5
Multicast models
Based on how the receivers treat the multicast sources, the multicast models include any-source multicast
(ASM), source-filtered multicast (SFM), and source-specific multicast (SSM).
•
ASM model
—In the ASM model, any sender can send information to a multicast group as a
multicast source, and receivers can join a multicast group (identified by a group address) and
obtain multicast information addressed to that multicast group. In this model, receivers do not know
the positions of the 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 model. To a sender, the two models appear
to have the same multicast membership architecture.
The SFM model functionally extends the ASM 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. Because
not all multicast sources are valid to receivers, they are filtered.
•
SSM model
—Users might be interested in the multicast data from only certain multicast sources. The
SSM model provides a transmission service that enables users to specify the multicast sources that
they are interested in at the client side.
The main difference between the SSM model and the ASM model is that in the SSM model,
receivers have already determined 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 issues:
•
Where should the multicast source transmit information to? (Multicast addressing.)
•
What receivers exist on the network? (Host registration.)
•
Where is the multicast source that will provide data to the receivers? (Multicast source discovery.)
•
How should information be transmitted to the receivers? (Multicast routing.)
IP multicast is an end-to-end service. The multicast architecture involves the following parts:
1.
Addressing mechanism
—A multicast source sends information to a group of receivers through a
multicast address.
2.
Host registration
—Receiver hosts can join and leave multicast groups dynamically. This
mechanism is the basis for management of group memberships.
3.
Multicast routing
—A multicast distribution tree (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. The TCP/IP stack must
support reception and transmission of multicast data.
