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Anycast is a network addressing and routing scheme whereby data is routed to the "nearest" or "best" destination as viewed by the routing topology. The term is intended to echo the terms unicast, broadcast and multicast.
On the Internet, anycast is usually implemented by using BGP to simultaneously announce the same destination IP address range from many different places on the Internet. This results in packets addressed to destination addresses in this range being routed to the "nearest" point on the net announcing the given destination IP address. In the past, Anycast was suited to connectionless protocols (generally built on UDP), rather than connection-oriented protocols such as TCP that keep their own state. However, there are many cases where TCP Anycast is now used. With TCP Anycast, there are cases where 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. These conditions are typically referred to as a "pop switch". To correct for this issue, there have been proprietary advancements within custom IP stacks which allow for healing of stateful protocols where it is required. However, without any technology to heal pop switches, systems like GeoDNS are more appropriate. 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; for example, DNS service is a distributed service over multiple geographically dispersed servers.
[edit] Use in DNSA number of the Internet root nameservers are implemented as clusters of hosts using anycast addressing. The C, F, I, J, K, L and M servers exist in multiple locations on different continents, using anycast address announcements to provide a decentralized service. This has accelerated the deployment of physical (rather than logical) root servers outside the United States. RFC 3258 documents how anycast is used to provide authoritative DNS services. Autonomica, Community DNS, DNSMadeEasy, Dynect, easyDNS, BlueCat Networks, Netriplex, Neustar Ultra Services (formerly UltraDNS), No-IP, Packet Clearing House, and many other authoritative name service providers have switched to an IP anycast environment to increase query performance and redundancy. The use of the IP anycast addressing helps in providing highly resilient DNS services. Recursive DNS services, such as DNS Advantage[1], OpenDNS[2], DynDNS Internet Guide[3]and Google Public DNS[4], also use anycast to distribute the load across their network. [edit] Use in IPv6 transitionIn IPv4 to IPv6 transitioning anycast addressing may be deployed to provide IPv4 compatibility to IPv6 hosts. This method, 6to4, uses a default gateway with the IP address 192.88.99.1, cf. RFC 3068. This allows multiple providers to implement 6to4 gateways without hosts having to know each individual provider's gateway addresses. [edit] Security of implementationsAnycast allows any operator whose routing information is accepted by an intermediate router to hijack any packets intended for the anycast address. While this at first sight appears insecure, it is no different from 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. [edit] ReliabilityAnycast is normally highly reliable, as it can provide automatic failover. Anycast applications typically feature external "heartbeat" monitoring of the server's function, and withdraw the route announcement if the server fails. In some cases this is done by the actual servers announcing the anycast prefix to the router over OSPF or another IGP protocol. If the servers die, the router will automatically withdraw the announcement. "Heartbeat" functionality is important because, if the announcement continues for a failed server, the server will act as a "black hole" for nearby clients; this failure mode is the most serious mode of failure for an anycast system. Even in this event, this kind of failure will only cause a total failure for clients that are closer to this server than any other, and will not cause a global failure. [edit] Denial of Service attacks and anycastAnycast on the Internet can help to distribute DDoS attacks and reduce their effectiveness: As traffic is routed to the closest node, a process over which the attacker has no control, the DDoS traffic flow will be distributed amongst the closest nodes. This can mean that not all nodes will be affected. This may be a reason to deploy anycast addressing. The effectiveness of this technique to divert attacks is questionable, however, because unicast addresses (used for maintenance) can be easy to obtain, at least on IPv6. RFC 2373 defines that "An anycast address must not be used as the source address of an IPv6 packet." Therefore, pinging an anycast address will return the unicast address of the closest node, since the reply must come from a unicast address. An attacker can then attack individual nodes from any location, bypassing anycast addressing methods. This same method works on some, but not all, IPv4 anycast addresses [1]. [edit] Local and global nodesIn some situations of anycast deployment on the Internet there is a difference between local and global nodes. Local nodes are often more intended to provide benefit for the direct local community. Local node announcements are often announced with the no-export BGP community to prevent peers from announcing them to their peers (i.e. the announcement is kept in the local area). Where both local and global nodes are deployed, the announcements from global nodes are often AS prepended (i.e. the AS is added a few more times) to make the path longer so that a local node announcement is preferred over a global node announcement. Both F and K root name servers currently use local and global nodes. [edit] References
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