/ˌɛm ɛl diː/
noun — "tracking who wants multicast traffic on IPv6 networks."
MLD (Multicast Listener Discovery) is a network protocol used in IPv6 environments to manage membership in multicast groups. It allows routers to discover which hosts on a local network segment are interested in receiving multicast traffic and to stop forwarding multicast packets where no listeners exist. Functionally, MLD serves the same role in IPv6 that IGMP serves in IPv4, but it is tightly integrated into the IPv6 protocol suite.
Technically, MLD operates using control messages exchanged between hosts and routers on a single link. Hosts send listener reports to indicate interest in specific multicast addresses, while routers periodically issue queries to confirm which multicast groups are still active. If no listeners respond for a given group, the router ceases forwarding multicast traffic for that group on that interface. MLD messages are carried using IPv6 control messaging rather than a standalone transport, which reduces protocol overhead and aligns multicast management directly with IPv6 neighbor and control mechanisms.
There are two primary versions of MLD. MLDv1 provides basic multicast group membership reporting and querying. MLDv2 adds support for source-specific multicast, allowing hosts to specify not only which multicast group they want to receive, but also which specific source addresses they trust. This improves efficiency and security by preventing unwanted multicast sources from being forwarded.
Key characteristics of MLD include:
- IPv6-native design: built specifically for IPv6 networks rather than adapted from IPv4.
- Listener-based control: routers forward multicast traffic only when listeners are present.
- Versioned evolution: MLDv1 for basic membership, MLDv2 for source-specific control.
- Bandwidth efficiency: prevents unnecessary multicast flooding on network segments.
- Local-link scope: operates between hosts and routers on the same network segment.
In practical workflows, MLD is essential for IPv6 multicast applications such as live video distribution, real-time data feeds, and enterprise multicast services. For example, when multiple devices on an IPv6-enabled LAN subscribe to a multicast video stream, each device signals its interest using MLD reports. The local router aggregates this information and forwards the multicast stream only to that network segment. When all listeners leave, multicast forwarding automatically stops, conserving bandwidth.
Conceptually, MLD acts like a headcount at a meeting: as long as people are present and interested, the presentation continues, but once the room empties, the projector turns off.
Intuition anchor: MLD makes multicast in IPv6 efficient and intentional, ensuring data flows only where it is explicitly wanted.