Multicast is a the delivery of information to multiple destinations simultaneously. Typically used to refer to IP Multicasting, which is a protocol for efficiently sending to multiple receivers at the same time on TCP-IP networks, by use of a multicast address. It's also commonly associated with audio/video protocols such as RTP.
By comparison with multicast, conventional point-to-single-point delivery is called unicast.
There are two basic kinds of multicast delivery:
- dense multicast
- sparse multicast
The IP multicast model is not the only model for multiparty communications. Some connection-oriented link layers, such as Asynchronous Transfer Mode, have built-in mechanisms for "point-to-multipoint" or "multipoint-to-multipoint" connections. This model generally assumes that the stations participating in the communication are known ahead of time, so that distribution trees can be generated and resources allocated by network elements in advance of actual data traffic. By contrast, the IP multicast model has been described by Internet architect Dave Clark as "You put packets in at one end, and the network conspires to deliver them to anyone who asks."
While IP multicast is an extremely satisfying conceptual model, it requires a great deal more state inside the network than IP unicast model of best-effort delivery does, and this has been the cause of some criticism. What is worse, no mechanism has yet been demonstrated that would allow the IP multicast model to scale to millions of senders and millions of multicast groups as would be necessary to make fully-general multicast applications practical in the commercial Internet. As of 2003, most efforts at scaling multicast up to large networks have concentrated on the simpler case of single-source multicast, which seems to be more computationally tractable.
For both this reason and also reasons of economics, multicast is not in general use in the commercial Internet.
However, some communities within the public Internet make regular use of multicast (see the MBONE for an example), and multicast is used for special applications within private IP networks such as Internet2.
IP multicast conferencing was first demonstrated on a wide scale when it was used to broadcast several sessions from the 23rd IETF in March, 1992 to researchers and interested observers around the world. Selected IETF sessions have continued to be multicast over the MBONE and private multicast networks since then.
IP multicast protocols
- Protocol independent multicast (PIM)
- Distance vector multicast routing protocol (DVRMP)
- Multicast OSPF (MOSPF)
- Multicast BGP (MBGP)
- Internet group management protocol (IGMP)