Hello Ashutosh
First of all, services like Netflix and Youtube are not streaming services. They are video on demand services. This means that they implicitly require unicast functionality. The video that you watch on Youtube is being watched only by you at that specific time and no one else. Someone else may click on it a few seconds before or after you, but this is a uniquely separate event.
Streaming services that are candidates for using multicast include online radio and the broadcasting of live events over the Internet. However there are two reasons why they are not implemented:
- ISPs in general filter PIM and IGMP packets that are necessary to make multicast work
- When expanding to large scale multicast, the overhead required outweighs the benefits of the service
Concerning the first, multicast requires a multicast routing protocol such as Protocol Independent Multicast (PIM) which is the most popular one, although there are others. In order for this protocol to function on a network, it must be available end to end. PIM doesn’t normally run between ISP boundaries as it would be pretty complicated to do so. The protocol itself wasn’t designed for such a scale.
Secondly, keep in mind that multicast implicitly requires that end user device actions impact the forwarding state of a multicast-enabled IP router on the Internet. This means that for any multicast event, all end users (which may be from thousands of devices to millions) must have some way to inform multicast routers on the Internet of how the multicast routing should take place. When we are talking about such a large scale, the huge overhead required negates the advantages that multicast would give. This is why unicast is preferred at such scales.
In general, multicast is a technology that is used within a private network. It has been designed for such scales and thus is not implemented over the Internet. Having said that, some ISPs do offer multicast services WITHIN THEIR NETWORKS for enterprise customers, but those are special case scenarios.
I hope this has been informative and helpful!
Laz