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Managing Multicast in an Internetwork

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is that broadcast traffic goes to all hosts on a subnet, whereas multicast traf-
fic only goes to the hosts that request it. The distinguishing factor that puts
multicast traffic so far ahead of broadcast traffic in utility is the ability to
specify which multiple hosts will receive the transmission.

This isn't done magically; it doesn't know who and where the recipients

are just because it's multicast traffic. As with any application, protocols are
needed to make things happen. Multicast works on the basis of host sub-
scription to groups.

Several methods and protocols have been developed and implemented to

facilitate multicast functionality within the internetwork:

Subscribing groups

Maintenance groups

Joining groups

Leaving groups

Each of these protocols and methods is used for specific tasks or to achieve
specific results within the multicast environment.

We will now look at these protocols and learn just where they fit in and

what they are needed for. We will begin with the most important, subscrip-
tion and group maintenance, and then move on to enhancements for multi-
cast deployment and distribution.

Subscribing and Maintaining Groups

For multicast traffic to reach a host, that host must be running an application
that sends a request to a multicast-enabled router informing the router that
it wishes to receive data belonging to the specified multicast group. If this
request were to never take place, the router wouldn't be aware that the host
was waiting for data for the specified group.

As an overview, a multicast-enabled router receives all group advertise-

ments and routes. It listens on all interfaces, waiting for a request from a host
to forward multicast group traffic. Once a host on an interface makes a
request to become a member of a group, the interface activates the requested
group on itself and only on itself. While the host is a member, multicast data
will be forwarded to that interface and any host subscribed to the group will
receive the data.

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