Anycast

Anycast is a network addressing and routing methodology in which a single destination address has multiple routing paths to two or more endpoint destinations. Routers will select the desired path on the basis of number of hops, distance, lowest cost, latency measurements or based on the least congested route. Anycast networks are widely used for content delivery network (CDN) products to bring their content closer to the end user. Anycast is a network addressing and routing methodology in which a single destination address has multiple routing paths to two or more endpoint destinations. Routers will select the desired path on the basis of number of hops, distance, lowest cost, latency measurements or based on the least congested route. Anycast networks are widely used for content delivery network (CDN) products to bring their content closer to the end user. The Internet Protocol and other network addressing systems recognize five main addressing methods: Anycast can be implemented by using Border Gateway Protocol (BGP). Multiple hosts (usually in different geographic areas) are given the same unicast IP address and different routes to the address are announced through BGP. Routers consider these to be alternative routes to the same destination, though they are actually routes to different destinations with the same address. As usual, routers select a route by whatever distance metric is in use (the least cost, least congested, shortest). Selecting a route in this setup amounts to selecting a destination. The pitfall of this approach is that a connection to an anycast address may fail because the network can change the routing of packets in mid-connection due to congestion or changes in the network, with the result that the destination changes mid-connection, though the new destination is not aware of the connection and is not maintaining the connection state. These conditions are typically referred to as a 'PoP switch'. With a normal unicast address a routing change would not be a problem, as this merely results in a different route to the same eventual destination. All packets within a connection do not normally need to follow the same path. But when the address is actually an anycast address masquerading as a unicast address (as in this design), a routing change could be a destination change. Because of the possibility of a 'PoP switch' in IPv4, anycast is generally used with connectionless protocols based on UDP, as a way to provide high availability and load balancing for stateless services. For example, one of the well-known applications of IPv4 anycast is the Domain Name System (DNS). This is well-suited to anycast because it is a UDP-based service providing connectionless access to domain name data which is replicated across multiple, geographically-dispersed, stateless servers, with severe demands for availability and scalability. Because anycast is more error-prone with connection-oriented protocols such as TCP where the destination maintains state, it is less commonly used with these protocols. Some custom IP stacks use proprietary methods to provide healing of stateful protocols when necessary, mitigating problems due to anycast PoP switches. These methods usually involve some form of Network function virtualization in form of packet rewriting as has been demonstrated by AWS HyperPlane, and Google Maglev and/or packet encapsulation as laid out by protocols like Generic Routing Encapsulation, IPOP. Anycast is supported explicitly in IPv6. .mw-parser-output cite.citation{font-style:inherit}.mw-parser-output .citation q{quotes:''''''''''''}.mw-parser-output .citation .cs1-lock-free a{background:url('//upload.wikimedia.org/wikipedia/commons/thumb/6/65/Lock-green.svg/9px-Lock-green.svg.png')no-repeat;background-position:right .1em center}.mw-parser-output .citation .cs1-lock-limited a,.mw-parser-output .citation .cs1-lock-registration a{background:url('//upload.wikimedia.org/wikipedia/commons/thumb/d/d6/Lock-gray-alt-2.svg/9px-Lock-gray-alt-2.svg.png')no-repeat;background-position:right .1em center}.mw-parser-output .citation .cs1-lock-subscription a{background:url('//upload.wikimedia.org/wikipedia/commons/thumb/a/aa/Lock-red-alt-2.svg/9px-Lock-red-alt-2.svg.png')no-repeat;background-position:right .1em center}.mw-parser-output .cs1-subscription,.mw-parser-output .cs1-registration{color:#555}.mw-parser-output .cs1-subscription span,.mw-parser-output .cs1-registration span{border-bottom:1px dotted;cursor:help}.mw-parser-output .cs1-ws-icon a{background:url('//upload.wikimedia.org/wikipedia/commons/thumb/4/4c/Wikisource-logo.svg/12px-Wikisource-logo.svg.png')no-repeat;background-position:right .1em center}.mw-parser-output code.cs1-code{color:inherit;background:inherit;border:inherit;padding:inherit}.mw-parser-output .cs1-hidden-error{display:none;font-size:100%}.mw-parser-output .cs1-visible-error{font-size:100%}.mw-parser-output .cs1-maint{display:none;color:#33aa33;margin-left:0.3em}.mw-parser-output .cs1-subscription,.mw-parser-output .cs1-registration,.mw-parser-output .cs1-format{font-size:95%}.mw-parser-output .cs1-kern-left,.mw-parser-output .cs1-kern-wl-left{padding-left:0.2em}.mw-parser-output .cs1-kern-right,.mw-parser-output .cs1-kern-wl-right{padding-right:0.2em}RFC 4291, which covers IPv6 addressing architecture, reserves Interface Identifier 0 within an IPv6 subnet as the 'Subnet Router' anycast address. In addition, RFC 2526 reserves a block of 128 Interface Identifiers within a subnet as anycast addresses.