3
Figure 3 Multicast transmission
As shown in
, the multicast source sends only one copy of the information to a multicast
group. Host B, Host D and Host E, which are receivers of the information, must join the multicast
group. The routers on the network duplicate and forward the information based on the distribution of
the group members. Finally, the information is correctly delivered to Host B, Host D, and Host E.
To summarize, multicast has the following advantages:
•
Advantages
over
unicast
—Multicast traffic is replicated and distributed until it flows to the
farthest-possible node from the source. The increase of receiver hosts will not remarkably
increase the load of the source or the usage of network resources
•
Advantages
over
broadcast
—Multicast data is sent only to the receivers that need it. This
reasonably uses network bandwidth and enhances network security. In addition, multicast data
is not confined to the same subnet.
Multicast features
Multicast transmission has the following features:
•
A multicast group is a multicast receiver set identified by an IP multicast address. Hosts join a
multicast group to become members of the multicast group before they can receive the
multicast data addressed to that multicast group. Typically, a multicast source does not need to
join a multicast group.
•
An information sender is called a "multicast source." A multicast source can send data to
multiple multicast groups at the same time. Multiple multicast sources can send data to the
same multicast group at the same time.
•
All hosts that have joined a multicast group become members of the multicast group. The group
memberships are dynamic. Hosts can join or leave multicast groups at any time. Multicast
groups are not subject to geographic restrictions.
•
Routers or Layer 3 switches that support Layer 3 multicast are called "multicast routers" or
"Layer 3 multicast devices." In addition to providing the multicast routing function, a multicast
router can also manage multicast group memberships on stub subnets with attached group
members. A multicast router itself can be a multicast group member.
For a better understanding of the multicast concept, you can compare multicast transmission to the
transmission of TV programs as shown in
.
