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Internet Group Management Protocol (IGMP)

Updated: January 21, 2005

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

Internet Group Management Protocol (IGMP)

The use of IP multicasting in TCP/IP networks is defined as a TCP/IP standard in RFC 1112, "Internet Group Management Protocol (IGMP)." In addition to defining address and host extensions for how IP hosts support multicasting, this RFC also defines the Internet Group Management Protocol (IGMP) version 1. RFC 2236, "Internet Group Management Protocol (IGMP), version 2" defines IGMP version 2. Both versions of IGMP provide a protocol to exchange and update information about host membership in specific multicast groups. Additionally, the Windows Server 2003 family supports IGMP version 3, described in the Internet draft entitled "Internet Group Management Protocol, version 3." With IGMP version 3, hosts can specify interest in receiving multicast traffic from specified sources or from all but a specific set of sources.

What is IP multicasting?

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

Other important aspects of IP multicasting include the following:

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

  • The ability of hosts to join multicast groups is performed through the sending of IGMP messages.

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

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

Multicast addressing

IP multicast addresses are reserved and assigned from within the Class D address range from 224.0.0.0 through 239.255.255.255. The following table is a partial list of well-known Class D addresses that are reserved for IP multicasting and registered with the Internet Assigned Numbers Authority (IANA).

 

IP multicast address Description

224.0.0.0

Base address (reserved).

224.0.0.1

The All Hosts multicast group that contains all systems on the same network segment.

224.0.0.2

The All Routers multicast group that contains all routers on the same network segment.

224.0.0.5

The Open Shortest Path First (OSPF) AllSPFRouters address. Used to send OSPF routing information to all OSPF routers on a network segment.

224.0.0.6

The OSPF AllDRouters address. Used to send OSPF routing information to OSPF designated routers on a network segment.

224.0.0.9

The RIP Version 2 group address. Used to send RIP routing information to all RIP v2 routers on a network segment.

224.0.1.24

WINS server group address. Used to support autodiscovery and dynamic configuration of replication for WINS servers. For more information, see WINS replication overview.

For a full and current listing of additional IP addresses that are reserved for multicasting, see the Internet Assigned Numbers Authority (IANA) Web site.

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

IP multicast addresses are mapped to a reserved set of media access control multicast addresses.

Note

IGMP messages

IGMP is used to exchange membership status information between IP routers that support multicasting and members of multicast groups. Host membership in a multicast group is reported by individual member hosts, and membership status is periodically polled by multicast routers.

IGMP message types are described in the following table.

 

IGMP message type Description

Host membership report

Sent when a host joins a multicast group to declare membership in a specific host group. IGMP host membership report messages are also sent in response to an IGMP host membership query sent by a router. For an IGMP version 3 host membership report message, the host can specify interest in receiving multicast traffic from specified sources or from all but a specific set of sources. Source-specific reporting prevents multicast-enabled routers from delivering multicast traffic to a subnet where there are no listening hosts.

Host membership query

Used by a multicast router to periodically poll a network for group members. For an IGMP version 3 host membership query message, the router can query the host's interest in receiving multicast traffic from a specified list of sources.

Leave group

Sent by a host when they leave a host group and are the last member of that group on the network segment.

IGMP messages are encapsulated and sent within IP datagrams as shown in the following illustration.

IGMP encapsulation in an IP datagram
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