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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.

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 architecture example

IGMP operates between a host 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 hosts sending multicasts to hosts that have registered through IGMP to receive them.

IGMP operates on the "network layer, just the same as other network management protocols like "ICMP.[1]

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. 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 IP multicast capability in the larger network.

IGMP is vulnerable to some attacks,[2][3][4][5] and firewalls commonly allow the user to disable it if not needed.


There are three versions of IGMP.[6] IGMPv1 is defined by "RFC 1112, IGMPv2 is defined by "RFC 2236 and IGMPv3 was initially defined by "RFC 3376 and has been updated by "RFC 4604 which defines both IGMPv3 and MLDv2. IGMPv2 improves IGMPv1 by adding the ability for a host to signal desire to leave a multicast group. IGMPv3 improves IGMPv2 by supporting "source-specific multicast[7] and introduces membership report aggregation.

These versions are backwards compatible. A router supporting IGMPv3 can support clients running IGMPv1, IGMPv2 and IGMPv3.

Packet structure[edit]

IGMP messages are carried in bare IP packets with "IP protocol number 2.[8] There is no transport layer used with IGMP messaging, similar to the "Internet Control Message Protocol.

There are several types of IGMP messages: Membership Queries (general and group-specific), Membership Reports, and Leave Group messages.

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.

IGMPv2 messages[edit]

IGMPv2 packet structure[9]
+ Bits 0–7 8–15 16–31
0 Type Max Resp Time Checksum
32 Group Address


Indicates the message type as follows: Membership Query (0x11), Membership Report (IGMPv1: 0x12, IGMPv2: 0x16, IGMPv3: 0x22), Leave Group (0x17)
Max Resp Time 
Specifies the time limit for the corresponding report. The field has a resolution of 100 milliseconds, the value is taken directly. This field is meaningful only in Membership Query (0x11); in other messages it is set to 0 and ignored by the receiver.
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 to following IP addresses:

IGMPv2 destination address[10]
Message Type Multicast Address
General Query All hosts (
Group-Specific Query The group being queried
Membership Report The group being reported
Leave Group All routers (

IGMPv3 membership query[edit]

IGMPv3 membership query[11]
bit offset 0–3 4 5–7 8–15 16–31
0 Type = 0x11 Max Resp Code Checksum
32 Group Address
64 Resv S QRV QQIC Number of Sources (N)
96 Source Address [1]
128 Source Address [2]
. . .
Source Address [N]


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.
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.
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 [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.


The "FreeBSD,[note 1] "Linux[note 2] and "Windows "operating systems support IGMP at the host side.

See also[edit]


  1. ^ IGMPv3 was added to FreeBSD in version 8.0.
  2. ^ IGMPv3 was added in the Linux 2.5 kernel series.


  1. ^ Forouzan, Behrouz A. (2012). Data Communications and Networking (5th ed.). New York, NY: McGraw-Hill. p. 658. "ISBN "0073376221. 
  2. ^ Spoofed IGMP report denial of service vulnerability.
  3. ^ "Fragmented IGMP Packet May Promote "Denial of Service" Attack". Dec 20, 2004. Archived from the original on 2005-12-13. 
  4. ^ IGMP Security Problem Statement and Requirements Archived 2006-10-13 at the "Wayback Machine..
  5. ^ "Vulnerability in TCP/IP Could Allow Denial of Service (MS06-007, 913446))". February 14, 2006. Archived from the original on 2007-02-05. 
  6. ^ IP Multicast Routing Configuration Guide, "Cisco, pp. 25–28, retrieved 2017-05-27 
  7. ^ "Internet Group Management Protocol Overview". Javvin. Archived from the original on 2010-11-10. Retrieved 2010-11-18. 
  8. ^ RFC 3376 Section 4
  9. ^ RFC 2236 Section 2
  10. ^ RFC 2236 Section 9
  11. ^ RFC 3376 Section 4.1
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