3Com 7757 Configuration Manual page 417

Multicast Address
Multicast routing: How is information transported?
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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:
Addressing mechanism: Information is sent from a multicast source to a group
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of receivers through multicast addresses.
Host registration: A receiving host joins and leaves a multicast group
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dynamically to implement membership registration.
Multicast routing: A router or switch establishes a packet distribution tree and
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transports packets from a multicast source to receivers.
Multicast application: A multicast source must support multicast applications,
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such as video conferencing. The TCP/IP protocol stack 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:
What destination should the information source send the information to in the
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multicast mode?
How to select the destination address, that is, how does the information source
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know who the user is?
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:
IP multicast address
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:
The membership of a group is dynamic. A host can join and leave a multicast
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group at any time.
A multicast group can be either permanent or temporary.
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Multicast Architecture 417