What Is an Anycast?

Anycast, also known as anycast, anycast, or anycast, is a new type of communication service defined in IPv6 and one of the three major communication methods in IPv6. Proposed, but RFC1546's description of anycast is just an experimental service ^[1] .

The current definition of anycast technology is not very clear, but

Anycasting was originally proposed and defined in RFC1546. Its original semantics is to identify a group of specific services provided by an Anycast address on the IP network.

The characteristics of anycast communication mode make it have a certain application prospect in IP networks. First, the distributed services share the same IP address, and transparent service positioning is performed at the IP layer. This makes various network services, especially application layer services, more transparent, such as DNS. In IPv6 networks, it can share one Well-known IP addresses, users do nt need special configuration and do nt care which DNS server they are accessing. Secondly, the routing system chooses the recent service, which shortens the service response time and reduces the network load. Finally, the same The services are distributed redundantly on the network. The routing system can provide a mechanism to select a path with a relatively light load and a relatively high bandwidth to forward packets. This brings two benefits to users:

1. Reduced the impact of DDOS attacks on users. When any member or several members of the anycast group are attacked, the router responsible for packet forwarding can decide which member the packet should be forwarded to according to the response time of each group member. In this way, the attacked member does not respond because Therefore, the message will not be forwarded there. At the same time, due to the transparency of service access provided by anycast, group members are relatively difficult to be attacked by DDOS.

2. Reduces the impact of network congestion on users. In the same way as above, when any group member is in a congested network segment, its response time is longer, and the packet can be forwarded to the member with better response.

Anycast communication mode also has the following characteristics ^[2] :

1. The transparent service positioning anycast address can be used as a service identifier. Anycast communication group members share the same anycast address, and at the same time, transparent service positioning is performed at the IP layer, which makes various network services highly transparent. For example, the DNS service, each name server can share an anycast address, and users do not need special configuration or know which DNS server provides services when receiving the domain name service.

2. Load balancing When any member of anycast is congested, its response time is long, and then the data packet can be forwarded to the currently responding group member (that is, the current optimal group member). Therefore, anycast technology can save routing and link resources, reduce the impact of network congestion on customers, increase resource utilization efficiency, and avoid single points of failure.

3. Flexible service If the anycast group member who is providing the service is disconnected due to movement or some other reason, you can find the group member closest to the user to continue to provide services through anycast routing among other group members, which greatly guarantees the reliability of the service. Sex. (4) Automatic host configuration only needs to assign an Anycast address to the DNS server. When the client moves from one network to another network, there is no need to reconfigure the local DNS server for the client. The client can use the Anycast address to connect to the DNS server of any network. Communication.

Although anycast has certain advantages in the above aspects, the current application of anycast is not as expected. The use of anycast by the application layer mainly includes automatic server selection and automatic service positioning. The automatic server selection can be used when multiple mirror servers need to be provided for certain application layer services, such as FTP services and online game services. Service automatic location can be used when the host does not need special configuration and uses a well-known service, such as DNS. The network layer implements anycast mainly based on the anycast routing table to transparently forward packets and provide services to the upper layer, so that the application layer does not need a special anycast program module. Anycast routing table records the Unicast address of all members of anycast group. When the router receives a packet pointing to anycast address, it first searches the anycast routing table to find the corresponding anycast group, and then forwards the message to this On a member of the group. In view of the global demand for anycast at the application layer, research on the global anycast mechanism at the network layer has been facilitated.