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Multicast Listener Discovery (MLD)

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

Multicast Listener Discovery (MLD)

The use of multicasting in IP networks is defined as a TCP/IP standard in RFC 1112, "Internet Group Management Protocol (IGMP)." This RFC defines address and host extensions for the way in which IP hosts support multicasting. The same concepts originally developed for the current version of IP, known as IP version 4 (IPv4), also apply to IPv6.

What is multicasting?

Multicast traffic is sent to a single address but is processed by multiple hosts. Multicasting is similar to a newsletter subscription. As only subscribers receive the newsletter when it is published, only host computers that belong to the multicast group receive and process traffic sent to the group's reserved address. The set of hosts listening on a specific multicast address is called a multicast group.

Other important aspects of multicasting include the following:

  • Group membership is dynamic, allowing hosts to join and leave the group at any time.

  • The joining of multicast groups is performed through the sending of group membership messages. In IPv6, Multicast Listener Discovery (MLD) messages are used to determine group membership on a network segment, also known as a link or subnet.

  • Groups are not limited by size and members can be spread out across multiple network segments (if connecting routers support the forwarding of multicast traffic and group membership information).

  • A host can send traffic to the group's address without belonging to the corresponding group.

IPv6 multicast addressing

IPv6 multicast addresses are reserved and assigned from the Format Prefix 1111 1111 (0xFF). The following table is a partial list of IPv6 multicast addresses that are reserved for IPv6 multicasting and registered with the Internet Assigned Numbers Authority (IANA).

IPv6 multicast address Description

FF02::1

The all-nodes address used to reach all nodes on the same link.

FF02::2

The all-routers address used to reach all routers on the same link.

FF02::4

The all-Distance Vector Multicast Routing Protocol (DVMRP) routers address used to reach all DVMRP multicast routers on the same link.

FF02::5

The all-Open Shortest Path First (OSPF) routers address used to reach all OSPF routers on the same link.

FF02::6

The all-OSPF designated routers address used to reach all OSPF designated routers on the same link.

FF02::1:FFXX:XXXX

The solicited-node address used in the address resolution process to resolve the IPv6 address of a link-local node to its link-layer address. The last 24 bits (XX:XXXX) of the solicited-node address are the last 24 bits of an IPv6 unicast address.

For a full and current listing of additional IPv6 addresses that are reserved for multicasting, see the Information Sciences Institute Web site document: Internet Protocol Version 6 Multicast Addresses.

A single IPv6 multicast address identifies each multicast group. Each group's reserved IPv6 address is shared by all host members of the group who listen and receive any IPv6 messages sent to the group's address.

IPv6 multicast addresses are mapped to a reserved set of media access control (MAC) multicast addresses. For information about the mapping of IPv6 multicast addresses to Ethernet MAC addresses, see RFC 2464, "Transmission of IPv6 Packets over Ethernet Networks."

Note

MLD messages

MLD is used to exchange membership status information between IPv6 routers that support multicasting and members of multicast groups on a network segment. Host membership in a multicast group is reported by individual member hosts, and membership status is periodically polled by multicast routers. MLD is defined in RFC 2710, "Multicast Listener Discovery (MLD) for IPv6."

MLD message types are described in the following table.

MLD message type Description

Multicast Listener Query

Sent by a multicast router to poll a network segment for group members. Queries can be general (requesting group membership for all groups), or specific (requesting group membership for a specific group).

Multicast Listener Report

Sent by a host when it joins a multicast group, or in response to an MLD Multicast Listener Query sent by a router.

Multicast Listener Done

Sent by a host when it leaves a host group and might be the last member of that group on the network segment.

MLD messages are sent as ICMPv6 messages.

Note

  • IPv6 is a rapidly evolving standard. The RFCs referenced might have been made obsolete by newer RFCs.