2-6
ASM model
In the ASM model, any sender can send information to a multicast group as a multicast source, and
numbers of receivers can join a multicast group identified by a group address and obtain multicast
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.
SFM model
The SFM model is derived from the ASM. From the view of a sender, the two models have the same
multicast membership architecture.
The SFM model functionally extends the ASM model: In the SFM model, the upper layer software
checks the source address of received multicast packets and permits or denies multicast traffic from
specific sources. Therefore, receivers can receive the multicast data from only part of the multicast
sources. From the view of a receiver, multicast sources are not all valid: they are filtered.
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/SFM 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 the receivers need to receive multicast data from? (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.
