Anycast

The term is intended to echo the terms unicast, broadcast and multicast.

• In unicast, there is a one-to-one association between network address and network endpoint: each destination address uniquely identifies a single receiver endpoint.
• In broadcast and multicast, there is a many-to-one association between network addresses and network endpoints: each destination address identifies a set of receiver endpoints, to which all information is replicated.
• In anycast, there is also a many-to-one association between network addresses and network endpoints: each destination address identifies a set of receiver endpoints, but only one of them is chosen at any given time to receive information from any given sender.

Anycast is best suited to connectionless protocols like UDP, rather than connection-oriented protocols such as TCP, since the receiver selected for any given source may change from time to time as optimal routes change, silently breaking any conversations that may be in progress at the time.

For this reason, anycast is generally used as a way to provide high availability and load balancing for stateless services such as access to replicated data.

Table of contents

1 Use of anycast to implement DNS 2 Use of anycast to implement IPv6 transition 3 Security of anycast 4 Reliability of anycast

Use of anycast to implement DNS

A number of the Internet root nameservers are implemented as large numbers of clusters of machines using anycast. The C, F, I, J and K servers exist in multiple locations on different continents, using anycast announcements to provide a decentralized service. As a result most of the physical, rather than nominal, root servers are now outside the United States.

Use of anycast to implement IPv6 transition

There is a 6to4 (IPv6 transition protocol) anycast default gateway available with the IP address 192.88.99.1. (See RFC 3068 for details.) This allows multiple providers to implement 6to4 gateways without hosts needing to know each individual provider's gateway addresses.

Security of anycast

Anycast allows any operator whose routing information is accepted by an intermediate router to hijack any packets intended for the anycast address. Whilst this at first sight appears insecure, it is no different to the routing of ordinary IP packets, and no more or less secure. As with conventional IP routing, careful filtering of who is and is not allowed to propagate route announcements is crucial to prevent man-in-the-middle or blackhole attacks.

Reliability of anycast