IGMP Fundamentals

IGMP Messages

Figure 1 IGMP networking

Figure 1 shows the IGMP message types.

IGMP Query message: This type of message is sent by a router to hosts to learn whether multicast receivers exist on a specific network segment. IGMP Query messages are sent only by queriers. IGMP Query messages are categorized into the following types:
- General Query message: It does not contain specific source or group information.
- Group-specific Query message: It contains specific multicast group information, but does not contain specific source information.
- Group-and-Source-Specific Query message: It contains both specific multicast source and group information.
IGMP Report message: It is sent by a host to an upstream device when the host wants to join a multicast group.
IGMP Leave message: It is sent by a host to an upstream device when the host wants to leave a multicast group.

IGMPv2 and IGMPv3 support leave messages, but IGMPv1 does not.

IGMP Querier and Non-Querier

An IGMP multicast device can either be a querier or a non-querier:

Querier

A querier is responsible for sending IGMP Query messages to hosts and receiving IGMP Report messages and Leave messages from hosts. A querier can then learn which multicast group has receivers on a specified network segment.
Non-querier

A non-querier only receives IGMP Report messages from hosts to learn which multicast group has receivers. Then, based on the querier's action, the non-querier identifies which receivers leave multicast groups.

Generally, a network segment has only one querier. Multicast devices follow the same principle to select a querier. The process is as follows (using DeviceA, DeviceB, and DeviceC as examples):

After IGMP is enabled on DeviceA, DeviceA considers itself a querier in the startup process by default and sends IGMP Query messages. If DeviceA receives IGMP Query messages from DeviceB that has a lower IP address, DeviceA changes from a querier to a non-querier. DeviceA starts the another-querier-existing timer and records DeviceB as the querier of the network segment.
If DeviceA is a non-querier and receives IGMP Query messages from the querier DeviceB, the another-querier-existing timer is updated; if DeviceA is a non-querier and receives IGMP Query messages from DeviceC that has a lower IP address than the querier DeviceB, the querier is changed to DeviceC, and the another-querier-existing timer is updated.
If DeviceA is a non-querier and the another-querier-existing timer expires, DeviceA changes to a querier.

IGMPv1 does not support querier election. An IGMPv1 querier is designated by the upper-layer protocol, such as PIM. In this version, querier election can be implemented only among multicast devices that run the same IGMP version on a network segment.

IGMP Implementation

IGMP enables a multicast router to identify receivers by sending IGMP Query messages to hosts and receiving IGMP Report messages and Leave messages from hosts. A multicast router forwards multicast data to a network segment only if the network segment has multicast group members. Hosts can decide whether to join or leave a multicast group.

As shown in Figure 2, IGMP-enabled DeviceA functions as a querier to periodically send IGMP Query messages. All hosts (Host A, Host B, and Host C) on the same network segment of DeviceA can receive these IGMP Query messages.

Figure 2 IGMP networking

When a host (for example, Host A) receives an IGMP Query message of a multicast group G, the processing flow is as follows:
- If Host A is already a member of group G, Host A replies with an IGMP Report message of group G at a random time within the response period specified by DeviceA.
  
  After receiving the IGMP Report message, DeviceA records information about group G and forwards the multicast data to the network segment of the host interface that is directly connected to DeviceA. Meanwhile, DeviceA starts a timer for group G or resets the timer if it has been started. If no members of group G respond to DeviceA within the interval specified by the timer, DeviceA stops forwarding the multicast data of group G.
- If Host A is not a member of any multicast group, Host A does not respond to the IGMP Query message from DeviceA.
When a host (for example, Host A) joins a multicast group G, the processing flow is as follows:

Host A sends an IGMP Report message of group G to DeviceA, instructing DeviceA to update its multicast group information. Subsequent IGMP Report messages of group G are triggered by IGMP Query messages sent by DeviceA.
When a host (for example, Host A) leaves a multicast group G, the processing flow is as follows:

Host A sends an IGMP Leave message of group G to DeviceA. After receiving the IGMP Leave message, DeviceA triggers a query to check whether group G has other receivers. If DeviceA does not receive IGMP Report messages of group G within the period specified by the query message, DeviceA deletes the information about group G and stops forwarding multicast traffic of group G.

