Chapter 3 - Multicast

Abstract

This chapter reviews the concept of multicast Internet Protocol (IP) delivery in which datagrams are delivered to groups of nodes across the Internet, regardless of the subnets to which the individual group members belong. In multicast, data are sent out to a special address that represents the group of all intended recipients. Stations that wish to receive multicast traffic for a particular distribution listen for datagrams targeted to that address, along with those that are sent to their own addresses. In practice, on many data-links, the multicast IP datagram is actually broadcast in a frame to all stations on the network, but because the datagram has a more specific address than the broadcast address for the subnetwork, the hosts are able to discriminate against unwanted packets within their IP code, throwing them away before too much processing is done. The main challenges of IP multicast are in determining the hosts that are the intended recipients of the multicast datagram and the place where the fan-out should happen. The Internet Group Management Protocol (IGMP) that is used to manage the groups of hosts is also described in this chapter.