3com 4210 PWR 9-Port 3CR17341-91-ME 用户手册

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) 

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.

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. 

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. 

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.