Message Processing Characteristics in Different IGMP Versions

IGMP Version	Characteristic
IGMPv1	IGMPv1 manages multicast groups by exchanging IGMP Query messages and IGMP Report messages. In IGMPv1, a host does not send an IGMP Leave message when leaving a multicast group, and a router deletes the record of a multicast group when the timer for maintaining the members in the multicast group expires. IGMPv1 provides only General Query messages.
IGMPv2	In IGMPv2, an IGMP Report message contains information about a multicast group, but does not contain information about a multicast source. A message contains the record of a multicast group. After a host sends an IGMP Report message of a multicast group to a router, the router notifies the multicast forwarding module of this join request. Then the multicast forwarding module can correctly forward multicast data to the host. IGMPv2 is capable of suppressing IGMP Report messages to reduce repetitive IGMP Report messages. This function works as follows: After a host (for example, Host A) joins a multicast group G, Host A receives an IGMP Query message from the router. Then the host randomly selects a value from 0 to the maximum response time (specified in the IGMP Query message) as the timer value. When the timer expires, Host A sends an IGMP Report message of group G to the router. However, if Host A receives an IGMP Report message of group G from another host in group G before the timer expires, Host A does not send an IGMP Report message of group G to the router. When a host leaves group G, the host sends an IGMP Leave message of group G to a router. Because of the Report message suppression mechanism in IGMPv2, the router cannot determine whether another host exists in group G. Therefore, the router triggers a query on group G. If another host exists in group G, the host sends an IGMP Report message of G to the router. If the router sends the query on group G for a specified number of times, but does not receive an IGMP Report message for group G, the router deletes information about group G and stops forwarding multicast data of group G. IGMPv2 provides General Query messages and Group-specific Query messages. NOTE: Both IGMP queriers and non-queriers can process IGMP Report message, while only queriers can forward IGMP Report messages. IGMP non-queriers cannot process IGMP Leave messages.
IGMPv3	An IGMPv2 Report message contains information about multicast groups, but does not contain information about multicast sources. Therefore, an IGMPv2 host can select a multicast group, but not a multicast source/group. IGMPv3 has resolved the problem. The IGMPv3 message from a host can contain multiple records of multicast groups, with each multicast group record containing multiple multicast sources. On the router side, the querier sends IGMP Query messages and receives IGMP Report and Leave messages from hosts to identify network segments that contain receivers and forward the multicast data to the network segments. In IGMPv3, source information in multicast group records can be filtered in either include mode or exclude mode: In include mode: If a source is included in a group record and the source is active, the router forwards the multicast data of the source. If a source is included in a group record but the source is inactive, the router deletes the source information and does not forward the multicast data of the source. In exclude mode: If a source is active, the router forwards the multicast data of the source, because there are hosts that require the multicast data of the source. If a source is inactive, the router does not forward the multicast data of the source. If a source is excluded in a group record, the router forwards the multicast data of the source. IGMPv3 does not have the Report message suppression mechanism. Therefore, all hosts joining a multicast group must reply with IGMP Report messages when receiving IGMP Query messages. In IGMPv3, multicast sources can be selected. Therefore, besides the common query and multicast group query, an IGMPv3-enabled device adds the designated multicast source and group query, enabling the router to find whether receivers require data from a specified multicast source.
Advantages of IGMPv2 over IGMPv1	IGMPv2 provides IGMP Leave messages, and thus IGMPv2 can manage members of multicast groups effectively. The multicast group can be selected directly, and thus the selection is more precise.
Advantages of IGMPv3 over IGMPv2	IGMPv3 allows hosts to select multicast sources, while IGMPv2 does not. An IGMPv3 message contains records of multiple multicast groups, and thus the number of IGMP messages is reduced on the network segment.

IGMP Group Compatibility

In IGMP group compatibility mode, a multicast device of a later IGMP version is compatible with the hosts of an earlier IGMP version. For example, an IGMPv2 multicast device can process join requests of IGMPv1 hosts; an IGMPv3 multicast device can process join requests of IGMPv1 and IGMPv2 hosts.

In IGMP group compatibility mode, if a multicast device receives IGMP Report messages from hosts running an earlier IGMP version, the multicast device automatically changes the version of the corresponding multicast group to be the same as that of the hosts and then operates in the earlier IGMP version. The process works as follows:

When an IGMPv2 multicast device receives an IGMPv1 Report message from a multicast group, the multicast device lowers the IGMP version of the multicast group to IGMPv1. Then, the multicast device ignores the IGMPv2 Leave messages of the multicast group.
When an IGMPv3 multicast device receives IGMPv2 Report messages from a multicast group, the multicast device lowers the IGMP version of the multicast group to IGMPv2. Then, the multicast device ignores the IGMPv3 BLOCK messages and the multicast source list in the IGMPv3 TO_EX messages. The multicast source-selecting function of IGMPv3 messages is then disabled.
When an IGMPv3 multicast device receives IGMPv1 Report messages from a multicast group, the multicast device lowers the IGMP version of the multicast group to IGMPv1. Then, the multicast device ignores the IGMPv2 Leave messages, IGMPv3 BLOCK messages, IGMPv3 TO_IN messages, and multicast source list in the IGMPv3 TO_EX messages.

If you manually change the IGMP version of a multicast device to a later version, the multicast device still operates in the original version if group members of the original version exist. The multicast device upgrades its IGMP version only after all group members of the original version leave.

Router-Alert Option for IGMP

Generally, a packet is sent to and processed by the routing protocol layer only if the packet's destination IP address is the IP address of a local interface. An IGMP packet's destination IP address is usually a multicast address, so that IGMP packets may not be sent to the routing protocol layer for processing.

To allow IGMP packets to be sent to the routing protocol layer, the Router-Alert option mechanism is used to mark protocol packets. If a packet contains the Router-Alert option, the packet must be sent to and processed by the routing protocol layer.

After a multicast device receives an IGMP packet:

If the multicast device does not check the Router-Alert option and sends the IGMP packet to the routing protocol layer, irrespective of whether the IGMP packet contains the Route-Alert option.
If the multicast device is configured to check the Router-Alert option, the multicast device sends the IGMP packet to the routing protocol layer only if the packet contains the Route-Alert option.