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1.5 Where Is Multicast?

The Multicast Backbone, or MBone, refers to the networks on the Internet that are enabled for multicast. The original MBone was built in the early 1990s as a network of multicast-enabled routers that were connected by tunnels. These routers were frequently UNIX servers running multicast routing software developed before router vendors had stable implementations of multicast software.

Tunnels allowed these early multicast-enabled "islands" to appear to be virtually connected to one another. Multicast packets were encapsulated within unicast packets and sent in the tunnel. Routers that were not multicast-enabled simply saw the unicast IP packet and routed it toward the tunnel destination. When the unicast packet reached the tunnel destination, the router decapsulated the unicast header to find the multicast packet within. If that packet had to be forwarded to another tunneled router, it was once again encapsulated and sent out another tunnel.

As router vendors implemented more stable multicast routing code, ISPs began to replace tunnels with native multicast routing in the late 1990s. Native multicast routing means routers forward raw multicast packets without encapsulating the multicast data within unicast packets. Most of the world's largest ISPs are multicast-enabled in at least some portion of their production networks today.

Multicast Internet Exchanges (MIXs) were built to connect multicast-enabled ISPs. MIXs are usually found in network access points (NAPs) where ISPs publicly peer with one another. A MIX enables ISPs to exchange multicast traffic on separate equipment from what is used for unicast peering. SprintNAP, in Pennsauken, New Jersey, and the NASA Ames Research Center Federal Internet Exchange (FIX-West), in Mountain View, California, contain two of the most popular MIXs used for public multicast peering.

Most people think of the old tunneled network of UNIX boxes when they hear the word "MBone," but it technically refers to any network that is multicast-enabled. Unanimous agreement has not been reached on a catchy word or phrase to colloquially refer to the native multicast-enabled portion of the Internet.

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