Multicast Listener Discovery (MLD)
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4/8/2010
Multicast Listener Discovery (MLD) enables you to manage subnet multicast membership for IPv6. MLD is a series of three Internet Control Message Protocols for IPv6 (ICMPv6) messages that replaces the Internet Group Management Protocol (IGMP) that is used for IPv4.
The use of multicasting in IP networks is defined as a TCP/IP standard in RFC 1112. This RFC defines addresses and host extensions for the way IP hosts support multicasting. The same concepts originally developed for IP version 4 (IPv4), also apply to IPv6.
MLD is defined in RFC 2710.
Multicasting overview
Multicast traffic is sent to a single address but is processed by multiple hosts. Hosts listening on a specific multicast address make up a multicast group, and they receive and process traffic sent to the group address.
Group membership is dynamic, allowing hosts to join and leave the group at any time. Groups are not limited by size and members can be from multiple network segments (links or subnets) if the connecting routers support forwarding of multicast traffic and group membership information.
A host can send traffic to a group address without belonging to the group. In fact, to join a group, a host sends a group membership message. Multicast routers periodically poll membership status.
Each multicast group is identified by one IPv6 multicast address. All group members who listen and receive IPv6 messages sent to the group address share the group address.
IPv6 multicast addressing
IPv6 multicast addresses are reserved, assigned, and registered with the Internet Assigned Numbers Authority (IANA). IPv6 multicast addresses have the Format Prefix 1111 1111 (0xFF). The following table shows some of the reserved IPv6 multicast addresses.
| 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 current list of IPv6 addresses that are reserved for multicasting, see RFC 2375.
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
Web addresses can change, so you might not be able to connect to the Web site or sites mentioned here.
MLD messages
MLD messages are used to determine group membership on a network segment, also known as a link or subnet. MLD messages are sent as ICMPv6 messages.
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 can request 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 is the last member of that group on the network segment. |
For a list of multicast addresses that have been 128-bit addresses that have been assigned or reserved, see Information Sciences Institute Web site.
See Also
Concepts
Core Protocols of IPv6
Internet Group Management Protocol (IGMP)