I tried to set rtcpMuxPolicy as "negotiate" when init a PeerConnection and by the setConfiguration() function with no luck. I checked the commit https://trac.webkit.org/changeset/202671/webkit , and found it did not add support of setting the rtcpMuxPolicy. Please add this support. Tested on Safari Technology Preview Release 38.
<rdar://problem/34332118>
Right, there is no support right now. We should be able to add it though.