| The history of telecommunication was characterized mainly by two technologies in the past hundred years (before the Internet era began): telephony, and radio and television broadcasting. These two technologies cover two fundamentally different communication areas or needs: Individual communication (unicast): Connections between two communication partners, where the direction of data exchange can be unidirectional (simplex) or bidirectional (duplex). Mass communication (broadcast): One station sends data to all stations reachable over a medium, where data distribution is normally unidirectional.
After these two technologies, the Internet followed as the third communication technology, changing the telecommunication world. Though the Internet was initially designed for individual communication, the protocols and mechanisms added at the beginning of the nineties introduced a new communication form: group communication (multicast). Multicast makes possible an efficient data distribution to several receivers. By contrast with the mass communication of (radio) broadcasting, where data is distributed to all participants within one single transmission medium, group communication delivers data units only to those receivers explicitly interested in this data. In addition, group communication in the Internet (IP multicast) enables each Internet computer to send data directly to the members of a multicast group. Consequently, in the designing of mechanisms and protocols, two specific tasks can be deduced for the functionality of group communication in Internet systems: The first task is solved by the Internet Group Management Protocol (IGMP), which has to be supported by each multicast-capable Internet computer. Section 17.3 introduces IGMP and its implementation in Linux systems. For the second task, we have to distinguish between end system and forwarding systems. Section 17.4 will discuss how both types are supported in Linux. As with Internet routing, group communication also separates clearly between forwarding and routing. There are different multicast routing algorithms, including the Distance Vector Multicast Routing Protocol (DVMRP), which will be introduced in Section 17.5.2 as a representative example for these algorithms, using the mrouted daemon. |
