Interconnection · Lesson 14

How does IGMP manage multicast groups?

IGMP lets IPv4 hosts tell multicast routers which groups they want to receive, so multicast traffic can be delivered where it is needed.

The group membership problem

Multicast needs a way to discover interested receivers.

An IPv4 multicast address identifies a group, not one host. A receiver can join that group when it needs the stream and leave when it no longer does.

IGMP operates between hosts and the multicast router on the local network. It does not carry the multicast application data and it does not replace the routing protocols that connect multicast-enabled networks.

IGMP is membership signalling.It tells the local multicast router whether receivers are interested in a group; the router then uses that state to decide where multicast packets should go.

Build the model

Four IGMP ideas

01

Join

A host sends a membership report for the multicast group it wants.

02

Query

A multicast router periodically asks whether receivers are still interested.

03

Report

One or more members answer, maintaining group state on the local network.

04

Leave

A host can signal that it is leaving; the router verifies whether anyone else remains.

Follow the exchange

Membership controls delivery

The router learns local interest through IGMP. Multicast traffic should only be forwarded onto a segment while that segment has an interested member.

IGMP membership lifecycle: a receiver reports membership to a multicast router, the router queries members, and multicast traffic flows while membership remains active.
IGMP keeps local multicast membership state current.

Knowledge check

Test your IGMP model

Answer six questions.

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