Multicast IPv6 Addresses

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4/8/2010

A multicast address identifies multiple interfaces, and is used for one-to-many communication. 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 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.

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

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The fields in the multicast address are shown in the following table.

Field Description

Format Prefix

IPv6 multicast addresses are identified with the Format Prefix of 1111 1111.

Flags

Indicates flags that are set on the multicast address. 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.

  • If 0, the multicast address is a permanently assigned, well-known multicast address allocated by the Internet Assigned Numbers Authority (IANA).
  • If 1, the multicast address is a not permanently assigned, or transient.

Scope

Indicates the scope of the IPv6 internetwork for which the multicast traffic is intended. 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 and scope field values are defined in RFC 2373:

Scope field value      Scope

1                                Node-local

2                                Link-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:

  • 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.

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, each group ID maps to a unique network interface multicast MAC address.

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 and link-local scopes, the following multicast addresses are defined:

  • FF01::2 (node-local scope all-routers address)
  • FF02::2 (link-local scope all-routers address)

The solicited-node address facilitates efficient querying of network nodes during address resolution. IPv6 uses the Neighbor Solicitation message to perform address resolution. In IPv4, the ARP Request frame is sent to the MAC-level broadcast, disturbing all nodes on the network segment regardless of whether a node is running IPv4. For IPv6, instead of disturbing all IPv6 nodes on the local link by using the local-link scope all-nodes address, the solicited-node multicast address is used as the Neighbor Solicitation message destination.

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.

The following steps show an example of how the solicited-node address is handled for the node with the link-local IPv6 address of FE80::2AA:FF:FE28:9C5A, and the corresponding solicited-node address is FF02::1:FF28:9C5A:

  1. 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.
  2. The node using the address of FE80::2AA:FF:FE28:9C5A is listening for multicast traffic at the solicited-node address FF02::1:FF28:9C5A. For interfaces that correspond to a physical network adapter, it has registered the corresponding multicast address with the network adapter.

As shown in this example, by using the solicited-node multicast address, address resolution that commonly occurs on a link can occur without disturbing all network nodes. In fact, very few nodes are disturbed during address resolution. Because of the relationship between the network interface MAC address, the IPv6 interface ID, and the solicited-node address, in practice, the solicited-node address acts as a pseudo-unicast address for efficient address resolution.

Concepts

IPv6 Addressing

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