Understanding MLD

 

The Multicast Listener Discovery (MLD) Protocol manages the membership of hosts and routers in multicast groups. IP version 6 (IPv6) multicast routers use MLD to learn, for each of their attached physical networks, which groups have interested listeners. Each routing device maintains a list of host multicast addresses that have listeners for each subnetwork, as well as a timer for each address. However, the routing device does not need to know the address of each listener—just the address of each host. The routing device provides addresses to the multicast routing protocol it uses, which ensures that multicast packets are delivered to all subnetworks where there are interested listeners. In this way, MLD is used as the transport for the Protocol Independent Multicast (PIM) Protocol.

MLD is an integral part of IPv6 and must be enabled on all IPv6 routing devices and hosts that need to receive IP multicast traffic. The Junos OS supports MLD versions 1 and 2. Version 2 is supported for source-specific multicast (SSM) include and exclude modes.

In include mode, the receiver specifies the source or sources it is interested in receiving the multicast group traffic from. Exclude mode works the opposite of include mode. It allows the receiver to specify the source or sources it is not interested in receiving the multicast group traffic from.

For each attached network, a multicast routing device can be either a querier or a nonquerier. A querier routing device, usually one per subnet, solicits group membership information by transmitting MLD queries. When a host reports to the querier routing device that it has interested listeners, the querier routing device forwards the membership information to the rendezvous point (RP) routing device by means of the receiver's (host's) designated router (DR). This builds the rendezvous-point tree (RPT) connecting the host with interested listeners to the RP routing device. The RPT is the initial path used by the sender to transmit information to the interested listeners. Nonquerier routing devices do not transmit MLD queries on a subnet but can do so if the querier routing device fails.

All MLD-configured routing devices start as querier routing devices on each attached subnet (see Figure 1). The querier routing device on the right is the receiver's DR.

Figure 1: Routing Devices Start Up on a Subnet
Routing Devices Start Up on a Subnet

To elect the querier routing device, the routing devices exchange query messages containing their IPv6 source addresses. If a routing device hears a query message whose IPv6 source address is numerically lower than its own selected address, it becomes a nonquerier. In Figure 2, the routing device on the left has a source address numerically lower than the one on the right and therefore becomes the querier routing device.

Note

In the practical application of MLD, several routing devices on a subnet are nonqueriers. If the elected querier routing device fails, query messages are exchanged among the remaining routing devices. The routing device with the lowest IPv6 source address becomes the new querier routing device. The IPv6 Neighbor Discovery Protocol (NDP) implementation drops incoming Neighbor Announcement (NA) messages that have a broadcast or multicast address in the target link-layer address option. This behavior is recommended by RFC 2461.

Figure 2: Querier Routing Device Is Determined
Querier Routing Device Is Determined

The querier routing device sends general MLD queries on the link-scope all-nodes multicast address FF02::1 at short intervals to all attached subnets to solicit group membership information (see Figure 3). Within the query message is the maximum response delay value, specifying the maximum allowed delay for the host to respond with a report message.

Figure 3: General Query Message Is Issued
General Query Message Is Issued

If interested listeners are attached to the host receiving the query, the host sends a report containing the host's IPv6 address to the routing device (see Figure 4). If the reported address is not yet in the routing device's list of multicast addresses with interested listeners, the address is added to the list and a timer is set for the address. If the address is already on the list, the timer is reset. The host's address is transmitted to the RP in the PIM domain.

Figure 4: Reports Are Received by the Querier Routing Device
Reports Are Received by the Querier
Routing Device

If the host has no interested multicast listeners, it sends a done message to the querier routing device. On receipt, the querier routing device issues a multicast address-specific query containing the last listener query interval value to the multicast address of the host. If the routing device does not receive a report from the multicast address, it removes the multicast address from the list and notifies the RP in the PIM domain of its removal (see Figure 5).

Figure 5: Host Has No Interested Receivers and Sends a Done Message to Routing Device
Host Has No Interested Receivers and
Sends a Done Message to Routing Device

If a done message is not received by the querier routing device, the querier routing device continues to send multicast address-specific queries. If the timer set for the address on receipt of the last report expires, the querier routing device assumes there are no longer interested listeners on that subnet, removes the multicast address from the list, and notifies the RP in the PIM domain of its removal (see Figure 6).

Figure 6: Host Address Timer Expires and Address Is Removed from Multicast Address List
Host Address Timer Expires and Address
Is Removed from Multicast Address List