Installing and Administering Internet Services

Chapter 9 307
Configuring mrouted
Overview of Multicasting
network, all hosts that participate in IP multicast. The addresses of
other well-known permanent multicast groups are published in the
“Assigned Numbers” RFC (RFC-1060, March 1990).
IP multicast addresses can be used only as destination addresses and
should never appear in the source address field of a datagram. It should
also be noted that ICMP (Internet Control Message Protocol) error
messages are not generated for multicast datagrams.
Since IP internet addressing is a software manifestation of the
underlying physical network, IP addresses must be mapped to physical
addresses that are understood by the hardware comprising the network.
As such, IP multicast addresses are mapped to 802.3/Ethernet multicast
addresses. The IP multicasting addressing scheme, like that of
Ethernet’s, uses the datagram’s destination address to indicate multicast
delivery.
When mapping an IP multicast address 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 since only 23 of the 28 bits representing the multicast address
are used.
Multicast Groups
A multicast group is comprised of hosts that have indicated their
intent to join the multicast group by listening to the same IP multicast
address. Group membership is dynamic in that 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.
Multicast addresses are often temporary in that they are assigned to
transient groups, such as when users run an application that
dynamically registers to specific multicast addresses, and are then
discarded when all members of the group have left. Some multicast
addresses may be well-known addresses assigned to permanent groups
that always exist, even when their membership is empty.
Both hosts and mrouted routers that participate in IP multicast 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