3Com 4210 PWR, Configuration Manual

Page: 191 / 567

Multicast Models 189
Multicast Models
Based on the multicast source processing modes, there are three multicast models:
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Any-Source Multicast (ASM)
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Source-Filtered Multicast (SFM)
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Source-Specific Multicast (SSM)
ASM model In the ASM model, any sender can become a multicast source and send
information to a multicast group; 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 a
multicast source 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. From the view of a sender, the
two models have the same multicast group membership architecture.
Functionally, the SFM model is an extension of the ASM model. In the SFM model,
the upper layer software checks the source address of received multicast packets
so as to permit or deny 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
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
The purpose of IP multicast is to transmit information from a multicast source to
receivers in the multicast mode and to satisfy information requirements of
receivers. You should be concerned about:
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Host registration: What receivers reside on the network?
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Technologies of discovering a multicast source: Which multicast source should
the receivers receive information from?
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Multicast addressing mechanism: Where should the multicast source transports
information?
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Multicast routing: How is information transported?
IP multicast is a kind of peer-to-peer service. Based on the protocol layer sequence
from bottom to top, the multicast mechanism contains addressing mechanism,
host registration, multicast routing, and multicast application:
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Addressing mechanism: Information is sent from a multicast source to a group
of receivers through multicast addresses.