Illustrated TCP/IP by Matthew G. Naugle Wiley Computer Publishing, John Wiley & Sons, Inc. ISBN: 0471196568   Pub Date: 11/01/98

Chapter 282
PIM—Dense Mode Operation

When a multicast datagram is received, its incoming interface is looked up in the unicast routing table; therefore, the router must be running some type of unicast routing protocol (remember the name, Protocol Independent Multicast). If the receiver interface is the one on which the router forwards unicast datagrams back to that subnet, the multicast datagram is accepted and forwarded to all ports except the incoming interface. If not, the datagram is simply discarded without error messages being sent (silently discarded). From here, the router checks for a forwarding state for the group address. If there is not an entry for the group address, the router adds one. The router checks the outgoing interface list to see whether it should forward the datagram. This list contains a listing of interfaces from which the router has heard group membership or PIM router messages. The PIM-DM router’s messages can be Hello, Prune, Join, or Graft. If there is an active interface(s), the router forwards the datagram out those interfaces. If no interfaces are indicated, a Prune message is sent.

The intended receiver router of that Prune message will be placed in the message (not in the IP header). The downstream router knows this address by doing an RPF lookup in the unicast routing table. When the receiver router receives this Prune request, it will schedule a deletion of that LAN interface for that group, which means it inserts a delay before deletion. It is waiting to see if any other routers respond. Other routers on the subnet will also receive this Prune message and will in turn send a Join message to that router, forcing it to cancel the deletion of the LAN interface for the (source, group) pair. Just because one router prunes doesn’t mean there aren’t other routers on that same LAN that want to continue receiving information for the group address.

PIM—Dense Mode Operation

No entries in the outgoing list could be a result of no group members on the interface and the router not receiving any PIM-Hello messages from other routers located on that subnet (this allows for leaf network detection, in that in absence of these messages, only multicast hosts reside on a subnet). A router will keep track of the leaf members (local-group database built by IGMP) and will also contain a listing of routers as well. When a router is not heard from within a specified amount of time, the router deletes that router’s entry from the list.

Pruned states for any multicast entry are eventually timed out, forcing all multicast datagrams to be forwarded on all interfaces again, until the multicast trees are pruned.