Multicast Architecture 545
SSM model
In the practical life, users may be interested in the multicast data from only certain
multicast sources. The SSM model provides a transmission service that allows users
to specify the multicast sources they are interested in at the client side.
The radical difference between the SSM model and the ASM model is that in the
SSM model, receivers already know the locations of the multicast sources by some
other means. In addition, the SSM model uses a multicast address range that is
different from that of the ASM model, and dedicated multicast forwarding paths
are established between receivers and the specified multicast sources.
Multicast Architecture IP multicast addresses the following questions:
■ Where should the multicast source transmit information to? (multicast
addressing)
■ What receivers exist on the network? (host registration)
■ Where is the multicast source from which the receivers need to receive
multicast data? (multicast source discovery)
■ How should information be transmitted to the receivers? (multicast routing)
IP multicast falls in the scope of end-to-end service. The multicast architecture
involves the following four parts:
1 Addressing mechanism: Information is sent from a multicast source to a group of
receivers through a multicast address.
2 Host registration: Receiver hosts are allowed to join and leave multicast groups
dynamically. This mechanism is the basis for group membership management.
3 Multicast routing: A multicast distribution tree (namely a forwarding path tree for
multicast data on the network) is constructed for delivering multicast data from a
multicast source to receivers.
4 Multicast applications: A software system that supports multicast applications,
such as video conferencing, must be installed on multicast sources and receiver
hosts, and the TCP/IP stack must support reception and transmission of multicast
data.
Multicast Addresses To allow communication between multicast sources and multicast group
members, network-layer multicast addresses, namely, multicast IP addresses must
be provided. In addition, a technique must be available to map multicast IP
addresses to link-layer multicast MAC addresses.
IPv4 multicast addresses
Internet Assigned Numbers Authority (IANA) assigned the Class D address space
(224.0.0.0 to 239.255.255.255) for IPv4 multicast. The specific address blocks and
usages are shown in Table 52.
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