3Com 4210 PWR, Руководство по настройке

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190 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.
Multicast Address 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:
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:
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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.
n
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The IP addresses of a permanent multicast group keep unchanged, while the
members of the group can be changed.
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There can be any number of, or even zero, members in a permanent multicast
group.
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Those IP multicast addresses not assigned to permanent multicast groups can
be used by temporary multicast groups.