9
IGMP Commands
Introduction
This chapter describes the Internet Group Management Protocol (IGMP) commands used with the BSR 2000™.
Internet Group Management Protocol (IGMP), part of the Internet Protocol (IP) suite, is used between hosts and routers to report dynamic multicast group membership. IP multicasting is the transmission of an IP datagram to a "host group" identified by a single IP destination address. Multicasting directs the same information packets to multiple destinations at the same time, versus unicasting, which sends a separate copy to each individual destination. Because the destinations receive the same source packet at once, delivery of the information takes place in a more timely manner.
As stated in RFC 1112, the membership of a host group is dynamic; that is, hosts may join and leave groups at any time. There is no restriction on the location or number of members in a host group, and a host may be a member of more than one group at a time.
There are three types of messages structures supported by IGMP to communicate with each other about the multicast traffic: “queries”, “reports”, and “leave group” messages. Query messages are used to discover which hosts are in which multicast groups. In response, the hosts sends a report message to inform the querier of a host’s membership. (Report messages are also used by the host to join a new group). Leave group messages are sent when the host wishes to leave the multicast group.
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