Anycast is a type of addressing in computer networks, in which you can address a single computer from an entire group of computers from a single address. It answers the one that is accessible via the shortest route. This technique is used in accordance with the OSI model to the network layer are used.
Anycast is realized by a distribution of several similar servers on physically separate IP networks. In practice it is often installed on any continent or in any country of a region at least one server. Each of these computer gets the same IP address and propagated a corresponding route via a routing protocol ( BGP in the Internet ). In case of failure or unavailability of the route disappears and all subsequent packets are routed to another server. The requested service can thus be performed even if one or more servers. This increases the availability increased. A server must also be addressed directly to the administration. Therefore, anycast servers have an additional unicast address in almost all cases.
From the perspective of a client between unicast and anycast no difference. Unlike multicast environments that produce packages for all group members, the sender generates only a single packet here. Anycast is a feature that finds in IPv6 support in particular. A packet with an anycast address is only one - in general the nearest - delivered to interface the group. Which one determines the entry in the routing table of the router nearest the sender.
This concept enables companies to use an isolated anycast address to deliver a package multiple routers of the provider, which member of a Anycastgruppe are - ie they have the same anycast address. If the company has several access points to the Internet, so the failure of one connection to the provider or the provider of a router can be immediately compensated for by another router of the same Anycastgruppe, which is then accessed via one of the alternative compounds. The station has, however, no way to choose the physical receiver ( the receiving interface), the target is determined solely by the routing protocol.
Analogously, in order to achieve load balancing, are always delivered in the packets from senders to the nearest server. If multiple servers a Anycastgruppe exist in different networks, can be achieved by means that the load is distributed to many servers without the knowledge or participation of the sender. This is used eg for the DNS root servers ( see below).
Most important advantages of anycast are load balancing and shorter access times. In addition, an almost complete transparency ( the user does not even notice that he communicates with an anycast server ) and excellent scalability ( it can always be added taken server or be removed). Since all servers involved must always provide identical data, the internal synchronization is expensive. Moreover, in case of fault, search is difficult because often it is not clear without further analysis, which server is responsible.
Anycast is used for example in some of the root name servers and top-level domain server as well as the IPv6 -in-IPv4 tunneling mechanism, 6to4. By anycast load balancing multiple distributed denial -of -service attacks on root nameservers 2006 and 2007 were largely unsuccessful.