Multicast

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Categories: Connectivity

This is when, on a computer network, one device sends a stream of packets to a group of devices, which starts out as a single stream which is then duplicated by the routers through which it passes as necessary.

It is an alternative to Unicast where a computer sends data to one other device only.

If you want to send the same stream to multiple devices, for instance for broadcasting a TV or radio channel, it is much more efficient compared to multiple unicast streams. But it does not allow a receiving device to stop or rewind the source and it prevents the sender from easily knowing how many devices are receiving the stream.

In a broadcast center, it is commonly used for IP ring mains. Broadcast audio and video over IP mostly uses IP multicast, and with the appropriate control software can duplicate the functionality of an XY Broadcast Router (Matrix, NOT a IT network router). When switching a route, what is happening under the lid is that a receiving device joins a multicast stream. Challenges include how to cleanly switch video because the video timing (position of the vertical blanking interval) is invisible to the IT networking layers. One option is for the receiving device to join the new stream before leaving the old one, and to switch the decoded video.

IP Multicast

With IP, there are ranges of addresses which are designated as multicast address. A computer that wants to send out a stream sets the IP destination address to the designated address.

When a receiving device (computer of other type of host) want to receive a stream, it does 2 things. First it sends an IGMP join query to the nearest router. Secondly it asks its network hardware to start receiving packets from the Ethernet address corresponding to the IP multicast address of the stream.

Ethernet Multicast

When IP multicast is carried over Ethernet, there is no physical destination Ethernet (MAC) address, so, a sending device (computer or router) makes one up based on the IP address.

What Ethernet switches do with Multicast depends on whether they support IGMP Snooping. Basic switches will flood Ethernet Multicast packets out of all ports.

See IGMP, IGMP Snooping