IP Multicast building blocks
The SENDERS send
Multicast Addressing - rfc1700
class D (224.0.0.0 - 239.255.255.255)
The RECEIVERS inform the routers what they want to
receive
Internet Group Management Protocol (IGMP) - rfc2236 ->
version 2
The routers make sure the STREAMS make it to the
correct receiving nets.
Multicast Routing Protocols (PIM-SM/SSM)
RPF  (reverse path forwarding) – against source address