Concept

Internet Group Management Protocol

Summary
The Internet Group Management Protocol (IGMP) is a communications protocol used by hosts and adjacent routers on IPv4 networks to establish multicast group memberships. IGMP is an integral part of IP multicast and allows the network to direct multicast transmissions only to hosts that have requested them. IGMP can be used for one-to-many networking applications such as online streaming video and gaming, and allows more efficient use of resources when supporting these types of applications. IGMP is used on IPv4 networks. Multicast management on IPv6 networks is handled by Multicast Listener Discovery (MLD) which is a part of ICMPv6 in contrast to IGMP's bare IP encapsulation. A network designed to deliver a multicast service using IGMP might use this basic architecture: IGMP operates between a host and a local multicast router. Switches featuring IGMP snooping also derive useful information by observing these IGMP transactions. Protocol Independent Multicast (PIM) is then used between the local and remote multicast routers to direct multicast traffic from hosts sending multicasts to hosts that have registered through IGMP to receive them. IGMP operates on the network layer (layer 3), just the same as other network management protocols like ICMP. The IGMP protocol is implemented on hosts and within routers. A host requests membership to a group through its local router while a router listens for these requests and periodically sends out subscription queries. A single router per subnet is elected to perform this querying function. Some multilayer switches include an IGMP querier capability to allow their IGMP snooping features to work in the absence of an IGMP-capable router in the layer 2 network. IGMP is vulnerable to some attacks, and firewalls commonly allow the user to disable it if not needed. There are three versions of IGMP. IGMPv1 is defined by , IGMPv2 is defined by and IGMPv3 was initially defined by . updates both IGMPv3 and MLDv2 to better support source-specific multicast.
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