176 Understanding IPv6, Second Edition

receive IPv6 multicast traffic regardless of their location on the IPv6 network. Multicast routers exchange host membership information by using a multicast routing protocol such as the Protocol Independent Multicast (PIM) protocol. Group membership is communicated either explicitly, by exchanging multicast address and subnet information, or implicitly, by informing upstream routers that no group members exist downstream from the source of the multicast traffic.

The goals of a multicast routing protocol include the following:

■Forward traffic away from the source to prevent loops.

■Minimize or eliminate multicast traffic to subnets that do not need the traffic.

■Minimize CPU and memory load on the router for scalability.

■Minimize the overhead of the routing protocol.

■Minimize the join latency, which is the time it takes for the first group member on a subnet to begin receiving traffic sent to the group.

Multicast routing is more complex than unicast routing. With unicast routing, unicast traffic is forwarded to a unique destination. Unicast routes can summarize ranges of unique destinations. Unicast routes in the network are comparatively consistent and need to be updated only when the topology of the IPv6 network changes.

With multicast routing, multicast traffic is forwarded to an ambiguous group destination. Multicast addresses represent individual multicast groups, and in general, cannot be summarized in the multicast forwarding table. The location of group members is not consistent, and the multicast forwarding tables of multicast routers might need to be updated whenever a multicast group member joins or leaves a multicast group.

Just as unicast routing protocols update the unicast IPv6 routing table, multicast routing protocols update the IPv6 multicast forwarding table of a router.

MLD Packet Structure

Unlike IGMPv2, MLD uses ICMPv6 messages instead of defining a separate message structure. Figure 7-1 shows the structure of an MLD message packet.

Figure 7-1 The structure of an MLD message packet

An MLD message packet consists of an IPv6 header, a Hop-by-Hop Options extension header, and the MLD message. The Hop-by-Hop Options extension header contains the IPv6 Router