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.