Multicast IPv6 addresses

Applies To: Windows Server 2003, Windows Server 2003 R2, Windows Server 2003 with SP1, Windows Server 2003 with SP2

Multicast IPv6 addresses

A multicast address identifies multiple interfaces. With the appropriate multicast routing topology, packets addressed to a multicast address are delivered to all interfaces that are identified by the address.

IPv6 multicast addresses have the Format Prefix (FP) of 1111 1111. An IPv6 address is simple to classify as multicast because it always begins with FF. Multicast addresses cannot be used as source addresses.

Beyond the FP, multicast addresses include additional structure to identify their flags, scope, and multicast group, as shown in the following illustration.

The fields in the multicast address are as follows:

• Flags

  The Flags field indicates flags that are set on the multicast address. The size of this field is 4 bits. As of RFC 2373, the only flag defined is the Transient (T) flag. The T flag uses the low-order bit of the Flags field. When set to 0, the T flag indicates that the multicast address is a permanently-assigned (well-known) multicast address allocated by the Internet Assigned Numbers Authority (IANA). When set to 1, the T flag indicates that the multicast address is a transient (not permanently assigned) multicast address.

• Scope

  The Scope field indicates the scope of the IPv6 internetwork for which the multicast traffic is intended. The size of this field is 4 bits. In addition to information provided by multicast routing protocols, routers use the multicast scope to determine whether multicast traffic can be forwarded.

• The following scopes are defined in RFC 2373:

  Scope field value	Scope
  1	Node-local
  2	Link-local
  5	Site-local
  8	Organization-local
  E	Global

  For example, traffic with the multicast address of FF02::2 has a link-local scope. An IPv6 router never forwards this traffic beyond the local link.

• Group ID

  The Group ID field identifies the multicast group and is unique within the scope. The size of this field is 112 bits. Permanently assigned group IDs are independent of the scope. Transient group IDs are only relevant to a specific scope. Multicast addresses from FF01:: through FF0F:: are reserved, well-known addresses.

To identify all nodes for the node-local and link-local scopes, the following multicast addresses are defined:

• FF01::1 (node-local scope all-nodes address)

• FF02::1 (link-local scope all-nodes address)

To identify all routers for the node-local, link-local, and site-local scopes, the following multicast addresses are defined:

• FF01::2 (node-local scope all-routers address)

• FF02::2 (link-local scope all-routers address)

• FF05::2 (site-local scope all-routers address)

With 112 bits in the Group ID, it is possible to have 2112 group IDs. However, because of the way in which IPv6 multicast addresses are mapped to Ethernet multicast MAC addresses, RFC 2373 recommends assigning the Group ID from the low order 32 bits of the IPv6 multicast address and setting the remaining original group ID bits to 0. By using only the low-order 32 bits in the group ID, each group ID maps to a unique Ethernet multicast MAC address.

Solicited-node address

The solicited-node address facilitates efficient querying of network nodes during address resolution. In IPv4, the ARP Request frame is sent to the MAC-level broadcast, disturbing all nodes on the network segment, including those that are not running IPv4. IPv6 uses the Neighbor Solicitation message to perform address resolution. However, instead of using the local-link scope all-nodes address as the Neighbor Solicitation message destination, which would disturb all IPv6 nodes on the local link, the solicited-node multicast address is used. The solicited-node multicast address consists of the prefix FF02::1:FF00:0/104 and the last 24-bits of the IPv6 address that is being resolved.

For example, for the node with the link-local IPv6 address of FE80::2AA:FF:FE28:9C5A, the corresponding solicited-node address is FF02::1:FF28:9C5A. To resolve the FE80::2AA:FF:FE28:9C5A address to its link layer address, a node sends a Neighbor Solicitation message to the solicited-node address of FF02::1:FF28:9C5A. The node that is using the address of FE80::2AA:FF:FE28:9C5A is listening for multicast traffic at the solicited-node address and, for interfaces that correspond to a physical network adapter, has registered the corresponding multicast address with the network adapter.

The result of using the solicited-node multicast address is that address resolution, which commonly occurs on a link, is not required to use a mechanism that disturbs all network nodes. In fact, very few nodes are disturbed during address resolution. In practice, because of the relationship between the Ethernet MAC address, the IPv6 interface ID, and the solicited-node address, the solicited-node address acts as a pseudo-unicast address for very efficient address resolution.