5
ASM model
In the ASM model, any sender can send information to a multicast group as a multicast source.
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 examines 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. To a
receiver, not all multicast sources are valid because 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.
In the SSM model, receivers have already determined the locations of the multicast sources. This is
the main difference between the SSM model and the ASM model. In addition, a different multicast
address range than the ASM/SFM model is used in the SSM model. 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 provides 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:
•
Addressing
mechanism
—A multicast source sends information to a group of receivers
through a multicast address.
•
Host
registration
—Receiver hosts can join and leave multicast groups dynamically. This
mechanism is the basis for management of group memberships.
•
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.
•
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.
Multicast addresses
Network-layer multicast addresses (multicast IP addresses) enables communication between
multicast sources and multicast group members. In addition, a technique must be available to map
multicast IP addresses to link-layer multicast MAC addresses.
IP multicast addresses
•
IPv4 multicast addresses:
