MLDv2 enhancements

MLDv2 is based on and backwards-compatible with MLDv1. MLDv2 provides hosts with enhanced control capabilities and enhances the MLD state.

MLDv2 has introduced IPv6 multicast source filtering modes (Include and Exclude). These modes allow a host to receive or reject multicast data from the specified IPv6 multicast sources. When a host joins an IPv6 multicast group, one of the following occurs:

• If the host expects IPv6 multicast data from specific IPv6 multicast sources like S1, S2, …, it sends a report with Filter-Mode denoted as "Include Sources (S1, S2, …)."

• If the host does not expect IPv6 multicast data from specific IPv6 multicast sources like S1, S2, …, it sends a report with Filter-Mode denoted as "Exclude Sources (S1, S2, …)."

As shown in Figure 100, the network has two IPv6 multicast sources, Source 1 (S1) and Source 2 (S2). Both of the sources can send IPv6 multicast data to IPv6 multicast group G. Host B wants to receive IPv6 multicast data addressed to G from Source 1 but not from Source 2.

In MLDv1, Host B cannot select IPv6 multicast sources when it joins IPv6 multicast group G. The IPv6 multicast streams from both Source 1 and Source 2 flow to Host B whether it needs them or not.

In MLDv2, Host B can explicitly express its interest in IPv6 multicast data destined to G from Source 1 but not from Source 2. Then, Host B receives only IPv6 multicast data from Source 1.

A multicast device that is running MLDv2 maintains the multicast address state for each multicast address on each attached subnet. The multicast address state consists of the following information:

• Filter mode—Device keeps tracing the Include or Exclude state.

• List of sources—Device keeps tracing the newly added or deleted IPv6 multicast source.

• Timers—Filter timers, which include the time that the device waits before switching to the Include mode after an IPv6 multicast address times out, and source timers for source recording.