Overview
The mrouted Routing Daemon
Normally, IP multicast addresses are mapped to 802.3 or Ethernet multicast addresses. The IP multicasting addressing scheme, similar to Ethernet’s scheme, uses the datagram’s destination address to indicate multicast delivery.
When an IP multicast address is mapped to an Ethernet multicast address, the
Multicast Groups
A multicast group comprises hosts with an intention to join the multicast group by listening to the same IP multicast address. Group membership is dynamic, that is, a host may join or leave a group at any time. A host may be a member of one or more groups simultaneously. Additionally, a host is allowed to send multicast datagrams to a group without being a member of the group.
You can assign multicast addresses to transient groups because the multicast address are often temporary. A typical transient group scenario is when users run an application that dynamically registers to specific multicast addresses, which are discarded later when all members of the group have left. Some multicast addresses may be assigned to permanent groups that always exist, even when their membership is empty.
Both hosts and mrouted routers that participate in IP multicasting use the Internet Group Management Protocol (IGMP) to communicate multicast group information among themselves. Hosts use IGMP to inform mrouted routers that they are joining a group. mrouted routers use IGMP to pass multicast routing information to other mrouted routers, and to check whether a host is still an active group member.
The underlying TCP/IP stack must support ICMP to participate in IP multicasting. While IGMP defines a standard for communicating information, it does not define a standard for how the multicast information is propagated among multicast routers. Consequently, DVMRP enables multicast routers to efficiently communicate group membership information among themselves. DVMRP uses IGMP messages to carry routing and group membership information. DVMRP
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