GloboTicket has really taken off, and all sorts of artists are selling
tickets to their shows via the GloboTicket system.
This is great for Globomantics employees as well who will get
access to virtually attend the concerts for free.
But how will such a multicast system work?
The first and most important part of any multicast deployment is the source.
This is the actual device sending some form of multicast stream
into the network. In the GloboTicket deployment,
a series of cameras at the venue will capture the concert
and send the data to the streaming server.
This server will use its video streaming software to send the feed into the
network so that interested Globomantics employees can tune in. In a lot of
ways, multicast is like local radio. Radio stations are the source of traffic
and generally do not change their position.
They send their transmission across the airwaves to radio
frequencies. Though the station is sending traffic out,
it has no way to measure or gauge the listeners tuning in at any given time.
When someone wants to listen to what that radio station is playing,
they tune into that radio frequency. The source is providing music,
news or whatever programming is on the schedule.
The receiver's radio is picking up the transmission from the
radio frequency over the air based on the tuner.
If the receiver no longer wants to listen,
they can change the channel or turn off their radio.
In the case of multicast, of course,
it's not radio waves, but the network carrying that transmission. The
source IP address will be the fixed IP of the sender.
The destination address will be a multicast group
address instead of a radio frequency.
The source still has no visibility on the receivers, and in that respect,
radio and multicast are remarkably similar.
The receiver looking for multicast traffic is akin to the radio tuner.
Signaling its interest in particular multicast groups is like tuning the
radio to that broadcast frequency. Receivers can provide some or all of
the information needed to request that traffic. In some deployments, the
receiver may only know the multicast group and be open to receive traffic
from any source that's sending. In others, the receiver knows exactly
what source it expects to be receiving and can request that directly.
If a receiver is unwilling or unable to request a regular
source, it will instead ask for any available source, sending
it to a multicast group of interest.
This is called any‑source multicast.
If a receiver is able to request a certain source,
it can provide that source address as part of the
request and receive only that stream. In the next section, we'll talk about both IGMP and MLD at a high level.