Overview

Multicast is referred to as "one-to-many" and "many-to-many" communication.

With multicast, a node sends one set of multicast packets into a distribution tree and the multicast packets are duplicated and sent to receivers as needed.

One example is a training video coming from a server in the data center going to multiple users. The server sends one multicast stream. This stream is duplicated, as needed, by multicast routers in the distribution tree and sent only to the desired receivers.

There are significant benefits to using multicast, especially with IPv6. Multicast provides efficient traffic flows from a sender group to multiple individual receivers. Multicast is very scalable. A key benefit of multicast in IPv6 is that it completely replaces legacy IPv4’s broadcasts.

FYI: This lesson continues from the Introduction to IPv6 Address Types lesson.

Scalability

If we want to send a video stream to 100 receivers, sending the traffic via unicast would mean that 100 individual streams would need to be sent from the servers to the receivers.

This would obviously cause a lot of processor overhead and a lot of bandwidth usage. Multicast is a much more efficient way for senders to reach receivers. A server would send one stream to the multicast address and interested receivers would simply join the multicast group.

If we suddenly had a request to add 50 more receivers to the video stream, this would be a big issue for a unicast solution. With multicast, however, the new receivers would only need to join the multicast group. (Depending on the setup, there may be a need for some setup on router(s) to extend the distribution tree).

The point is the same, multicast is very scalable.

Performance

IPv4’s broadcasts waste bandwidth because traffic is sent to disinterested nodes. In addition, these disinterested nodes must inspect each broadcast packet and then decide to ignore it.

Security

A rogue device could be used to generate and intercept broadcasts. A hacker could do things like initiate a Denial of Service Attack by sending numerous broadcasts to overwhelm all of the nodes in a subnet. A Man in the Middle attack could also be launched by intercepting broadcasts then posing as a legitimate device. Broadcasts are a security risk!

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