Multicast network architecture
IP multicast: principles and protocols

A little history

In the early 1990s, the very first routers began to appear. Often experimental and capable of routing IP multicast traffic, most routers on IP networks were not, however, capable of handling multicast flows.

To be routed, multicast packets had to be encapsulated in unicast packets and sent through "tunnels" established between routers implementing multicast processing functions. When they arrived at a multicast router, the unicast packets were decapsulated and the original multicast packets returned. This collection of non-contiguous multicast networks interconnected by multicast tunnels gave rise to the Multicast Backbone or MBONE, the global multicast network. In the late 1990s, the first ISPs began to replace these tunnels and implement native multicast routing.

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