Internet Group Management Protocol

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 on the network layer, just the same as other network management protocols like ICMP.

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, and firewalls commonly allow the user to disable it if not needed.

Standards

There are three versions of IGMP, as defined by Request for Comments (RFC) documents of the Internet Engineering Task Force (IETF). 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 over IGMPv1 by adding the ability for a host to signal desire to leave a multicast group. IGMPv3 improves over IGMPv2 mainly by supporting source-specific multicast and Membership Report aggregation.

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 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

Where:

Type 

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:

IGMPv3 membership query

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 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.

Implementations

The FreeBSD, Linux and Windows operating systems support IGMP at the host side.