the network know how to route to you? In the world of anycast how does having multiple servers with the same IP address work?

Anycast works by having servers dynamically interact with upstream routing nodes and essentially sharing a single IP address. Multiple servers advertise the same IP address to their upstream router so when a request is sent to an anycasted IP address the routers will direct it to a single machine on the network that is closest. If a server crashed or is turned off it will stop advertising the IP address and traffic will be rerouted automatically elsewhere.

Anycast is not suitable for absolutely everything. We use it for DNS services at Bath and we are in good company. The most popular and well used DNS service on the Internet is Google's public DNS service using IP addresses of and globally. With approximately 400 billion requests a day being handled you can imagine this will not be achievable by a couple of servers no matter how large. The addresses are mapped to the nearest operational server situated all over the world by anycast routing.'

Posted in: Networks


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