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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.
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 in
more detail in “Membership queries” later in this chapter.
Note that Membership Reports are never generated for the “all-hosts” group
address of 224.0.0.1. Although every multicast-enabled host should listen for traffic
on that multicast address, no host should ever report its membership in that
group.
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