Multicast is the communication between a single sender and multiple receivers on a network. Multicast technologies are used to reduce network traffic when there are many receivers that want to listen to or view the same source simultaneously by delivering a single stream of information to hundreds of recipients. For an n party video conference, a (n-1) times lower bandwidth is needed. "Multicast" is typically used to refer to IP Multicast using of a multicast address. IPv6 supports unicast, multicast, and anycast, but Broadcast in IPv6 has disappeared as a term, but is considered one form of multicast. But there are also other protocols implemented the multicast concept such as Asynchronous Transfer Mode (ATM) which has built-in mechanisms for "point-to-multipoint" or "multipoint-to-multipoint" connections.
The multicast is based on the concept of group, which is an arbitrary group of receivers with an interest to receive the same data stream. This group of receivers can be located anywhere on the Internet. The main multicast technologies and protocols used in the TCP/IP are as follows:
Internet Group Management Protocol (IGMP and IGMPv3) - used between a host and its nearest router and the host use it to tell the router what multicast groups it wants to listen to. Multicast-aware switches snoop on the IGMP traffic to avoid multicast flooding.
Protocol-Independent Multicast (PIM) - used to distribute multicast traffic mainly within an organization to avoid multicast routing loops. There are two PIM variants, dense mode (PIM-DM) and sparse mode (PIM-SM). Sparse mode is the one that is normally used.
Multiprotocol Border Gateway Protocol (MBGP) - a small extension of BGP to handle multicast routing. MBGP enables support for noncongruent unicast and multicast topologies, which is sometimes needed at exchange points.
Multiprotocol Open Shortest Path First (MOSPF) - an extension of OSPF to handle multicast routing. It is not widely deployed so far.
Multicast Source Discovery Protocol (MSDP) - used to distribute the knowledge about all the multicast sources to all of the RP's in the Internet, which is required for proper operation. However, this does not scale well if the number of multicast sources would increase severalfold.
Source Specific Multicast (SSM) - scales well even using the IP v4, and also makes multicasting more reliable for broadcasters to use. Its main use is where there are many users listening to few sources. SSM does not require IP Multicast Address Management and it is ideal for Internet Broadcast Applications.
Explicit Multicast (Xcast) - Xcast can support a very large number of small multicast sessions. This is achieved by explicitly encoding the list of destinations in the data packets, instead of using a multicast address. It is a very good protocol for video conferencing.
Other multicast technologies, which are not based on IP Multicast, are quite popular however, like Internet Relay Chat and PSYC. Real-Time Transport Protocol (RTP) is the Audio/video protocol that associated with multicasting.
Multicast Illustration
Multicast Illustration
Related Terms: Broadcast, Anycast, Unicast, RTP, MOSPF, IGMP, SSM, PIM-SM, PIM-DM, Xcast, MBGP, MSDP