Illustrated TCP/IP by Matthew G. Naugle Wiley Computer Publishing, John Wiley & Sons, Inc. ISBN: 0471196568   Pub Date: 11/01/98

Chapter 180
ICMPv6 and Multicast

•  Group Membership messages
The functions of IGMP were moved into ICMPv6. The ICMPv6 Group Membership messages are used to convey information about multicast group membership from nodes to their neighboring routers. Please refer to Part Six for more information on the IGMP functions. This function of ICMPv6 allows for IGMP messages to be sent. These are Group Membership messages for Query, Reports, and Reduction (or leaving a group with a termination message). Due to the dynamic nature of the IPv6 and its Neighbor Discovery protocols (routers and hosts), IGMP functions were moved into the ICMP protocol suite. For example, when a node initializes (in an IPv6 environment), it must immediately join the all-nodes multicast address on that interface, as well as the solicited-node multicast address corresponding to each of the IP addresses assigned to the interface.
In the IPv6 header the Destination Address is set as follows: In a Group Membership Query message, the multicast address of the group being queried, or the Link-Local All-Nodes multicast address. In a Group Membership Report or a Group Membership Reduction message, the multicast address of the group being reported or terminated.

The hop limit is set to 1 to ensure this message does not leave the local subnetwork. The ICMPv6 fields are set as follows:

Type:

130—Group Membership Query

131—Group Membership Report

132—Group Membership Reduction

Code: 0
The first 16 bits after the ICMP header are used for the Maximum Response Delay. In Query messages, it is the maximum time that responding Report messages may be delayed, in milliseconds. In Report and Reduction messages, this field is initialized to 0 by the sender and ignored by receivers. The next 16 bits are unused and they are initialized to 0 by the sender and ignored by receivers. The rest of the message is filled with the Multicast Address, which is the address of the multicast group to which the message is being sent. In Query messages, the Multicast Address field may be 0, implying a query for all groups.