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

C

HAPTER

 15: M

ULTICAST

 O

VERVIEW

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Host registration: A receiving host joins and leaves a multicast group 
dynamically using the membership registration mechanism. 

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Multicast routing: A router or switch transports packets from a multicast source 
to receivers by building a multicast distribution tree with multicast routes. 

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Multicast application: A multicast source must support multicast applications, 
such as video conferencing. The TCP/IP protocol suite must support the 
function of sending and receiving multicast information. 

As receivers are multiple hosts in a multicast group, you should be concerned 
about the following questions: 

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What destination should the information source send the information to in the 
multicast mode? 

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How to select the destination address? 

These questions are about multicast addressing. To enable the communication 
between the information source and members of a multicast group (a group of 
information receivers), network-layer multicast addresses, namely, IP multicast 
addresses must be provided. In addition, a technology must be available to map IP 
multicast addresses to link-layer MAC multicast addresses. The following sections 
describe these two types of multicast addresses: 

Internet Assigned Numbers Authority (IANA) categorizes IP addresses into five 
classes: A, B, C, D, and E. Unicast packets use IP addresses of Class A, B, and C 
based on network scales. Class D IP addresses are used as destination addresses of 
multicast packets. Class D address must not appear in the IP address field of a 
source IP address of IP packets. Class E IP addresses are reserved for future use. 

In unicast data transport, a data packet is transported hop by hop from the source 
address to the destination address. In an IP multicast environment, there are a 
group of destination addresses (called group address), rather than one address. All 
the receivers join a group. Once they join the group, the data sent to this group of 
addresses starts to be transported to the receivers. All the members in this group 
can receive the data packets. This group is a multicast group. 

A multicast group has the following characteristics: 

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The membership of a group is dynamic. A host can join and leave a multicast 
group at any time. 

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A multicast group can be either permanent or temporary. 

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A multicast group whose addresses are assigned by IANA is a permanent 
multicast group. It is also called reserved multicast group. 

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