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The
fundamental algorithm to construct these source-based trees is referred to as
Reverse Path Broadcasting (RPB).
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The
RPB operation is very simple.
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For
each source, if a packet arrives on a link that the local router believes to be
on the shortest path back toward the packet's source, then the router forwards
the packet on all interfaces except the incoming interface.
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If
the packet does not arrive on the interface that is on the shortest path back
toward the source, then the packet is discarded.
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The
interface over which the router expects to receive multicast packets from a
particular source is referred to as the "parent" link.
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The
outbound links over which the router forwards the multicast packet are called
"child" links for this source.
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The
key benefit to reverse path broadcasting is that it is reasonably efficient and
easy to implement.
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It
does not require that the router know about the entire spanning tree, nor does
it require a special mechanism to stop the forwarding process (as flooding does).
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In
addition, it guarantees efficient delivery since multicast packets always
follow the "shortest" path from the source station to the destination
group.
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Finally,
the packets are distributed over multiple links, resulting in better network utilization
since a different tree is computed for each source.
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One
of the major limitations of the RPB algorithm is that it does not take into
account multicast group membership when building the delivery
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