The spec here [changed](https://github.com/w3c/webrtc-pc/pull/2353) two years ago, and `"negotiate"` was dropped due to "[lack of implementer interest](https://www.w3.org/TR/2018/CR-webrtc-20180927/)". Even though it's what Firefox implements, this wasn't caught at TPAC 2019 because we didn't have the prerequisite WebIDL in place, we simply just implemented this by default, in spite of the [spec at the time](https://www.w3.org/TR/2018/CR-webrtc-20180927/#dom-rtcconfiguration-rtcpmuxpolicy) requiring that `"require"` be the default value. We also didn't implemented any of the transport objects, and this would have required and extra one: `rtcpTransport`. Other browsers never supported this. `"require"` remains the default, but is [now](https://w3c.github.io/webrtc-pc/#rtcrtcpmuxpolicy-enum) also the sole option.
Bug 1339203 Comment 2 Edit History
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The spec here [changed](https://github.com/w3c/webrtc-pc/pull/2353) two years ago, and `"negotiate"` was dropped due to "[lack of implementer interest](https://www.w3.org/TR/2018/CR-webrtc-20180927/)". Even though it's what Firefox implements, this wasn't caught at TPAC 2019 because we didn't have the prerequisite WebIDL in place, we simply just implemented this by default, in spite of the [spec at the time](https://www.w3.org/TR/2018/CR-webrtc-20180927/#dom-rtcconfiguration-rtcpmuxpolicy) requiring that `"require"` be the default value. We also didn't implement any of the transport objects, and this would have required and extra one: `rtcpTransport`. Other browsers never supported this. `"require"` remains the default, but is [now](https://w3c.github.io/webrtc-pc/#rtcrtcpmuxpolicy-enum) also the sole option.