Internet standard multicast
In Internet Standard Multicast (ISM), a receiver can receive messages from any source and from multiple sources. In ISM, receivers are not aware of sources; they express interest in receiving a traffic stream by subscribing to the group of interest (G).
In the ISM model, the multicast router discovers and tracks all multicast sources that are sending messages to an address, maintaining information about all sources, and routing data from all sources to all interested receivers. The router does this by maintaining a state table of (S, G) entries. When the number of sources is large, the burden on the multicast router can become heavy.