Microsoft KB Archive/193907

= Windows NT 4.0 SP4 Supports IGMP Version 2 =

Article ID: 193907

Article Last Modified on 11/1/2006

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APPLIES TO


 * Microsoft Windows NT Workstation 4.0 Developer Edition
 * Microsoft Windows NT 4.0 Service Pack 4

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This article was previously published under Q193907



SUMMARY
Windows NT 3.51 and later supports Internet Group Management Protocol (IGMP) version 1. The Service Pack 4 update for Windows NT 4.0 provides support for IGMP version 2.



MORE INFORMATION
IGMP is used by IP hosts to report their host group memberships to any immediately-neighboring multicast routers. IGMP messages are encapsulated in IP datagrams, with an IP protocol number of 2. Internetwork forwarding of IP multicast datagrams is handled by multicast routers. Windows NT currently does not support multicast forwarding.

IP multicasting is the transmission of an IP datagram to a "host group," a set of zero or more hosts identified by a single IP destination address. The membership of a group is dynamic; that is, hosts may join and leave groups at any time.

A permanent group has a well-known, administratively assigned IP address. It is the address, not the membership of the group, that is permanent. Host groups are identified by class D IP addresses (from 224.0.0.0 to 239.255.255.255). 224.0.0.0 is guaranteed not to be assigned to any group, and 224.0.0.1 is assigned to the permanent group of all IP hosts (including gateways). There is no multicast address for all hosts on the total Internet. RFC 1700 has a list of permanent Internet multicast addresses.

There are two types of IGMP message of concern to host-router interaction in IGMP v1:


 * Host Membership Query
 * Host Membership Report

Multicast routers send Host Membership Query messages to discover which host groups have members on their attached local networks. The multicast router needs to know this information because, if there is a host group with no members in a segment, it will not forward multicast packets for that group to the segment. Queries are addressed to the all-hosts group (address 224.0.0.1).

Hosts respond to a Query by generating Host Membership Reports, reporting each host group to which they belong on the network interface from which the Query was received. Routers do not know -- and do not need to know -- which hosts belong to a group, only that at least one host belongs to a group on a particular network. When a host joins a new group, it should immediately transmit a Report for that group.

There are three types of IGMP message of concern to the host-router interaction in IGMP v2:


 * Membership Query
 * Version 2 Membership Report
 * Leave Group

There is an additional type of message, for backwards-compatibility with IGMP v1:


 * Version 1 Membership Report

IGMP v2 allows a computer to inform the router that it is leaving a group. This update enables the router to determine if there are no more members in a group and then runs a command to stop forwarding multicast packets on to the link. This update is useful when users are frequently joining and leaving groups.

When a host leaves a multicast group, if it was the last host to reply to a Query with a Membership Report for that group, it should send a Leave Group message to the all-routers multicast group (224.0.0.2).

An IGMP v2 host may be placed on a subnet where the querying router has not yet been upgraded to IGMP v2. The IGMP v1 router will send General Queries with the Max Response Time set to 0. This must be interpreted as a value of 100 (10 seconds). An IGMP v2 host may suppress Leave Group messages on a network where the query is using IGMP v1.

An IGMP v2 router may be placed on a subnet where at least one router on the subnet has not yet been upgraded to IGMP v2. The query must use IGMP v1. The use of IGMP v1 must be administratively configured.

For additional information, check the "IGMPLevel" Windows NT registry parameter in the following article in the Microsoft Knowledge Base:

120642 TCP/IP and NBT Configuration Parameters for Windows 2000 or Windows NT

For additional information, also see the following RFCs:


 * RFC 1054 - Host Extensions for IP Multicasting
 * RFC 1112 - Internet Group Management Protocol, Version 2

Additional query words: IGMP 2.0 4.0

Keywords: kbfea kbinfo kbnetwork KB193907

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