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Group Membership Protocols Overview
Multicast group membership protocols allow a router to know when a host on a directly attached subnet, typically a LAN, wants to receive traffic from a certain multicast group. Even if more than one host on the LAN wants to receive traffic for that multicast group, the router has to send only one copy of each packet for that multicast group out on that interface because of the inherent broadcast nature of LANs. Only when the router is informed by the multicast group membership protocol that there are no interested hosts on the subnet can the packets be withheld and that leaf pruned from the distribution tree.
There is one standard IP multicast group membership protocol: the Internet Group Management Protocol (IGMP). However, IGMP has several versions that are supported by hosts and routers. There are currently three versions of IGMP:
- IGMPv1—The original protocol defined in RFC 1112. An explicit join message is sent to the router, but a timeout is used to determine when hosts leave a group. This process wastes processing cycles on the router, especially on older or smaller routers.
- IGMPv2—Defined in RFC 2236. Among other features, IGMPv2 adds an explicit leave message to the join message so that routers can more easily determine when a group has no interested listeners on a LAN.
- IGMPv3—Defined in RFC 3376. Among other features, IGMPv3 optimizes support for a single source of content for a multicast group, or source-specific multicast (SSM). (RFC 1112 supported both many-to-many and one-to-many multicast, but one-to-many is considered the more viable model for the Internet at large.)
Although the various versions of IGMP are backward compatible, it is common for a router to run multiple versions of IGMP on LAN interfaces because backward compatibility is achieved by dropping back to the most basic of all versions run on a LAN. For example, if one host is running IGMPv1, any router attached to the LAN running IGMPv2 drops back to IGMPv1 operation, effectively eliminating the IGMPv2 advantages. Running multiple IGMP versions ensures that both IGMPv1 and IGMPv2 hosts find peers for their versions on the router.
