In early IP networks, a packet could be sent to either a single device (unicast) or to all devices (broadcast); a single transmission destined for a group of devices was not possible. IPv6 uses multicast for those purposes for which IPv4 used broadcast; consequently, IPv6 does not support broadcast.

Applications can use multicast transmissions to enable efficient communication between groups of devices. Data is transmitted to a single multicast IP address and received by any device that needs to obtain the transmission.

An IPv6 router uses Multicast Listener Discovery (MLD) protocol to discover the following information:

• The presence of multicast listeners (nodes wanting to receive multicast packets) on its directly attached links
• Which multicast addresses are of interest to those listeners

MLD provides this information to the multicast routing protocol the router is using. This ensures that multicast packets are delivered to all links where there are interested receivers. MLD is derived from IGMPv2.

MLD has a router function and a listener function. The router function discovers the presence of multicast listeners and ensures delivery of multicast packets to listeners. The listener function informs routers when it starts listening for a multicast address, when it stops listening for a multicast address, and when it responds to queries about multicast addresses. z/OS® Communications Server implements the listener function.

When a listener starts listening for a multicast address on an interface, it sends an MLD report message for that address on that interface.

When a listener stops listening for a multicast address on an interface, it sends a single MLD done message.

A router sends an MLD query message to query listeners about multicast addresses. A specific query is sent to listeners for a specific multicast address on a receiving interface. A general query is sent to listeners for all multicast addresses on a receiving interface. These query messages contain a maximum response delay (MRD). The MRD causes listeners to delay report messages and not send them if another listener reports first. If no reports for the address are received from the link after the response delay of the last query has passed, the routers on the link assume that the address no longer has any listeners there; the address is therefore deleted from the list and its disappearance is made known to the multicast routing component.

If you configure IP security for IPv6, see z/OS Communications Server: IP Configuration Guide for information about filter rules for MLD packets.