IGMP Technical Summary (1)

IGMP is short for Internet Group Management Protocol (Internet Group Management Protocol. It is a protocol for Managing IP multicast members in the TCP/IP protocol family. It is used to establish and maintain group members between IP hosts and directly adjacent multicast routers. How does IGMP work? What is the difference between different versions? This blog will introduce you in detail...

1. IGMP version

So far, IGMP has three versions:

· IGMPv1 (defined by RFC 1112)/· IGMPv2 (defined by RFC 2236)/· IGMPv3 (defined by RFC 3376)

All versions of IGMP support the ASM (Any-Source Multicast, Any Source Multicast) model. IGMPv3 can be directly applied to the SSM (Source-Specific Multicast, specifying the Source Multicast) model, IGMPv1 and IGMPv2 can be applied to SSM models only with the support of igmp ssm Mapping technology.

2. IGMPv1 Working Mechanism

IGMPv1 manages multicast group members based on query and response mechanisms.

When multiple multicast routers exist in a CIDR block, because they can all receive IGMP Membership Report messages (Membership Report Message) from the host ), therefore, it is sufficient for one router to send an IGMP Query Message. In this case, a query machine (Querier) election mechanism is required to determine which router is used as the IGMP query machine.

For IGMPv1, the multicast routing protocol (such as PIM) selects the unique multicast information forwarder DR (Designated Router) as the IGMP queryer.

IGMP Query

As shown in, assume that Host B and Host C want to receive multicast data sent to multicast group G1, and Host A wants to receive multicast data sent to multicast group G2, the basic process for adding a host to a multicast group and maintaining the multicast group membership is as follows:

(1) The host will send an IGMP member relationship report packet to the multicast group to which it is to be added to declare the addition, instead of waiting for the IGMP query packet sent by the IGMP queryer;

(2) The IGMP queryer periodically sends IGMP Query Packets (the destination address is 224.0.0.1) to all hosts and routers in the primary network segment in multicast mode );

(3) After receiving the query message, pay attention to one of the Host B and Host C of G1 (this depends on who's delay timer times out first) -- for example, Host B first sends an IGMP member relationship report packet to G1 in multicast mode to declare it belongs to G1. Because all hosts and routers in the gossip network segment can receive the report sent from Host B to G1, when Host C receives the report, it will not send the same report packet for G1, because the IGMP Router (Router A and Router B) already know that there are hosts interested in G1 in the gossip network segment. This mechanism is called the IGMP member relationship report Suppression Mechanism on the host, which helps reduce the information traffic of the Region network segment;

(4) At the same time, because Host A is concerned with G2, it will still send A report packet to G2 in multicast mode to declare that it belongs to G2;

(5) After the above query and response process, the IGMP router understands that there are members of G1 and G2 in the gossip network segment, so it is generated by the multicast routing protocol (such as PIM) (*, G1) and (*, G2) multicast forwarding items serve as the basis for forwarding multicast data. "*" indicates any Multicast Source;

(6) When the multicast data from the multicast source to G1 or G2 is routed to the IGMP router through the Multicast Route, because the IGMP router has (*, G1) and (*, g2) multicast forwarding item, so the multicast data is forwarded to the multicast CIDR block, and the recipient's host can receive the multicast data.

IGMPv1 does not specifically define the packet that leaves the multicast group. When the host running IGMPv1 leaves a multicast group, no report packet is sent to the multicast group to be left. When no multicast group member exists in the CIDR Block, the IGMP router will not receive any report packets sent to the multicast group, the IGMP router then deletes the multicast forwarding item corresponding to the multicast group after a period of time.