NDI (Network Device Interface) is a new network-based video and audio distribution protocol (video over IP on Ethernet-based LAN) developed by NewTek. Detailed description of NDI and its benefits can be found on the Official NDI page. It can be used TouchDesigner-to-TouchDesigner or with numerous other software tools and hardware products that support NDI.
The NDI protocol automatically discovers NDI video sources using mDNS, which is a multi-cast service discovery protocol. If your NDI sources are on networks not reachable via multi-cast, you'll need to specify the IP of the source machines in the NDI In TOPs parameters.
Video sent over NDI is sent as YUV 4:2:2 format, and is compressed. The video can optionally include an alpha channel as well.
TouchDesigner is compiled against the Version 3 of the NDI protocol, so multi-cast servers are supported. For help on how to enable multi-cast for NDI, refer to the NDI documentation. Control for multi-cast is done outside of TouchDesigner.
NDI supports sources tagging themselves as being parts of one or more named 'groups'. When an NDI receiver searches for sources, it can specify groups it's interested in to reduce the number of sources it lists.
NDI provides a useful mobile application to stream NDI from phones. Search for Newtek NDI on the stores.