Internet Core Protocols: The Definitive Guide

Membership queries

Another commonly seen IGMP message is the “Membership Query.” Queries are sent by multicast routers in order to verify that each of the multicast groups that are currently being forwarded still have listening members on the local network (by default, the interval between queries is 125 seconds). If no hosts respond to a Membership Query, then the router can stop forwarding multicast datagrams for any of the multicast groups that no longer have any active listeners on that network.

Typically, only one multicast router issues these queries. Any other multicast routers on the network then takes a passive role, watching for Membership Reports, but not sending any queries. The “query router” is elected by having the lowest-numbered IP address. If that router stops issuing queries, then the multicast router with the next-higher IP address will become the query router.

Hosts respond to Membership Queries with standard Membership Reports, and the multicast routers use this information to determine which multicast addresses should be forwarded to the local network. If no hosts respond for a multicast address that is already being forwarded, then the multicast router can assume that nobody wants that traffic anymore and can stop forwarding remote datagrams for that multicast address to this network.

Membership Queries are sent to the "all-hosts” address of 224.0.0.1. With IGMPv2, routers are also allowed to send group-specific queries to test for membership in a particular ...