3Com 10014299 manual IP Multicast Routing Protocols

IP Multicast Routing Protocols 501

the IP multicast environment, the destination address of a data packet is not one address but a group, forming a group address. All the information receivers are added to a group, and once they access the group, data flowing to the destination address begin to transmit to the receivers of that particular group. All the group members can receive the data packet. Therefore, to get the data packet, they have to become group members first. The data packet transmitter is not required to be a group member. In the multicast environment, data will be sent to all the group members, and the subscribers who are not group members will not receive the data packets.

Generally, IP multicast has the following features:

■The membership of the host group is dynamic .There is no restriction on the location or the number of members in the host group. Independent hosts access or leave the multicast group at any time. These members can be anywhere on the Internet. One host can be a member of several multicast groups simultaneously.

■One host can send data packets to a multicast group even though it is not a group member. When sending the message to all the IP hosts in a multicast group, it is necessary to send a message to the group address only, just like unicast.

■There is no need for the router to save the membership for all the hosts. It is only necessary to know whether there is any host belonging to a certain multicast group on the network segment. The physical interface is located on the network segment. The host can only save the multicast groups it has joined.

IP Multicast Routing The multicast protocol includes two parts. One part is the Internet Group

ProtocolsManagement Protocol (IGMP) acting as the IP multicast basic signaling protocol. The other part includes the multicast routing protocols such as DVMRP, PIM-SM, PIM-DM, which implement IP multicast flow routing.

Internet Group IGMP is a simple protocol for the support of multicast transmission. IGMP is a Management Protocol simple leave/join protocol that allows end-user nodes and their multicast-enabled

(IGMP) routers to exchange messages that describe the wishes of hosts to participate in multicast groups. It defines the multicast membership establishment and maintenance mechanism between hosts and routers, and it is the foundation of the entire IP multicast.

IGMP informs routers about the group members, and enables routers to know the information about other members within the group through the hosts directly connected to them. Application programs can learn that information coming from one data source goes to a specific group. If a LAN subscriber announces that it has joined a certain multicast group via IGMP, the multicast routers in the LAN propagate this information by the multicast routing protocol, and finally add this LAN as a branch to the multicast tree. When the host, as a member of a certain group, begins to receive information, the routers periodically carry out queries on this group, and check whether the group members are still participating. As long as there is a host still participating, routers can continue to receive data. Only after all the subscribers in the LAN exit this multicast group, are the related branches deleted from the multicast tree.