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

Chapter 287
A Host Sending to a Group

When a host transmits a multicast packet to a specific group, the designated router (chosen by IGMPv2) forwards the multicast datagram as a unicast datagram to the RP. This unicast datagram is the multicast datagram encapsulated as a PIM-SM-Register packet. This type of packet informs the RP of a new source. The RP strips off the encapsulated (register) headers and redistributes the multicast datagram out the delivery tree. The active RP for that source transmits PIM-JOIN messages back to the source station’s DR. The routers lying between the source’s DR and the RP maintain the path information by the received PIM-JOIN messages. This is done so that when nonregistered encapsulated packets are received, they will know what interfaces to forward them on. The RP will send the unicast datagram back out as a multicast datagram across the group’s multicast tree. The source’s DR will continue to encapsulate the multicast datagrams and send them to the RP. When the DR receives a Register-Stop message from the RP (the RP sends these messages if the RP has no downstream receivers for the group or for that source) it will also send Register-Stop messages if the RP has already joined the (S,G) tree and is receiving the data packets natively (unencapsulated). A timer is used by the DR and if this timer expires, it will start to resend the multicast datagrams encapsulated in Register messages.

The slide shows the sequence of a host sending to a group.

A Host Sending to a Group