Internet Group Management Protocol (IGMP) Windows 2000 provides level 2 (full) support for IP multicasting (IGMP version 2), as described in RFC 1112 and RFC 2236. The introduction to RFC 1112 provides a good overall summary of IP multicasting. The text reads: "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. A multicast datagram is delivered to all members of its destination host group with the same 'best-effort' reliability as regular unicast IP datagrams; that is, the datagram is not guaranteed to arrive intact to all members of the destination group or in the same order relative to other datagrams. "The membership of a host group is dynamic; that is, hosts may join and leave groups at any time. There is no restriction on the location or number of members in a host group. A host may be a member of more than one group at a time. A host need not be a member of a group to send datagrams to it. "A host group may be permanent or transient. A permanent group has a well-known, administratively assigned IP address. It is the address�not the membership of the group�that is permanent; at any time a permanent group may have any number of members, even zero. Those IP multicast addresses that are not reserved for permanent groups are available for dynamic assignment to transient groups that exist only as long as they have members. "Internetwork forwarding of IP multicast datagrams is handled by multicast routers that may be co-resident with, or separate from, Internet gateways. A host transmits an IP multicast datagram as a local network multicast that reaches all immediately-neighboring members of the destination host group. If the datagram has an IP time-to-live greater than 1, the multicast router(s) attached to the local network take responsibility for forwarding it towards all other networks that have members of the destination group. On those other member networks that are reachable within the IP time-to-live, an attached multicast router completes delivery by transmitting the datagram as a local multicast."
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