3
Figure 3 Multicast transmission
, Host B, Host D and Host E are receivers of the information. They 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:
•
Because multicast traffic flows to the farthest-possible node from the source before it is
replicated and distributed, an increase in the number of hosts will not increase the load of the
source and will not remarkably add to the usage of network resources.
•
Because multicast data is sent only to the receivers that need it, multicast uses network
bandwidth reasonably and enhances network security. In addition, data broadcast is confined to
the same subnet, but multicast is not.
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, and 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 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 with
the transmission of TV programs.