What is IGMP in the Computer Network?

Internet Group Management Protocol (IGMP) is a network layer protocol used to manage multicast group memberships in IPv4 networks. IGMP allows devices to join multicast groups and enables routers to efficiently distribute multicast traffic only to networks that have interested receivers.

Unlike multicast routing protocols that determine paths for multicast traffic, IGMP focuses solely on group membership management. It provides multicast routers with information about which hosts on their directly connected networks want to receive traffic for specific multicast groups.

How IGMP Works

IGMP operates locally between hosts and their directly connected multicast routers. Each multicast router maintains a list of active multicast groups for which at least one member exists on each connected network segment.

Without IGMP, routers would need to flood all multicast traffic to every network segment, creating unnecessary network congestion. IGMP enables efficient multicast delivery by maintaining accurate membership information.

IGMP Message Types

Joining a Multicast Group

When a host application wants to receive multicast traffic for a specific group, the host sends an IGMP Membership Report message. The protocol requires sending the report twice within a short interval to ensure reliability in case the first message is lost.

If this is the first member for that group on the network segment, the multicast router adds the group to its active list and begins forwarding traffic for that group to the network.

Leaving a Multicast Group

When a host no longer has any applications interested in a multicast group, it sends an IGMP Leave Group message. However, the router cannot immediately remove the group from its list, as other hosts might still be interested.

The router responds by sending a Group-Specific Query message, allowing remaining interested hosts to respond. If no membership reports are received within the specified timeout period, the router removes the group from its active list.

Conclusion

IGMP is essential for efficient multicast communication in IPv4 networks, enabling routers to maintain accurate group membership information. This prevents unnecessary network flooding and ensures multicast traffic reaches only networks with active receivers.