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Reverse Path
Multicasting (RPM) is an enhancement to Reverse Path Broadcasting and Truncated
Reverse Path Broadcasting.
RPM creates a
delivery tree that spans only
1) subnetworks
with group members, and
2) routers and
subnetworks along the shortest path to those subnetworks.
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RPM
allows the source-based "shortest-path" tree to be "pruned"
so that datagrams are only forwarded along branches that lead to active members
of the destination group.
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When
a multicast router receives a packet for a (source, group) pair, the first
packet is forwarded following the TRPB algorithm across all routers in the
internetwork.
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Routers
on the edge of the network (which have only leaf subnetworks) are called leaf routers.
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The
TRPB algorithm guarantees that each leaf router will receive at least the first
multicast packet.
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If
there is a group member on one of its leaf subnetworks, a leaf router forwards
the packet based on this group membership information.
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If
none of the subnetworks connected to the leaf router contain group members, the
leaf router may transmit a "prune" message on its parent link,
informing the upstream router that it should not forward packets for this
particular (source, group) pair on the child interface on which it received the
prune message.
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Prune
messages are sent just one hop back toward the source.
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An
upstream router receiving a prune message is required to store the prune
information in memory.
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If
the upstream router has no recipients on local leaf subnetworks and has
received prune messages from each downstream neighbor on each of the child
interfaces for this (source, group) pair, then the upstream router does not
need to receive any more packets for this (source, group) pair.
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Therefore,
the upstream router can also generate a prune message of its own, one hop further
back toward the source.
Figure: Grafting
to cancel Pruning
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A
host may later decide to join a multicast group after a prune message has been
sent by its leaf router. In this case the leaf router may send graft message
to its upstream router to cancel its earlier prune message
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The
above figure shows the graft message flow when a host attached to router 6
wants to join the group.
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Subsequently,
router 1 will forward the multicast packets to router 4, which will forward the
multicast packets to router 6. then the multicast packets arrive at the host.
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