Multicast, the original English text is Multicast, and some people translate it into multicast. It is one of the three communication methods in IP networks (the other two are broadcast-Broadcast, and point-to-point communication is also called Unicast-Unicast). In the application environment of industrial fieldbus industrial Ethernet, multicast technology is the most commonly used one to ensure that multiple communication slave stations belonging to a communication master station can receive real-time information from the master station. The same information, that is, the time at which each slave receives the information is relatively consistent.
Multicast technology mainly relies on a multicast address (including multiple destination addresses) as the destination address of the information, and uses the most efficient transmission strategy: information is only transmitted once on each network link, only in the chain. This information will be copied when the road branches. So how does multicast technology manage multicast groups consisting of multiple destination addresses? The following common multicast technology protocols have provisions: Internet Group Management Protocol (IGMP), Protocol Independent Multicast (PIM), Distance Vector Multicast Routing Protocol (DVMRP), Multicast OSPF (MOSPF), Multicast BGP (MBGP), Multicast Source Discovery Protocol (MSDP), and Multicast Listener Discovery (MLD). Due to the length of the relationship, I first introduced the IGMP-Internet Group Management Protocol, which is the Internet Group Management Protocol, because it is the most commonly used multicast technology supported by all industrial Ethernet switching products.
The Internet Group Management Protocol is used between IP hosts and their immediate neighboring multicast agents to support the allocation of temporary multicast group addresses and the addition and deletion of multicast group members. IP multicast is defined as the transmission of an IP datagram (packet) to a "(destination) host group. A "host group" consisting of zero or more hosts is identified by a single IP destination address. A multicast datagram is delivered to all members of its destination host group, with the same "besteffort-UDP" security as a regular unicast IP datagram, which means that the datagram is not guaranteed to achieve its purpose. All members of the group, or not in the same order as other datagrams. The number of members of the host group is dynamic; that is, the host can participate in and leave the group at any time. There is no limit to the number or location of members in a host group, but members are limited to those hosts that have dedicated access keys. A host may be a member of multiple groups at the same time. A host that is not a member of a group can also send datagrams to it. The host group may be permanent or temporary. The permanent group has a well-known, officially assigned IP address. It is an address, not a member of the group, that is, permanent; at any time, a permanent group may have many members, and may even have zero members. On the other hand, a temporary group is dynamically assigned an address when it is created by a request from a host. When its members fall to zero and the temporary group is to be dismissed, its address can be redistributed. The creation of temporary groups and the maintenance of group membership information are the responsibility of a "multicast proxy" (an entity that exists within an Internet gateway or other dedicated host). At least one multicast proxy is directly connected to each IP network or subnet that supports IP multicast. The host requests to create a new group, join or leave the existing group by exchanging messages with the neighbor agent. The multicast agent is also responsible for the interconnection network delivery of multicast IP datagrams. When sending a multicast IP datagram, the host transmits it to a local area network multicast address, which identifies all neighbor members of the destination host group. If the group has members in other networks, the multicast agent becomes a secondary receiver for local multicast and relays the datagram to the agents on other networks through the Internet gateway system. Finally, the agent on the other network transmits the datagram as a local multicast to the neighbors of their own destination group.
At present, the vast majority of industrial Ethernet switching equipment used in the field is a Layer 2 switching device, that is, it does not support Layer 3 technologies such as the multicast technology just mentioned. All of these IE switches use IGMP Snooping technology - by dynamically configuring Layer 2 ports to suppress the proliferation of multicast traffic, so that multicast traffic is only forwarded to those ports associated with IP multicast devices. As its name implies, IGMPsnooping requires an IE switch to probe the IGMP transport intent between the host and the Layer 3 device and record the ports of the multicast group and its members. When an IE switch receives a report from a host that IGMP reports to join a particular multicast group, the switch adds the port number to which the host is connected in its forwarding entry; when the IE switch When a report from a host that wants to leave a particular multicast group is received, it removes the switch port number it is connected from. The switch also periodically deletes its multicast group member entries if it does not receive an IGMP multicast member report from the multicast client. Layer 3 switching devices in the network, such as Layer 3 switches or routers, periodically send IGMP general queries to all VLANs (virtual local area networks). Once the IGMP snooping function on the IE switch is turned on, the switch responds to the interrogation of the Layer 3 switching device based on the MAC multicast group's join request for the connected device, and for each VLAN, each MAC multicast group is in its Create a corresponding entry in the forwarding entry.
Due to space reasons, the introduction to IGMP ends here. Experts are welcome to correct me.
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