IP multicast provides an effective method of many-to-many communication. Delivering unicast datagrams is fairly simple. Normally, IP packets are sent from a single source to a single recipient. The source inserts the address of the target host in the IP header destination field of an IP datagram; intermediate routers (if present) simply forward the datagram toward the target in accordance with their respective routing tables.
Sometimes, distribution needs individual IP packets be delivered to multiple destinations (like audio or video streaming broadcasts). Multicast is a method of distributing datagrams sourced from one or more hosts to a set of receivers that may be distributed over different (sub) networks. This makes delivery of multicast datagrams significantly more complex.
Multicast sources can send a single copy of data using a single address for the entire group of recipients. The routers between the source and recipients route the data using the group address route. Multicast packets are delivered to a multicast group. A multicast group specifies a set of recipients who are interested in a particular data stream and is represented by an IP address from a specified range. Data addressed to the IP address is forwarded to the members of the group. A source host sends data to a multicast group by specifying the multicast group address in the datagram’s destination IP address. A source does not have to register to send data to a group nor do they need to be a member of the group.
Routers and Layer 3 switches use the Internet Group Management Protocol (IGMP) to manage membership for a multicast session. When a host wants to receive one or more multicast sessions it sends a join message for each multicast group it wants to join. When a host wants to leave a multicast group, it sends a leave message.
To extend multicast to the Internet, the multicast backbone (Mbone) is used. The Mbone is layered on top of portions of the Internet. These portions, or islands, are interconnected using tunnels. The tunnels allow multicast traffic to pass between the multicast-capable portions of the Internet. As more and more routers on the Internet are multicast-capable (and scalable), the unicast and multicast routing table converge.
The original Mbone was based on Distance Vector Multicast Routing Protocol (DVMRP) and was very limited. The Mbone is, however, converging around the following protocol set:
IGMP
Protocol Independent Multicast (Sparse Mode) (PIM-SM)
Border Gateway Protocol with multi-protocol extensions (MBGP)
Multicast Source Discovery Protocol (MSDP)