Multicast Addresses - 3Com 7700 Configuration Manual

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HAPTER ULTICAST

Multicast Addresses

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ROTOCOL
Figure 1 Comparison Between the Unicast and Multicast Transmission
Unicast
Server
Multicast
Server
Note: A multicast source does not necessarily belong to a multicast group. It only
sends data to the multicast group and it is not necessarily a receiver. Multiple
sources can send packets to a multicast group simultaneously.
A router that does not support multicast may exist on the network. A multicast
router can encapsulate multicast packets in unicast IP packets by tunneling and
sending them on to the neighboring multicast router. The neighboring multicast
router removes the unicast IP header and continues the multicast transmission.
Multicast advantages:
Enhanced efficiency by reducing network traffic and relieving server and CPU
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loads.
Optimized performance decreases traffic redundancy.
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Distributed applications make multipoint applications possible.
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The destination addresses of multicast packets use Class D IP addresses ranging
from 224.0.0.0 to 239.255.255.255. Class D addresses cannot appear in the
source IP address fields of IP packets.
During unicast data transmission, a packet is transmitted from the source address
to the destination address with the "hop-by-hop" principle of the IP network.
However, a packet has more than one destination address in a mutli-cast
environment, i.e., a group of addresses. All the information receivers join a group.
Once a receiver joins the group, data flowing to the group is sent to the receiver
immediately. All members in the group can receive the packets. Membership of a
multicast group is dynamic, that is, hosts can join and leave groups at any time.
A multicast group can be either permanent or temporary. Part of addresses in the
multicast group are reserved by the IANA and are known as the permanent
multicast group. IP addresses of a permanent group are unchanged but the
members in the group can change. The number of members in a permanent
multicast group can be random or even 0. Those IP multicast addresses that are
not reserved for permanent multicast groups can be used by temporary groups.
Receiver
Receiver
Receiver
Receiver
Receiver
Receiver