IPv6 Multicast and Anycast Addressing
(Page 5 of 5)
Anycast addresses are a new, unique type of address that is new to IP in IPv6; the IPv6 implementation is based on the material in RFC 1546, Host Anycasting Service. Anycast addresses can be considered a conceptual cross between unicast and multicast addressing. Where unicast says send to this one address and multicast says send to every member of this group, anycast says send to any one member of this group. Naturally, in choosing which member to send to, we would for efficiency reasons normally send to the closest oneclosest in routing terms. So we can normally also consider anycast to mean send to the closest member of this group.
The idea behind anycast is to enable functionality that was previously difficult to implement in TCP/IP. Anycast was specifically intended to provide flexibility in situations where we need a service that is provided by a number of different servers or routers but don't really care which one provides it. In routing, anycast allows datagrams to be sent to whichever router in a group of equivalent routers is closest, to allow load sharing amongst routers and dynamic flexibility if certain routers go out of service. Datagrams sent to the anycast address will automatically be delivered to the device that is easiest to reach.
Perhaps surprisingly, there is no special anycast addressing scheme: anycast addresses are the same as unicast addresses. An anycast address is created automatically when a unicast address is assigned to more than one interface.
Like multicast, anycast creates more work for routers; it is more complicated than unicast addressing. In particular, the further apart the devices that share the anycast address are, the more complexity is created. Anycasting across the global Internet would be potentially difficult to implement, and IPv6 anycasting was designed for devices that are proximate to each other, generally in the same network. Also, due to the relative inexperience of the Internet community in using anycast, for the present time anycast addresses are used only by routers and not individual hosts.
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