Basic Concepts
This section describes basic multicast concepts,
including multicast group, multicast source, multicast group member,
multicast router, and multicast distribution tree.
On the network shown in
Figure 1, Interested Users are interested in receiving data from Source.
Multicast needs to be deployed.
Figure 1 Diagram of data transmission in multicast mode
- Multicast group: an arbitrary group of receivers that want to
receive a particular data stream. Hosts that want to receive the data
stream for a particular group must join the group.
- Multicast source: a sender of data streams, for example, Source
in Figure 1. One multicast
source can send data to multiple multicast groups at the same time
and multiple multicast sources can send data to one multicast group
at the same time. Multicast sources do not need to join any multicast
group.
- Multicast group member: a host that successfully joins a multicast
group, for example, Interested User in Figure 1. Multicast group members
are dynamic. Hosts can join or leave a multicast group at any time.
Multicast group members can reside anywhere on networks.
- Multicast router: the router that supports multicast function
on the network is called multicast router. The multicast router
described in this manual indicates the FW running the multicast
function. A multicast router cannot only route packets to receivers
but also manage multicast group members on the leaf network connected
to user hosts. A multicast router can also be a multicast group member.
- Multicast distribution tree: based on member distribution, the
multicast routing protocol sets up routing in the shape of a tree
for forwarding data packets to multiple destinations. Packets are
copied and distributed at the farthest branch from the source and
are sent to members of the multicast group.
The following example compares multicast transmission
to a TV channel:
The multicast group is the link between a sender and a receiver,
like a TV channel.
The TV station acts as the multicast source and sends packets
to the channel.
The TV is the receiver host. Audiences turn on the TV and select
the program of their choice on a channel, which is like hosts joining
a group. The TV plays the channel's programs, which is like hosts
receiving data sent to the group.
Audiences can turn the TV on or off or switch channels randomly,
just as hosts dynamically join or leave a multicast group.