Content

| 7.3 |  | Routing Multicast Traffic |
| 7.3.1 |  | Overview |

The campus network is composed of a collection of subnetworks connected by routers. When the source of a video data stream is located on one subnet and the host devices are located on different subnets, there needs to be a way of determining how to get from the source to the destinations. This is the function of the Internet Protocol (IP).

Each host on the Internet has an address that identifies the physical location of the host. Part of the address identifies the subnet on which the host resides, and part identifies the individual host on that subnet. Routers periodically send routing update messages to adjacent routers, conveying the state of the network as perceived by that particular router. This data is recorded in routing tables that are then used to determine optimal transmission paths for forwarding messages across the network.

Unicast transmission involves transmission from a single source to a single destination. The transmission is directed toward a single physical location that is specified by the host address. This routing procedure is relatively straightforward because of the binding of a single address to a single host.

Routing multicast traffic is a more complex problem. A multicast address identifies a particular transmission session, rather than a specific physical destination. An individual host is able to join an ongoing multicast session by using IGMP to communicate this desire to the subnet router.

Because the number of receivers for a multicast session can potentially be quite large, the source should not need to know all the relevant addresses. Instead, the network routers must somehow be able to translate multicast addresses into host addresses. The basic principle involved in multicast routing is that routers interact with each other to exchange information about neighboring routers.