What's the Leave Flow

Normal leave

As defined by IGMPv2, the router must send a group-specific query message after it receives the leave message from a host, and it considers that the host does not need the data of the group until the query times out. The following figure illustrates the flow of a normal leave (the same to IGMPv3).

Figure 1 Flow of a normal leave 

Two IGMP messages are transmitted in the case of a channel switching, one for leaving the original multicast group and one for joining the new multicast group. Therefore, traffic of two multicast groups exists on the subscriber line before the original multicast group is stopped. If the subscriber line does not reserve sufficient bandwidth for carrying the traffic of two multicast groups, traffic overflow (packet loss) will occur. For example, if video streams are carried, pixelation will occur.

Fast leave

When the device receives the leave message from a multicast user, it immediately stops forwarding the messages of the user. The following figure illustrates the flow of a fast leave (the same to IGMPv3).

Figure 2 Flow of a fast leave

Fast leave based on the MAC address

When the device generates a multicast group membership table, it not only records the multicast user but also records the MAC addresses of the multicast group members of the multicast user. A maximum of eight MAC addresses are supported for each multicast user. When the device receives a leave message, it first deletes the MAC addresses in the multicast group membership table, and it stops forwarding the messages of the group only when all the MAC addresses of the multicast user are deleted. The following figure illustrates the flow of a fast leave based on the MAC address (the same to IGMPv3).

Figure 3 Flow of a fast leave based on the MAC address (STB2 has silently left.)

NOTE:

The silent leaving of an STB indicates that the STB exceptionally leaves a program. For example, the STB is powered off or goes offline.

Figure 4 Flow of a fast leave based on the MAC address (STB2 is online.)

In summary, the three leave modes have their advantages and disadvantages. You can choose any of them according to actual needs and can set the leave mode for a multicast user.

	User-side Multi-STB Supported or Not	Bandwidth Occupation Time
Normal leave	Yes The STB quantity is not limited.	Aged upon reception of group-specific query
Fast leave	No	Released immediately
Fast leave based on the MAC address	Yes One multicast user supports a maximum of eight STBs at a time.	Released immediately

The following configurations are recommended and can be adopted by different users according to their home network topologies.

HG Function	STB Quantity	Reserved Bandwidth	Normal Leave	Fast Leave	Fast Leave Based on the MAC Address
No IGMP	One	Insufficient		√	√
		Sufficient	√	√	√
	Several	Insufficient			√ (Less than eight)
		Sufficient	√		√ (Less than eight)
IGMP snooping	One	Insufficient		√	√
		Sufficient	√	√	√
	Several	Insufficient			√ (Less than eight)
		Sufficient	√		√ (Less than eight)
IGMP proxy	One	Insufficient		√	√ (Not limited)
		Sufficient	√	√	√ (Not limited)
	Several	Insufficient		√	√ (Not limited)
		Sufficient	√	√	√ (Not limited)

Global Leave

As defined in TR101, the global leave message is an IGMP message with an all-zero group IP address, which indicates leaving all the groups.

• Network side

When the network topology changes, the device sends the global leave message to the upper-layer multicast router. After receiving the message, the upper-layer multicast router immediately sends the general query message, with the maximum response time set to the maximum time of responding to the group-specific query message. The device, after receiving the query message, responds to the upper-layer multicast router with the join message of the interested group. In this way, the multicast service can recover more quickly. Here, the network topology change events include ring network switching, line up/down, and active/standby port switching in a protect group.

NOTE:

• If the device is interconnected with a network device that does not support the global leave message, multicast services may be interrupted during the network topology change. Therefore, it is recommended that the global leave function be manually disabled on the device.
• The device supports sending of the global leave message only in IGMPv2.

• User side

When the STB is powered on immediately after a sudden power-off, because the STB cannot remember the previously-watched program, the bandwidth of the previously-watched program and the program resources are released only after the general query ages.
If the STB supports the global leave function, the STB sends a global leave message after it is re-powered on. After receiving the message, the device sends a general query message, with the maximum response time set to the maximum time of responding to the group-specific query message. If the multicast user is a fast-leave or MAC-based fast-leave user, the device releases all program resources of this multicast user. If the user is a normal-leave user, the device sends a group-specific query message and releases the program resources after the group-specific query times out.

NOTE:

Only the IGMPv2 global leave messages can be processed.

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