Internet Core Protocols: The Definitive Guide

Membership reports

The process of registering with a router is very straightforward. Whenever an application that needs to participate in a specific multicast group is started, an IGMP “Membership Report” is sent by the host to the multicast address in use by the application. RFC 2236 states that multicast hosts should resend the first Membership Report within a few moments, in case the first one was lost or damaged.

For example, servers running the multicast-enabled Simple Network Time Protocol (SNTP) can use the multicast group address of 224.0.1.1 to automatically pass time-synchronization data across the network to SNTP-enabled clients. Whenever an SNTP system is started, it will immediately send a couple of IGMP Membership Reports to the 224.0.1.1 multicast address, as illustrated in Figure 4.3. Any multicast routers on the local network will see this report, and use this information when building their multicast-forwarding maps.

Figure 4.3. An SNTP-enabled system announcing its presence to the network

Membership Reports are also sent in response to IGMP Membership Queries, which are periodically sent by multicast routers on the local network in order to determine which multicast groups still have active memberships. Whenever a query is received, at least one host from each multicast group on the local network must respond with a Membership Report. This procedure is described ...