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310 CHAPTER 18: MULTICAST PROTOCOL
Figure 78 Comparison between the unicast and multicast transmission
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 tunnelling 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
loads.
■ Optimized performance decreases traffic redundancy.
■ Distributed applications make multipoint applications possible.
Multicast Addresses 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. A packet
has more than one destination address in a multi-cast environment, that is, 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.
Server
Unicast
Server
Multicast
Receiver
Receiver
Receiver
Receiver
Receiver
Receiver