The Internet Group Management Protocol (IGMP) is a communications protocol used by hosts and adjacent routers on IP networks to establish multicast group memberships. IGMP is an integral part of IP multicast.
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 uses ICMPv6 messaging in contrast to IGMP's bare IP encapsulation.
Architecture
A network designed to deliver a multicast service using IGMP might use this basic architecture:
IGMP operates between the client computer and a local multicast router. Switches featuring IGMP snooping 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 the multicast server to many multicast clients.
IGMP operates above the network layer, though it does not actually act as a transport protocol.
Host and router implementations
The IGMP protocol is implemented on a particular host and within a router. A host requests membership to a group through its local router while a router listens for these requests and periodically sends out subscription queries.The FreeBSD,Linux and Windows operating systems support IGMP at the host side.
Security
IGMP is vulnerable to some attacks, and firewalls commonly allow the user to disable it if not needed.Packet structure
IGMP messages are carried in bare IP packets with IP protocol number 2. There is no transport layer used with IGMP messaging, similar to Internet Control Message Protocol for example.IGMPv3 membership query
Membership Queries are sent by multicast routers to determine which multicast addresses are of interest to systems attached to its network. Routers periodically send General Queries to refresh the group membership state for all systems on its network. Group-Specific Queries are used for determining the reception state for a particular multicast address. Group-and-Source-Specific Queries allow the router to determine if any systems desire reception of messages sent to a multicast group from a source address specified in a list of unicast addresses.
Where:
Max Resp Code
This field specifies the maximum time (in 1/10 second) allowed before sending a responding report. If the number is below 128, the value is used directly. If the value is 128 or more, it is interpreted as an exponent and mantissa.Checksum
This is the 16-bit one's complement of the one's complement sum of the entire IGMP message.Group Address
This is the multicast address being queried when sending a Group-Specific or Group-and-Source-Specific Query. The field is zeroed when sending a General Query.Resv
This field is reserved. It should be zeroed when sent and ignored when received.S (Suppress Router-side Processing) Flag
When this flag is set, it indicates to receiving routers that they are to suppress the normal timer updates.QRV (Querier's Robustness Variable)
If this is non-zero, it contains the Robustness Variable value used by the sender of the Query. Routers should update their Robustness Variable to match the most recently received Query unless the value is zero.QQIC (Querier's Query Interval Code)
This code is used for specify the Query Interval value (in seconds) used by the querier. If the number is below 128, the value is used directly. If the value is 128 or more, it is interpreted as an exponent and mantissa.Number of Sources (N)
This field specifies the number of source addresses present in the Query. For General and Group-Specific Queries, this value is zero. For Group-and-Source-Specific Queries, this value is non-zero, but limited by the network's MTU.Source Address [i]
The Source Address [i] fields are a vector of n IP unicast addresses, where n is the value in the Number of Sources (N) field.IGMPv2 messages
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