3Com 10014299 manual IP Multicast

502CHAPTER 33: IP MULTICAST

Multicast Routing The group address in the multicast protocol is a virtual address. Therefore, unlike Protocol unicast, data packets cannot be routed directly from the data source to the

specific destination address. The multicast application program sends the data packet to a group of receivers instead of a single receiver .

Multicast routing establishes a cyclic data transmission path from one data source end to multiple receiving ends. The task of the multicast routing protocol is to establish a distribution tree structure. The multicast routers can adopt many methods to establish a data transmission path distribution tree. Protocol Independent Multicast (PIM) is the protocol that allows multicast routers to identify other multicast routers that will receive the packets. Depending on actual network conditions, the multicast routing protocol can be divided into two kinds - dense mode and sparse mode.

Protocol Independent The dense mode of the multicast routing protocol is suitable for small networks Multicast--Dense Mode with abundant bandwidth. Suppose that each subnet in the network has at least a

(PIM-DM))pair of receiving sites interested in multicast. Therefore, multicast data packet are distributed to all the sites in the network. Together with this process there is consumption of the related resources (bandwidth and the CPU of the router). To decrease the consumption of these precious network resources, the dense mode of the multicast routing protocol “prunes” the branches that do not have multicast data forwarding, and retains only the branches that contain the receiving sites.

To enable the receiving sites with the multicast forwarding demand in the pruned branches to receive multicast data flow, the pruned branches can return to forwarding state periodically. To reduce the time delay for the pruned branch to recover to the forwarding state, the dense mode of the multicast routing protocol adopts a grafting mechanism to actively add to the multicast distribution tree. This cyclic diffusion and pruning phenomenon is the feature of the dense mode of the multicast routing protocol. Generally, the data packet forwarding path in the dense mode is an “active tree” with the source being its root and the group members being its leaves.

The typical routing protocol in the dense mode includes Protocol-Independent Multicast-Dense Mode (PIM-DM) and Distance Vector Multicast Routing Protocol (DVMRP).

Protocol Independent Dense mode uses the flood-prune technology, which is not applicable for a WAN. Multicast-Sparse Mode In a WAN, multicast receivers are sparse and the sparse mode is used. In sparse

(PIM-SM)mode, all hosts do not need to receive multicast packets unless there is an explicit request for the packets by default. A multicast router must send a join message to the rendezvous point (RP), which is created in the network as the virtual place for data exchange. The RP corresponds to the group that receives the multicast data traffic from the specified group. The join message passes routers and finally reaches the root, the RP. The path that the join message used 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 because they are not updated, PIM sparse mode sends join messages to branches periodically to maintain the multicast distribution tree.