3Com 10014298 PIM-SM Protocol-Independent Multicast Sparse Mode, RPF Reverse Path Forwarding

IP Multicast Overview 113

The multicast routing creates a loop-free data transmission path from one data source to multiple receivers. The task of the multicast routing protocol is to create 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 dense mode is suitable for small networks. It assumes that each subnet in the network contains at least one receiver who is interested in the multicast source. Multicast packets are flooded to all points of the network. Subsequent resources (such as bandwidth and CPU of routers) are consumed. In order to decrease the consumption of these precious network resources, 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. The periodical 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 and the receiver, it is also called the shortest path tree (SPT).

■PIM-SM (Protocol-Independent Multicast Sparse Mode)

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 multicast packets to multiple external interfaces so that the packets can be forwarded 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. This check mechanism is the basis for most multicast routing protocols , which is known as a 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