13.1 Introduction

13.1 Introduction

Section 12.4 provided an overview of IP multicasting and described how class D IP addresses are mapped into Ethernet addresses. We briefly mentioned how multicasting occurs on a single physical network, but said complications occur when multiple networks are involved and the multicast datagrams must pass through routers.

In this chapter we'll look at the Internet Group Management Protocol (IGMP), which is used by hosts and routers that support multicasting. It lets all the systems on a physical network know which hosts currently belong to which multicast groups. This information is required by the multicast routers, so they know which multicast datagrams to forward onto which interfaces. IGMP is defined in RFC 1112 [Deering 1989].

Like ICMP, IGMP is considered part of the IP layer. Also like ICMP, IGMP messages are transmitted in IP datagrams. Unlike other protocols that we've seen, IGMP has a fixed- size message, with no optional data. Figure 13.1 shows the encapsulation of an IGMP message within an IP datagram.

Figure 13.1. Encapsulation of an IGMP message within an IP datagram.

IGMP messages are specified in the IP datagram with a protocol value of 2.