Internet Group Management Protocol (IGMP)
Internet Group Management Protocol (IGMP) allows communication between multicast transmitters and receivers.
Eos communication in Multi-Console Sessions is built on multicast data distribution, and both Streaming ACN (sACN) and v4 of Art-Net use multicast to transmit address levels. Configuring a control network to manage the efficient distribution of multicast traffic reduces unnecessary data transmission and ensures that data only reaches the appropriate destinations.
Overview
IGMP software on a managed switch must be present for Multicast transmission within a network. All IGMP versions and settings must match across switches in an entertainment control network.
When devices want to send or receive data to or from a universe, they send multicast packets. IGMP-enabled managed switches dynamically respond to requests for information about universes by creating IGMP groups for each one. Each group contains a list of all devices receiving information about that universe. When new data is sent to a universe, the switch distributes the information to all devices in the multicast group for that universe.
| IGMP Group | Sender | Receiver | |
|---|---|---|---|
| A | Universe 1 | Eos | 1 |
| B | Universe 2 | Eos | 2 |
For example, Eos is sending information to two DMX gateways via a switch. Gateway 1 is assigned to universe 1, and generating a multicast packet requesting universe 1 data which prompts the switch to create IGMP group A, containing gateway 1 as a subscriber of universe 1 information. Gateway 2 is assigned to universe 2, which similarly prompts its addition to newly-created IGMP group B. When Eos sends information to universe 1, the switch routes the universe 1 data to gateway 1. When Eos sends information to universe 2, the switch similarly uses group B to direct the data to gateway 2.
In larger non-entertainment control networks, IGMP is usually handled by a router. IGMP is generally not generated or handled by unmanaged switches or network hubs.
IGMP Groups
Network switches can manage varying numbers of total IGMP groups. The number of groups needed in a system is based on the quantity of devices, universes, and connections in the network.
A unique IGMP group is required for the following:
- Each Eos Family and ETCnomad device.
- Each sACN and Art-Net4 universe.
- Each DMX or show control gateway.
- Each Concert connection.
IGMP Querier
An IGMP querier is a device that sends query messages to discover which devices belong to which multicast groups. This is a crucial role that facilitates accurate and updated multicast membership information throughout the network. Unless other equipment has been specified, ETC recommends using a managed network switch as the IGMP querier in entertainment control networks.
There can be only one active IGMP querier on a network. Multiple backup queriers may be configured and elected to take over in the event the active querier stops working.
The querier sends out two different messages:
- General Membership Query - sent to all devices to identify which devices on the network are subscribed to which multicast groups. The destination IP address is 224.0.0.1 in IGMP v1 and v2, and 224.0.0.22 in IGMP v3.
- Group Specific Membership Query - sent to specific multicast groups to identify if that group has any devices on the network. The destination IP address is the multicast address for the group.
In IGMP version 3, the querier may also make changes to group memberships when it receives a relevant join or leave message from a host.
IGMP Host
An IGMP host is any receiving device that wishes to be part of a multicast group for a particular universe.
Hosts send out two different messages:
- Membership Reports - sent to inform the network switch that the host wishes to join a multicast group. These reports can be sent in response to a query or as an unsolicited request. The destination IP address is the multicast address the host wishes to subscribe to; for example, multicast address 239.255.0.1 for sACN universe 1.
- Leave Group - sent when the host wishes to leave a multicast group, including a list of the sources (or groups) the host wants to unsubscribe from. The destination IP address is 224.0.0.2 in IGMP v2 and v3.
In IGMP version 1, there is no process to leave a group; the host simply does not reply to the group specific membership query. The querier times out after not seeing a response and the multicast address is removed from the IGMP group.
Configuring IGMP
IGMP querier messages can be configured via the querier interval and robustness values.
- Querier Interval - the amount of time between separate general membership query transmissions.
- Querier Robustness - the quantity of general membership query packets sent with each transmission.
Packet Forwarding
IGMP packet forwarding takes multicast packets that are not being forwarded to specific destinations (i.e. no device has subscribed to the group), and will Broadcast them to all devices. Certain devices have been observed to not fully comply with IGMP registration, or do not send modern IGMP registration requests. If packet forwarding is disabled, these devices will not join IGMP groups, and will not receive packets (though ping and other tools may indicate them as online).
In systems where large amounts of multicast data is being generated but not subscribed to (like a fully patched system with no gateways online), the network may become inundated with broadcast traffic. This is a rare scenario, and should not be considered in a typical system design.
Net3 Show Control and I/O Gateways, CEM+ processors, and other legacy ETC devices do not send registration requests, and require IGMP packet forwarding to be enabled at the port or switch level.