Multicasting is a way to send a message to multiple recipients. In many applications it is better than broadcasting, since multicasting imposes less overhead on hosts that are not participating in the communication. The simple host membership reporting protocol (IGMP) is the basic building block for multicasting.
Multicasting on a LAN or across closely connected LANs uses the techniques we've described in this chapter. Since broadcasting is often restricted to a single LAN, multicasting could be used instead of broadcasting for many applications that use broadcasting today.
A problem that has not been completely solved , however, is multicasting across wide area networks. [Deering and Cheriton 1990] propose extensions to common routing protocols to support multicasting. Section 9.13 of [Perlman 1992] discusses some of the problems with multicasting across WANs.
[Casner and Deering 1992] describe the delivery of audio for an IETF meeting across the Internet using multicasting and a virtual network called the MBONE (multicasting backbone).