1. sdp video codec生成

InternalEncoderFactory::InternalEncoderFactory() {  supported_codecs_.push_back(cricket::VideoCodec(kVp8CodecName));  if (webrtc::VP9Encoder::IsSupported())    supported_codecs_.push_back(cricket::VideoCodec(kVp9CodecName));  if (webrtc::H264Encoder::IsSupported()) {    cricket::VideoCodec codec(kH264CodecName);    // TODO(magjed): Move setting these parameters into webrtc::H264Encoder    // instead.    codec.SetParam(kH264FmtpProfileLevelId,                   kH264ProfileLevelConstrainedBaseline);    codec.SetParam(kH264FmtpLevelAsymmetryAllowed, "1");    supported_codecs_.push_back(std::move(codec));  }  supported_codecs_.push_back(cricket::VideoCodec(kRedCodecName));  supported_codecs_.push_back(cricket::VideoCodec(kUlpfecCodecName));  if (IsFlexfecAdvertisedFieldTrialEnabled()) {    cricket::VideoCodec flexfec_codec(kFlexfecCodecName);    // This value is currently arbitrarily set to 10 seconds. (The unit    // is microseconds.) This parameter MUST be present in the SDP, but    // we never use the actual value anywhere in our code however.    // TODO(brandtr): Consider honouring this value in the sender and receiver.    flexfec_codec.SetParam(kFlexfecFmtpRepairWindow, "10000000");    flexfec_codec.AddFeedbackParam(        FeedbackParam(kRtcpFbParamTransportCc, kParamValueEmpty));    flexfec_codec.AddFeedbackParam(        FeedbackParam(kRtcpFbParamRemb, kParamValueEmpty));    supported_codecs_.push_back(flexfec_codec);  }}

以上是生成sdp中支持的codec，除此之外还会生成rtx codec，创建的函数为CreateRtxCode，代码如下：

static void AppendVideoCodecs(const std::vector
    
     & input_codecs,                              std::vector
     
      * unified_codecs) {  for (VideoCodec codec : input_codecs) {    const rtc::Optional
      
        payload_type =        NextFreePayloadType(*unified_codecs, codec.name);    if (!payload_type)      return;    codec.id = *payload_type;    // TODO(magjed): Move the responsibility of setting these parameters to the    // encoder factories instead.    if (codec.name != kRedCodecName && codec.name != kUlpfecCodecName &&        codec.name != kFlexfecCodecName)      AddDefaultFeedbackParams(&codec);    // Don't add same codec twice.    if (FindMatchingCodec(*unified_codecs, codec))      continue;    unified_codecs->push_back(codec);    // Add associated RTX codec for recognized codecs.    // TODO(deadbeef): Should we add RTX codecs for external codecs whose names    // we don't recognize?    if (CodecNamesEq(codec.name, kVp8CodecName) ||        CodecNamesEq(codec.name, kVp9CodecName) ||        CodecNamesEq(codec.name, kH264CodecName) ||        CodecNamesEq(codec.name, kRedCodecName)) {      const rtc::Optional
       
         rtx_payload_type =          NextFreePayloadType(*unified_codecs);      if (!rtx_payload_type)        return;      unified_codecs->push_back(          VideoCodec::CreateRtxCodec(*rtx_payload_type, codec.id));    }  }}