H3C Technologies H3C WX6000 Series Access Controllers - Manual (Page 312)

28-6
information addressed to that multicast group. In this model, receivers are not aware of the position of
multicast sources in advance. However, they can join or leave the multicast group at any time.
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:
z
Where should the multicast source transmit information to? (multicast addressing)
z
What receivers exist on the network? (host registration)
z
Where is the multicast source from which the receivers need to receive multicast data? (multicast
source discovery)
z
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 28-2 .