To receive a redundant stream, this is all that is required. However
to send a redundant stream, the sender needs to know which codecs are
recommended for the primary and secondary (and tertiary, etc)
encodings. This information is specific to the redundancy format,
and is specified using an additional attribute "fmtp" which conveys
format-specific information. A session directory does not parse the
values specified in an fmtp attribute but merely hands it to the
media tool unchanged. For redundancy, we define the format
parameters to be a slash "/" separated list of RTP payload types.
Thus a complete example is:
m=audio 12345 RTP/AVP 121 0 5
As such, we need RTCRtpCodecParameters.parameters to define the "fmtp", which is basically a list of packets encapsulated within the red codec.
For for the red, I propose a sender and receiver parameter inside RTCRtpCodecParameters.parameters that contains: sequence<payloadtype> payloadTypes - the list of payload types encapsulated within the red packet [as defined within https://tools.ietf.org/html/rfc2198 Section 5 re fmtp]
So for example let's say red was payload 100 and contained a "0" payload type and a "5" payload type: