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

Chapter 284
PIM—Sparse Mode (PIM-SM)

PIM-SM was designed to restrict multicast traffic only to those routers that have a need for the multicast packet. In PIM-SM, a specific router (for redundancy and scalability, some PIM-SM implementations allow for more than one rendezvous point, but that is beyond the scope of this book) is known as the rendezvous point (RP). Senders and receivers join a multicast group by registering at the rendezvous router. Routers find out their RP (explained later on slide 313) and then send received multicast datagrams as unicast datagrams to the RP. The RP router redistributes multicast datagrams out the group trees that it has built. A rendezvous point is simply an IP address of a single router and is used by senders to announce themselves and for receivers to find out about new senders for a group.

All routers running PIM periodically (every 30 seconds by default) transmit Hello messages to each other, for the purpose of discovering other PIM routers using 224.0.0.13 (ALL_ PIM_ROUTERS group address). This is local multicast that is in the range of not being allowed to traverse a router. When a PIM router receives this message, it stores the IP address for that neighbor. Each PIM router entry will have its own timer for repeat Hello messages. This time is included in the received Hello message and the router will note this time in its table (set to 3.5 * Hello Period (30 seconds) or default to 105 seconds). If the router does not periodically hear from the neighbor, it will time-out and delete that neighbor from the table. When the DR (selected by IGMPv2) receives a new entry (a new router), it unicasts its most recent RP address information to the new neighbor.

A router known as the designated router (the DR, usually an IGMPv2 function) is responsible for sending Join/Prune commands to the RP on behalf of its local receivers and sources. The choice of the DR is not based on the IGMP querier, nor is it based on the long-term, last-hop router for the group. The router with the highest IP address within all the received Hello messages is elected DR. The last-hop router is the last router to receive multicast messages before they are delivered to the local receivers. If this is the case, then this router will be the DR.