Manual

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Chapter 29 IPv6 Multicast Protocol
29.1 PIM-DM6
29.1.1 Introduction to PIM-DM6
PIM-DM6(Protocol Independent Multicast, Dense Mode) is the IPv6 version of Protocol
Independent Multicast Dense Mode. It is a Multicast Routing Protocol in dense mode which
adapted to small network. The members of multicast group are relatively dense under this kind
of network environment. There is no difference compared with the IPv4 version PIM-DM
except that the addresses it uses are IPv6 addresses. Thus we don’t differentiate between
PIM-DM and PIM-DM6 in this chapter. All PIM-DM in the text without specific explanation
refers to IPv6 version PIM-DM.
As a result of continuous development of IPv6 network, it has the network environment of
nonsupport IPv6 multicast sometimes, so it needs to do the IPv6 multicast operation by tunnel.
Therefore, our PIM-DM6 supports configuration on configure tunnel, and passes through
nonsupport IPv6 multicast network by single cast packet of IPv4 encapsulation.
The working process of PIM-DM can be summarized as: Neighbor Discovery, Flooding-Prune,
and Graft.
1. Neigh hour Discovery
When PIM-DM router is started at beginning, Hello message is required to discover neighbors.
The network nodes running PIM-DM use Hello message to contact each other. PIM-DM Hello
message is sent periodically.
2. Flooding-Prune
PIM-DM assumes that all hosts on the network are ready to receive multicast data. When
certain multicast source S begins to send data to a multicast group G, after receiving the
multicast packet, the router will make RPF examination first according to the unicast table. If
the check passes, the router will create a (S, G) table item and forward the multicast packet to
all downstream PIM-DM nodes (Flooding). If the RPF examination fails, i.e. the multicast
packet is inputted from the incorrect interface, and then the message is discarded. After this
procedure, every node will create an (S, G) item in the PIM-DM multicast domain. If there is no
multicast group member in the downstream nodes, then a Prune message is sent to upstream
nodes notifying not to forward data to this multicast group any more. After receiving Prune
message, the corresponding interfaces will be deleted from the output interface list
corresponding with the multicast-forwarding item (S, G). Through this process, a SPT (Shortest
Path Tree) is established with source S as root. Prune process is started by a sub-router.
The process above is called Flooding-Prune process. Each pruned node also provides
overtime mechanism at the same time. In case of overtime of prune, the router will restart
flooding-prune process. Flooding-prune of PIM-DM is conducted periodically