What is multicast? How it works with NDI? 

NDI supports multicast-based video sources using multicast UDP with forwards error correction to correct for packet loss. It is important to be aware that using multicast on a network that is not configured correctly is very similar to a “denial of service” attack on the entire network; for this reason, multicast sending is disabled by default. 

Every router that we have tested has treated multicast traffic as if it was broadcast traffic by default. Because most multicast traffic on a network is low bandwidth, this is of little consequence, and generally allows a network router to run more efficiently because no packet filtering is required. 

What this means, though, is that every multicast packet received is sent to every destination on the network, regardless of whether it was needed there or not. Because NDI requires high bandwidth multicast, even with a limited number of sources on a large network, the burden of sending this much data to all network sources can cripple the entire network’s performance. 

To avoid this serious problem, it is essential to ensure that every router on the network has proper multicast filtering enabled. This option is most commonly referred to as “IGMP snooping”. This topic is described in detail at https://en.wikipedia.org/wiki/IGMP_snooping. If you are unable to find a way to enable this option, we recommend that you use multicast NDI with all due caution. 

Another important cautionary note is that a software application like NDI will subscribe to a multicast group, and will unsubscribe from it when it no longer needs that group. 

Unlike most operations in the operating system, the un-subscription step is not automated by the OS; once you are subscribed to a group, your computer will continue to receive data until the router sends an IGMP query to verify whether it is still needed. This happens about every 5 minutes on typical networks. 

The result is that if you launch an NDI multicast stream and kill your application without closing the NDI connection correctly, your computer will continue to receive the data from the network until this timeout expires.