Question

Explain differences between IPv6 Anycast and Multicast. Give practical examples.

Answer 1

1. An IPv6 anycast address is an address that can be assigned to more than one interface (typically different devices). In other words, multiple devices can have the same anycast address. A packet sent to an anycast address is routed to the “nearest” interface having that address, according to the router’s routing table.

For example, servers A, B, and C in below figure could be DHCPv6 servers with a direct Layer 3 connection into the network. These servers could advertise the same /128 address. The router nearest the client request would then forward packets to the nearest server identified in the routing table.

Anycast is a bit special since Unicast layers on top of it. Anycast is announcing the same network in different parts of the network, in order to decrease the network hops needed to get to that network.

Ex: The 6to4 IPv6 transition protocol uses Anycast. 6to4 gateways announce their presence on a specific IP, 192.88.99.1. Clients looking to use a 6to4 gateway send traffic to 192.88.99.1 and trust the network to deliver the connection request to a 6to4 router.

NTP services for especially popular NTP hosts may very well be anycasted, but I don't have proof of this. There is nothing in the protocol to prevent it.

Other services use Anycast to improve data locality to end users. Google does Anycast with its search pages in some places (and geo-IP in others). The Root DNS servers use Anycast for similar reasons. ServerFault itself just might go there, they do have datacenters in New York and Oregon, but hasn't gone there yet.

2. Multicast is a technique in which a device sends a single packet to multiple destinations simultaneously (one-to-many). (Remember that a unicast address sends a single packet to a single destination [one-to-one].) Multiple destinations can actually be multiple interfaces on the same device, but they are typically different devices.

Ex: Inside a corporate network, Multicast can deliver live video to multiple nodes without having to have massive bandwidth on the part of the server delivering the video feed. This way you can have a video server feeding a 720p stream on only a 100Mb connection, and yet still serve that feed to 3000 clients.

HP printers announce their presence on a multicast group by default. With the right tools, it makes it real easy to learn what printers are available on your network.

The NTP protocol also allows a multicast method (IP 224.0.1.1) for announcing time sources to areas beyond just the one subnet.