The origins of IP multicasting go back to 1986. However, multicasting in its current form was introduced only in 1989. During 20 years of IP multicasting, the service has been evolving continuously – new multicast transport protocols have been designed, new group management protocols have been developed and new transport protocols and multicast applications have appeared. Nowadays, IP multicast is a mature solution, and concepts and protocols designed for multicasting are also used in non-multicast services.
Key Terms in this Chapter
Group Management Protocol: The bottom level of multicast management system hierarchy, responsible for joining and leaving multicast group. Group management protocols operate between receiver (“leaf” of the delivery tree) and nearest multicast router. Nowadays, there are two group management protocols, IGMP for IPv4 and MLD for IPv6.
Multicast Address: A single IP address that identifies a group of receivers. Multicast addresses belong to an IP address space separate from other (unicast, anycast, or broadcast) addressing.
Multicast Group: A group of zero, one or more receivers, which are interested in receiving the same information sent to the common IP destination address.
Multicast Delivery Tree: A tree structure, described multicast packets route from the sender to receivers. The sender is a root of the tree, and receivers are leaves.
Multicast Transport Protocol: Reliable or realtime multicast-enabled transport protocol; all protocol’s mechanisms are multicast-oriented. Examples of multicast transport protocols are RTP for real-time multimedia delivery and PGM for reliable data transmission. Note
Source-Filtered Multicast Service: A model of IP multicast delivery. Current models allow receiver to receive and accept data originating from any sender (any-source multicast, ASM), a specific set of senders (source-filtered multicast, SFM), or one specific sender (source-specific multicast, SSM).
Multicast Router: An IP router, which supports IP multicasting. Multicast routers forward IP packets (both unicast and multicast) and provide necessary replication of multicast packets to a set of destinations. To route multicast traffic, additional multicast routing table, separate from the unicast one, is used.
Multicast Routing Protocol: Routing protocol for multicast delivery. Multicast routing protocols function as the upper level of multicast management system hierarchy, responsible for (re)building multicast delivery tree. The best known multicast routing protocols are PIM-SM (a sparse mode protocol, working with the assumption that receivers are sparsely distributed in a given domain) and PIM-DM (a dense mode protocol, working with the assumption that receivers are densely located in a given domain).
Multicast Backbone: Experimental and temporary global multicast network intended to interconnect “multicast islands” over the current (non-multicast) Internet. The first multicast backbone, mbone, network was based on IPv4 (IP version 4). The next, m6bone, is based on IPv6.
Multicast: A many-to-many (M-to-N) transmission scheme, where M senders disseminate information to N receivers. Multicast transmissions are not broadcasting to all possible receivers, but they are addressed to the group of receivers--disseminated data is received only by members of the multicast group.