7. IGMP: multipoint addressing

Multipoint or IP multicasting enables datagrams to be sent to multiple destinations with high performance. The IP protocol uses Class D addresses to indicate that it is a multipoint transmission, and relies on the network service if it exists.

Broadcast groups are dynamic: a machine can join or leave a group at any time, the host needing only be able to send and receive datagrams in multicast. This IP function is not limited to the physical subnet, but gateways also propagate group membership information and manage routing so that each machine receives a copy of every datagram sent to the group.

Machines communicate their group membership to gateways, using the Internet Group Management Protocol (IGMP). The protocol is designed to be efficient and optimize the use of network resources. In most cases, the IGMP traffic introduced is a periodic...