Function find_matching_rtx_capability 

fn find_matching_rtx_capability<'a>(
    format: &MediaFormat,
    negotiated_associated_payload_type: PayloadType,
    capabilities: &'a MediaCapabilities,
) -> Option<&'a MediaCodecCapability>

RTX matching has one extra constraint beyond ordinary codec matching:

the router RTX capability must be associated with the already negotiated primary payload type, because RFC 4588 binds RTX to a specific primary PT through apt. https://www.rfc-editor.org/rfc/rfc4588.html#section-8.1