The Internet Group Management Protocol (IGMP) is a

communications protocol A communication protocol is a system of rules that allows two or more entities of a communications system to transmit information via any kind of variation of a physical quantity. The protocol defines the rules, syntax, semantics and synch ...

used by

hosts A host is a person responsible for guests at an event or for providing hospitality during it. Host may also refer to: Places * Host, Pennsylvania, a village in Berks County People *Jim Host (born 1937), American businessman *Michel Host ...

and adjacent routers on IPv4 networks to establish multicast group memberships. IGMP is an integral part of

IP multicast IP multicast is a method of sending Internet Protocol (IP) datagrams to a group of interested receivers in a single transmission. It is the IP-specific form of multicast and is used for streaming media and other network applications. It uses speci ...

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 Video on demand (VOD) is a media distribution system that allows users to access videos without a traditional video playback device and the constraints of a typical static broadcasting schedule. In the 20th century, broadcasting in the form of ...

and

gaming Gaming may refer to: Games and sports The act of playing games, as in: * Legalized gambling, playing games of chance for money, often referred to in law as "gaming" * Playing a role-playing game, in which players assume fictional roles * Playing ...

, and allows more efficient use of resources when supporting these types of applications. IGMP is used on

IPv4 Internet Protocol version 4 (IPv4) is the fourth version of the Internet Protocol (IP). It is one of the core protocols of standards-based internetworking methods in the Internet and other packet-switched networks. IPv4 was the first version d ...

networks. Multicast management on

IPv6 Internet Protocol version 6 (IPv6) is the most recent version of the Internet Protocol (IP), the communications protocol that provides an identification and location system for computers on networks and routes traffic across the Internet. I ...

networks is handled by

Multicast Listener Discovery Multicast Listener Discovery (MLD) is a component of the Internet Protocol Version 6 (IPv6) suite. MLD is used by IPv6 routers for discovering multicast listeners on a directly attached link, much like Internet Group Management Protocol (IGMP) is ...

(MLD) which is a part of

ICMPv6 Internet Control Message Protocol version 6 (ICMPv6) is the implementation of the Internet Control Message Protocol (ICMP) for Internet Protocol version 6 (IPv6). ICMPv6 is an integral part of IPv6 and performs error reporting and diagnostic fun ...

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_basic_architecture

IGMP operates between a host and a local multicast router.

Switches In electrical engineering, a switch is an electrical component that can disconnect or connect the conducting path in an electrical circuit, interrupting the electric current or diverting it from one conductor to another. The most common type of ...

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 In the seven-layer OSI model of computer networking, the network layer is layer 3. The network layer is responsible for packet forwarding including routing through intermediate routers. Functions The network layer provides the means of tran ...

(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 switch A multilayer switch (MLS) is a computer networking device that switches on OSI layer 2 like an ordinary network switch and provides extra functions on higher OSI layers. The MLS was invented by engineers at Digital Equipment Corporation. Swit ...

es 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.

Versions

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 Source-specific multicast (SSM) is a method of delivering multicast packets in which the only packets that are delivered to a receiver are those originating from a specific source address requested by the receiver. By so limiting the source, SSM re ...

. IGMPv2 improves IGMPv1 by adding the ability for a host to signal desire to leave a multicast group. IGMPv3 improves IGMPv2 by supporting

and introduces membership report aggregation. These versions are backwards compatible. A router supporting IGMPv3 can support clients running IGMPv1, IGMPv2 and IGMPv3. IGMPv1 uses a query-response model. Queries are sent to 224.0.0.1. Membership reports are sent to the group's multicast address. IGMPv2 accelerates the process of leaving a group and adjusts other timeouts. Leave-group messages are sent to 224.0.0.2. A group-specific query is introduced. Group-specific queries are sent to the group's multicast address. A means for routers to select an IGMP querier for the network is introduced. IGMPv3 introduces

capability. Membership reports are sent to 224.0.0.22.

Messages

There are several types of IGMP messages: ;General membership queries :Sent by multicast routers to determine which multicast addresses are of interest to systems attached to the network(s) they serve to refresh the group membership state for all systems on its network. ;Group-specific membership queries :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 ;Membership reports :Sent by multicast receivers in response to a membership query or asynchronously when first registering for a multicast group ;Leave group messages :Sent by multicast receivers when specified multicast transmissions are no longer needed at the receiver IGMP messages are carried in bare IP packets with

IP protocol number This is a list of the IP protocol numbers found in the field ''Protocol'' of the IPv4 header and the ''Next Header'' field of the IPv6 header. It is an identifier for the encapsulated protocol and determines the layout of the data that immediately ...

2. Similar to the

Internet Control Message Protocol The Internet Control Message Protocol (ICMP) is a supporting protocol in the Internet protocol suite. It is used by network devices, including routers, to send error messages and operational information indicating success or failure when commun ...

, there is no

transport layer In computer networking, the transport layer is a conceptual division of methods in the layered architecture of protocols in the network stack in the Internet protocol suite and the OSI model. The protocols of this layer provide end-to-e ...

used with IGMP messaging.

IGMPv2 messages

Where: ; Type : Indicates the message type as follows : ; Max Resp Time : Specifies the required responsiveness of replies to a Membership Query (0x11). This field is meaningful only in Membership Query; in other messages it is set to 0 and ignored by the receiver. The field specifies time in units of 0.1 second (a field value of 10 specifies 1 second). Larger values reduce IGMP traffic burstiness and smaller values improve protocol responsiveness when the last host leaves a group. ; 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. The message is sent using the following IP destination addresses:

IGMPv3 membership query

Where: ; Max Resp Code : This field specifies the maximum time (in 1/10 second increments) 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 to 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 : The Source Address fields are a vector of n IP unicast addresses, where n is the value in the Number of Sources (N) field.

Implementations

The

FreeBSD FreeBSD is a free and open-source Unix-like operating system descended from the Berkeley Software Distribution (BSD), which was based on Research Unix. The first version of FreeBSD was released in 1993. In 2005, FreeBSD was the most popular ...

,IGMPv3 was added to FreeBSD in version 8.0.

Linux Linux ( or ) is a family of open-source Unix-like operating systems based on the Linux kernel, an operating system kernel first released on September 17, 1991, by Linus Torvalds. Linux is typically packaged as a Linux distribution, whi ...

IGMPv3 was added in the Linux 2.5 kernel series. and

Windows Windows is a group of several proprietary graphical operating system families developed and marketed by Microsoft. Each family caters to a certain sector of the computing industry. For example, Windows NT for consumers, Windows Server for se ...

operating system An operating system (OS) is system software that manages computer hardware, software resources, and provides common daemon (computing), services for computer programs. Time-sharing operating systems scheduler (computing), schedule tasks for ef ...

s support IGMP at the host side.

Notes

References

{{Authority control Internet protocols Internet Standards Internet layer protocols Network layer protocols