Internet Core Protocols: The Definitive Guide

Membership Queries and Reports

If a network does not have a multicast router, then the multicast-enabled hosts on that network can only participate with other multicast-enabled hosts that are also on that network. In that kind of environment, users will never see a Membership Query, since those messages are only issued by multicast routers when they need to find out which hosts are actively listening for specific multicast group addresses.

However, if the hosts on the network wish to exchange multicast data with hosts on other networks (either internal to the organization or in far-away places), then multicast routers are required. In that scenario, there will be several Membership Queries issued by the router.

As described in Managing Group Memberships earlier in this chapter, at least one host will respond to each of these queries with a Membership Report. The type of report that is issued will depend on whether or not the hosts and routers are running IGMPv1 or IGMPv2. Most systems today are running IGMPv2.

The process of sending Membership Queries and Membership Reports is shown in Figure 4.17.

Figure 4.17. A simple exchange of Membership Query and Membership Report messages

Notice that the first packet contains an IGMPv2 Membership Query, which is sent to the "all-hosts” address of 224.0.0.1. Every multicast-enabled host is required to monitor that group address, allowing them ...