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Overview
The mrouted Routing Daemon
Chapter 120
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 low-order 23 bits of the IP multicast address are placed into
the low-order 23 bits of the special Ethernet multicast address. The
hexadecimal value of the special Ethernet multicast address is
01-00-5E-00-00-00. The resultant Ethernet address, however, is not
unique, because only 23 out of the 28 bits representing the multicast
address are used.
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