This chapter discusses the special use of multicasts and the protocols required to distribute multicasts throughout an internetwork. Multicasting enables a packet to be distributed from a source to any number of destinations automatically, with the responsibility for packet distribution placed on intelligent network nodes rather than the source itself. For certain types of applications this can be a very efficient way of distributing data to many receivers simultaneously (specifically, highly asymmetrical applications, such as market data feeds, videoconferencing, audio conferencing, database replication, distributed computation, and real-time workgroups). Historically, these applications, either broadcasted data or used multiple TCP unicast sessions from the data feed, to each client. As we have seen already, broadcasts are generally a bad idea on internetworks for several reasons. The unicast solution does not scale and makes poor use of the available bandwidth, since the feed is required to send the same information n times, often along the same physical links. IP multicast, therefore, provides an ideal delivery mechanism for this class of application. With the rise of the Internet, multicasting is likely to play an increasing role in both application and router implementation over the next few years.
The Internet Engineering Task Force (IETF) has introduced a number of standards to provide the infrastructure for multicast support, broadly divided into three areas: multicast addressing, host registration, and multicast routing. Chapter 2 covers the use of IP addresses for multicast use. The Internet Group Management Protocol (IGMP), as described in this chapter, handles registration. Multicast routing, also described in this chapter, is composed of several standards, as follows:
Distance-Vector Multicast Routing Protocol (DVMRP)
Multicast Open Shortest Path First (MOSPF), which is an extension to OSPF
Protocol Independent Multicast (PIM)
Core-Based Trees (CBT)
A related area of interest is support for real-time applications that use IP multicast, as well as the ability to deliver quality of service guarantees. We will, therefore, briefly cover some of the new protocols introduced in this area also. Readers wishing to keep up to date on this area should refer to .