Yup, the format is called SDP. It's an old, crufty, and mostly effective way to specify not only the port and IP, but also whether a TCP fallback is being used,

Yup, the format is called SDP. It's an old, crufty, and mostly effective way to specify not only the port and IP, but also whether a TCP fallback is being used, video and audio formats, and other things you can mostly ignore when just using data channels. See the "answer" section of 5.2.3 of [1], for details of some of the other magic numbers (which your JS library should handle for you). You can also check out about:webrtc to see SDPs made by other websites, like the Janus demo page. [1] https://tools.ietf.org/html/draft-ietf-rtcweb-sdp-03 https://tools.ietf.org/html/draft-ietf-rtcweb-sdp-03