In this example, it is sending an MPEG transport stream over UDP to the loopback address 127.0.0.1:9988 but that could just as easily be the IP address and port of another Mac: ffmpeg \ The video capture/sending and the video receipt/playback can be done on the same machine, or ffmpeg can send to a remote IP. On the 2012 MBP, the first video device (index 0) is FaceTime HD Camera (Built-in) and the first audio device (index 0) is Built-in Microphone.
List the audio and video input devices with ffmpeg -f avfoundation -list_devices true -i '' ffplay version 4.3.2 (my executable was named ffplay4).I successfully implemented something close to this (not true RTSP, but instead RTP streaming over UDP) with the following hardware and software:
