Additionally, multicast is often used on corporate networks to deliver live video of important corporate events, such as when the CEO speaks to thousands of remote employees. For these and many other applications, multicast has enjoyed tremendous growth in recent years on IP VPN networks. However, on the Internet, multicast deployment has been an undeniable disappointment. While roughly 10% of the Internet is enabled for multicast, given the "all or nothing" nature of multicast, in most cases that 10% might as well be 0%.
A new hope: AMT
As the efforts toward deploying multicast on Internet networks stalled, a new solution emerged. Multicast advocates began to recognize and accept the reality that "encouraging" all networks to deploy the protocols necessary to support multicast was simply not practical. These advocates then noticed that IPv6 shared the same "all or nothing" properties of multicast.
Like multicast, isolated pockets of IPv6-enabled networks existed like islands within the ocean of the (IPv4-only) Internet. IPv6 architects had spent considerable effort developing transition mechanisms that would allow these IPv6 "islands" to connect to one another across the abyss of v6-disconnectedness. Multicast architects decided to leverage/steal one such idea and apply it to the multicast problem. This solution became known as Automatic IP Multicast Without Explicit Tunnels, or AMT.
AMT has enjoyed wide popularity since its inception and is seen by multicast advocates as the last, best hope for Internet multicast. AMT accepts the reality that unicast-only networks do exist, and simply allows end users to "hop" over those networks. To accomplish this, AMT uses tunnels to connect users on unicast-only networks to content on multicast-enabled networks.
To support this model, multicast-enabled providers deploy AMT Relays at the edge of their networks. These relays act as the tunnel endpoints that "translate" native (untunneled) multicast content to users on unicast-only networks. An AMT Relay can be a standalone server, or more commonly, can be functionality added to existing edge routers on a multicast network.
Users on the unicast-only network sit behind AMT Gateways, which use an autodiscovery mechanism known as anycast to locate the nearest relay and then initiate a multicast tunnel to this relay. The gateway then requests a multicast stream of interest through the tunnel.