A host running IGMP will inform IGMP routers as to which multicast addresses it is listening to by sending it IGMP reports when it joins a multicast group. An IGMP router will send IGMP queries to IGMP hosts periodically to discover the multicast addresses the host is listening to as the following examples show:

When an interface is opened, i.e. xn_interface_open is called, the IGMP host joins the all host group. The IGMP router sends queries to the all host group; therefore, all IGMP hosts need to listen to this multicast address.
When a host joins a multicast group via xn_interface_mcast(ADD_ENTRY, SET_ENTRY) or setsockopt(IP_ADD_MEMBERSHIP), the IGMP host sends a report to the multicast address which was added. The device driver is also set up to listen for packets sent to this multicast address.
When an IGMP host receives a QUERY on an interface, it starts a timer for each multicast address it is listening to. If the timer is already running, it is not restarted.
When a report timer for a multicast expires, the IGMP host sends a REPORT to that multicast address on the interface.

If IGMP is enabled, xn_rt_add must be called to add the multicast address to the routing table before adding the address to multicast group via setsockopt and xn_interface_mcast.

Left TCP/IP Networking

TCP

Down Routing Table