In this lesson, we will talk about the specific operations of MLD. We will see;
• MLD Querier Selection
• MLD General Query, Group Query and Report Mechanism
• MLD Multicast Group Joining Operation
• MLD Multicast Group Leaving Operation
• MLDv2 Source and Group Specific Query Operation
• MLD SSM Mapping
Let’s start to learn MLD Operation, with nice and catchy visuals.
Multicast Listener Discovery (MLD) uses Query and Report mecahanisms. Hosts sends reports and a multicast router sends Quaries. In the MLD Multicast Network, one of the router sends this multicast Queries. The Multicast Router that sends this Quaries is called “MLD Quarier”.
Before sending Query messages, MLD Querier need to be selected. The selection of MLD Querier has some steps. Let’s check these steps one by one.
1. All Multicast routers send a General Query message to the FF02::1 becouse each router think that it is MLD Querier. “I am MLD Querier”.
2. After receiving General Queries, all the routers compares the source IP address with their Link-Local IPv6 addresses.
3. The Multicast Router that has the lowest IPv6 Link-Local address become MLD Querier.
4. After the MLD Querier election, all the other Multicast Routers starts Querier timer. Every time that the MLD Querier sends queriers, these timers are reset. If the time is up, then a new Querier election occurs.
With Query and Report operation, MLD Querior learns the multicast group members. This Query and Report operation is take place with the below steps:
1. MLD Querier sends a periodic General Query or Group specific Query to the destination FF02::1. By sending this, it asks, “Are there anybody want to join a Multicast Group?”.
2. One of the hosts in the segment sends a Report message that means “Yes, I want to receive that multicast group traffic”.
There is also an additional suppression mechanism that reduces the traffic. Only one host send a report for a multicast group, the others are suppressed. This is done with a timer that each host starts after the query message. The host whose timer finishes first, send Report message to MLD Querior and the other hosts. The hosts that receive Report message do not send Report message again, they are suppressed.