3Com SuperStack 4 Configuration Manual page 157

IP Multicast Overview 169
a distribution tree architecture. A multicast router can use multiple methods to
build up a path for data transmission, i.e., the distribution tree.
PIM-DM (Protocol-Independent Multicast Dense Mode, PIM-DM) PIM
dense mode is suitable for small networks. It assumes that each subnet in the
network contains at least one receiver interested in the multicast source. As a
result, multicast packets are flooded to all points of the network, consuming
network bandwidth and increasing router processing. To reduce network resource
consumption, branches that do not have members send Prune messages toward
the source to reduce the unwanted/unnecessary traffic. To enable the receivers to
receive multicast data streams, the pruned branches can be restored periodically to
a forwarding state. To reduce latency time, the PIM dense mode uses the prune
mechanism to actively restore multicast packet forwarding. Periodic flood and
prune are characteristics of PIM dense mode. Generally, the forwarding path in
dense mode is a "source tree" rooted at the source with multicast members as the
branches. Since the source tree uses the shortest path from the multicast source to
the receiver, it is also called the shortest path tree (SPT).
PIM-SM (Protocol-Independent Multicast Sparse Mode, PIM-SM) Dense
mode uses the flood-prune technology, which is not applicable for WAN. In WAN,
multicast receivers are sparse and therefore the sparse mode is used. In sparse
mode, hosts need not receive multicast packets unless, by default, there is an
explicit request for the packets. A multicast router must send a join message to the
RP (Rendezvous Point, which needs to be built into the network and is a virtual
place for data exchange) corresponding to the group for receiving the multicast
data traffic from the specified group. The join message passes routers and finally
reaches the root, i.e., the RP. The join message becomes a branch of the shared
tree. In PIM sparse mode, multicast packets are sent to the RP first, and then are
forwarded along the shared tree rooted at the RP and with members as the
branches. To prevent the branches of the shared tree from being deleted, PIM
sparse mode sends join messages to branches periodically to maintain the
multicast distribution tree.
To send data to the specified address, senders register with the RP first before
forwarding data to the RP. When the data reaches the RP, the multicast packets are
replicated and sent to receivers along the path of the distribution tree. Replication
only happens at the branches of the distribution tree. This process can be repeated
automatically until the packets reach the destination.
Forwarding IP Multicast In the multicast model, the source host sends information to the host group
Packets represented by the multicast group address within the destination address fields of
the IP packets. The multicast model must forward the multicast packets to multiple
external interfaces so that the packets can be sent to all receivers.
RPF (Reverse Path Forwarding)
To ensure that a multicast packet reaches the router along the shortest path, the
multicast must depend on the unicast routing table or a unicast routing table
independently provided for multicast to check the receiving interface of multicast
packets. This check mechanism is the basis for most multicast routing protocols
performing multicast forwarding, which is known as RPF (Reverse Path
Forwarding) check. A multicast router uses the source address from the multicast
packet to query the unicast routing table, or the independent multicast routing
table, to determine that the incoming interface on which the packet arrives is the