Anycast.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. 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 one-to-many 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 one-to-many 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, On the Internet, anycast is usually implemented by using BGP (Border Gateway Protocol) The routing protocol that is used to span autonomous systems on the Internet. It is a robust, sophisticated and scalable protocol that was developed by the Internet Engineering Task Force (IETF). 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. Anycast is best suited to connectionless protocols like UDP UDP (uridine diphosphate): see uracil. (User Datagram Protocol) A protocol within the TCP/IP protocol suite that is used in place of TCP when a reliable delivery is not required. , rather than connection-oriented protocols such as TCP (1) (Transmission Control Protocol) The reliable transport protocol within the TCP/IP protocol suite. TCP ensures that all data arrive accurately and 100% intact at the other end. , 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 Also called "RAS" (reliability, availability, serviceability) or "fault resilient," it refers to a multiprocessing system that can quickly recover from a failure. There may be a minute or two of downtime while one system switches over to another, but processing will continue. and load balancing The fine tuning of a computer system, network or disk subsystem in order to more evenly distribute the data and/or processing across available resources. For example, in clustering, load balancing might distribute the incoming transactions evenly to all servers, or it might redirect them for stateless Refers to software that does not keep track of configuration settings, transaction information or any other data for the next session. When a program "does not maintain state" (is stateless) or when the infrastructure of a system prevents a program from maintaining state, it cannot take services such as access to replicated data, Use of anycast to implement DNS (Domain Name System) A system for converting host names and domain names into IP addresses on the Internet or on local networks that use the TCP/IP protocol. For example, when a Web site address is given to the DNS either by typing a URL in a browser or behind the 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 de·cen·tral·ize v. de·cen·tral·ized, de·cen·tral·iz·ing, de·cen·tral·iz·es v.tr. 1. To distribute the administrative functions or powers of (a central authority) among several local authorities. service. As a result most of the physical, rather than nominal, root servers are now outside the United States United States, officially United States of America, republic (2005 est. pop. 295,734,000), 3,539,227 sq mi (9,166,598 sq km), North America. The United States is the world's third largest country in population and the fourth largest country in area. RFC (Request For Comments) A document that describes the specifications for a recommended technology. Although the word "request" is in the title, if the specification is ratified, it becomes a standards document. 3258 (http://www ietforg/rfc/rfc3258.txt) documents how anycast is used to provide authoritative DNS service. 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. (Sec RFC 3068 (http://www,ietf.org/rfc/rfc3068,txt) 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 than 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 Anycast 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. 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 arc closer to this server than any other, and will not cause a global failure. |
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