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Information about Anycast

Routing Schemes
anycast
broadcast
multicast
unicast




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 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 (generally built on UDP), rather than connection-oriented protocols such as TCP that keep their own state, 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 stateful protocols where it is required that an entire session will use the same server, 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.

Use of anycast to implement DNS

A number of the Internet root nameservers are implemented as large numbers of clusters of machines using anycast. The C, F, I, J, K and M servers exist in multiple locations on different continents, using anycast announcements to provide a decentralized service. As a result most of the physical, rather than nominal, root servers are now outside the United States. RFC 3258 documents how anycast is used to provide authoritative DNS services.

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. (See RFC 3068 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. 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.

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. 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.

(D)DoS and anycast

Anycast on the internet can help to distribute DDoS attacks and reduce their effectiveness. As traffic is routed to the closest node (and the attacker has no control over this behaviour) the DDoS traffic flow will be distributed amongst the closest nodes. This often means that not all nodes will be affected. This is often an important reason to deploy anycast.

The effectiveness of this can be decreased, however, because unicast addresses (used for maintenance) are easy to recover. An attacker can then attack every node from any location, just as if there was no anycast and all nodes were separate servers.

Local vs Global

In 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.

External links

Routing (or routeing) is the process of selecting paths in a network along which to send data or physical traffic. Routing is performed for many kinds of networks, including the telephone network, the Internet, and transport networks.
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In computer networking, broadcasting refers to transmitting a packet that will be received (conceptually) by every device on the network. In practice, the scope of the broadcast is limited to a broadcast domain. Contrast unicasting and multicasting.
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Multicast is the delivery of information to a group of destinations simultaneously using the most efficient strategy to deliver the messages over each link of the network only once, creating copies only when the links to the destinations split.
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In computer networks, unicast is the sending of information packets to a single destination. "Unicast" is derived from the word broadcast, as unicast is the extreme opposite of broadcasting.
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In computing, an address space defines a range of discrete addresses, each of which may correspond to a physical or virtual memory register, a network host, peripheral device, disk sector or other logical or physical entity.
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Routing (or routeing) is the process of selecting paths in a network along which to send data or physical traffic. Routing is performed for many kinds of networks, including the telephone network, the Internet, and transport networks.
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In computer networks, unicast is the sending of information packets to a single destination. "Unicast" is derived from the word broadcast, as unicast is the extreme opposite of broadcasting.
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In computer networking, a broadcast address is an IP address that allows information to be sent to all machines on a given subnet rather than a specific machine. The exact notation can vary by operating system, but the standard is laid out in RFC 919.
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Multicast is the delivery of information to a group of destinations simultaneously using the most efficient strategy to deliver the messages over each link of the network only once, creating copies only when the links to the destinations split.
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One-to-many in communication is the act of publishing or broadcasting from one sender to many receivers.

One-to-many (also known as "to-many") relationships are often used when managing databases.
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Internet is a worldwide, publicly accessible series of interconnected computer networks that transmit data by packet switching using the standard Internet Protocol (IP). It is a "network of networks" that consists of millions of smaller domestic, academic, business, and government
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The Border Gateway Protocol (BGP) is the core routing protocol of the Internet. It works by maintaining a table of IP networks or 'prefixes' which designate network reachability among autonomous systems (AS). It is described as a path vector protocol.
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An IP address (Internet Protocol address) is a unique address that certain electronic devices use in order to identify and communicate with each other on a computer network utilizing the Internet Protocol standard (IP)—in simpler terms, a computer address.
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In telecommunications, connectionless describes communication between two network end points in which a message can be sent from one end point to another without prior arrangement.
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User Datagram Protocol (UDP) is one of the core protocols of the Internet protocol suite. Using UDP, programs on networked computers can send short messages sometimes known as datagrams (using Datagram Sockets) to one another.
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A connection-oriented networking protocol is one which identifies traffic flows by some connection identifier rather than by explicitly listing source and destination addresses.
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The Transmission Control Protocol (TCP) is one of the core protocols of the Internet protocol suite. TCP provides reliable, in-order delivery of a stream of bytes, making it suitable for applications like file transfer and e-mail.
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A stateless server is a server that treats each request as an independent transaction that is unrelated to any previous request.

Advantages

This simplifies the server design because it does not need to dynamically allocate storage to deal with conversations in progress or
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On the Internet, the Domain Name System (DNS) associates various sorts of information with so-called domain names; most importantly, it serves as the "phone book" for the Internet by translating human-readable computer hostnames, e.g. en.wikipedia.
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root name server is a DNS server that answers requests for the root namespace domain, and redirects requests for a particular top-level domain (TLD) to that TLD's nameservers.
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Internet Protocol version 6 (IPv6) is a network layer protocol for packet-switched internetworks. It is designated as the successor of IPv4, the current version of the Internet Protocol, for general use on the Internet.
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6to4 (sometimes written 6 to 4) is a system that allows IPv6 packets to be transmitted over an IPv4 network (generally the IPv4 internet) without the need to configure explicit tunnels.
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In cryptography, a man-in-the-middle attack (MITM) is an attack in which an attacker is able to read, insert and modify at will, messages between two parties without either party knowing that the link between them has been compromised.
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In computer networking a packet drop attack or blackhole attack is a type of denial-of-service attack accomplished by selectively dropping packets for a particular network destination.
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The Open Shortest Path First (OSPF) protocol is a hierarchical interior gateway protocol (IGP) for routing in Internet Protocol, using a link-state in the individual areas that make up the hierarchy.
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An interior gateway protocol (IGP) is a routing protocol that is used within an autonomous system.

In contrast an exterior gateway protocol is for determining network reachability between autonomous systems (AS) and makes use of IGPs to resolve route within an AS.
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The Border Gateway Protocol (BGP) is the core routing protocol of the Internet. It works by maintaining a table of IP networks or 'prefixes' which designate network reachability among autonomous systems (AS). It is described as a path vector protocol.
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The Réseaux IP Européens Network Coordination Centre (RIPE NCC) is the Regional Internet Registry (RIR) for Europe, the Middle East and parts of Central Asia.
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The Réseaux IP Européens Network Coordination Centre (RIPE NCC) is the Regional Internet Registry (RIR) for Europe, the Middle East and parts of Central Asia.
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