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