Multicasting is the Internet analog of broadcasting. Broadcasting per se is impractical on the Internet because the Internet selects destinations at the source, not the receiver. True broadcasts on the Internet would quickly saturate the medium as each "transmitter" would have to send packets directed to thousands or millions of potential recipients. Current Internet streaming audio "broadcasts" are actually unicasts where a recipient contacts the transmitter and arranges for a constant stream of data to be directed to his or her computer. Although Unicasting is feasible, widespread use would place heavy demands on transmitters and Internet backbones.

Multicasting uses the Internet, but makes a number of accommodations designed to get broadcast type material from sources to destinations while minimizing capacity loads. Packets use the UDP protocol rather than TCP. UDP does not include confirmation of reception and retransmission of missing material. Multicast requires downstream nodes to replicate packets to multiple destinations so that each node that is connected to multiple downstream nodes receives one copy of a multicast packet and sends out a copy to each connected node that it needs to go to.

Multicast uses dynamically assigned IP addresses in the "Class D" range to A single receiver may use multiple IP addresses for different functions. These addresses are over and above the receiver's normal IP addresses used for HTML, FTP, etc. Usage is defined in RFC 3171. Much of the routing control is accomplished by intermediate control points known as Routing Points (RPs). Multicasts can have multiple sources that are combined on the fly -- thereby permitting video conferencing and similar applications.

Multicasts are useful for presentations, multiuser games, as well as, potentially, distribution of entertainment.

Successful multicasting requires that all ISPs involved support multicasting by replicating packets for all users on their network(s). Some do. Some don't. As of 2001, Most don't. Multicast can be tunneled in packets of another message type, but most of the advantages are lost thereby. Link 120306

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