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To deliver multicast packets to all receivers,
designated routers construct a tree that connects all members of an
IP multicast group. A distribution tree specifies a unique
forwarding path between the subnet of the source and each subnet
containing members of the multicast group.
A distribution tree has just enough
connectivity so that there is only one, loop-free path between every
pair of routers.
Because each router knows which of its lines
belong to the tree, the router can copy an incoming multicast
datagram onto all the outgoing branches. This action generates the
minimum needed number of datagram copies. Because messages are
replicated only when the tree branches, the number of copies of the
messages transmitted through the network is minimized.
Because multicast groups are dynamic, with
members joining or leaving a group at any time, the distribution
tree must be dynamically updated. Branches that contain new members
must be added. Branches in which no listeners exist must be
discarded, or pruned.
There are two basic tree construction techniques:
source-specific trees and shared, or center-specific trees.
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