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What is Anycast?

The Internet Protocol (IP) uses three types of addressing schemes: Unicast, Multicast, and Anycast.

A Unicast address is used to identify a single unique host. It is used to send data to a single destination. In computer networking, unicast communication is a one-to-one transmission from one point in the network to another. 

A Multicast address is used to deliver data to a group of destinations (a one-to-many transmission). IP multicast group addresses are represented by class-D IP addresses reserved specifically for multicast communications, ranging from through Any IP packet sent to a multicast address is delivered to only those hosts that have joined that particular IP Multicast group, resulting in less network traffic, thereby reducing bandwidth and network overhead. If the host hasn’t joined the group, the receiver ignores the packets at the hardware level, eliminating platform software resource consumption in that network element. IPv6 multicast replaces broadcast addresses that were supported in IPv4. 

Anycast, also known as IP Anycast or Anycast routing, is an IP network addressing scheme that allows multiple servers to share the same IP address, allowing for multiple physical destination servers to be logically identified by a single IP address. Based on the location of the user request, the anycast routers send it to the server in the network based on a least-cost analysis that includes assessing the number of hops, shortest distance, lowest transit cost, and minimum latency measurements to optimize the selection of a destination server.

Why use Anycast?

CDN service providers use Anycast to efficiently distribute content for faster network access in CDN networks. An anycast-enabled CDN assigns the same IP address to multiple edge servers, relying on IP routing to deliver requests to the servers that are nearby in the network to the clients originating the requests. Another CDN anycast methodology used is where anycast-based CDN load balancing provides access to replicated media content. In conjunction with routing protocols, Anycast can optimally route content requests to any one of the replicated content server nodes to maintain service scalability.

Another well-known Anycast use case is when used by the Domain Name System (DNS). The entry point to DNS is through DNS root servers, where all DNS requests can be processed. These servers are hosted as clusters of DNS servers that use Anycast addressing. This does two things: it allows routing algorithms to determine the “nearest” advertised location and send packets to it and creates built-in redundancy when a local DNS root server may be down. Many commercial DNS providers have switched to an IP anycast environment to increase DNS query performance and redundancy and provide efficient load balancing.

Anycast is associated with the core routing capabilities of the Border Gateway Protocol (BGP). The BGP anycast IP address or prefix is advertised from multiple locations. This route propagates across the Internet, enabling BGP to “advertise” awareness of the shortest path to the advertised prefix and publish multiple secondary sources paths to reach the destination IP address. This enables picking an anycast server “relatively close” to the location of a user’s data request.

ThousandEyes can be used to monitor critical network topology, including CDNs, DNS, load balancers, and routing elements that utilize anycast communications. If you are currently relying on a CDN provider to serve your content, it is critical to monitor and validate your CDN provider’s performance. By monitoring anycast-based CDNs, performance metrics, such as end-to-end latency, can be readily measured to understand which data center is best serving content.

If you are interested in learning more about how our customers use ThousandEyes to monitor their anycast CDNs, take a look at Twitter's experience. For more information on how ThousandEyes can help you better optimize your network in combination with cloud services and understand what’s critical to meet your needs, please visit www.thousandeyes.com.

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