附：

mediasoup不会向WebRTC发送RTX重传流的SR报文的证据：

void Transport::SendRtcp(uint64_t nowMs)
{
	...

	for (auto& kv : this->mapConsumers)
	{
		auto* consumer = kv.second;

		// 1.GetRtpStreams返回的是媒体流。
		for (auto* rtpStream : consumer->GetRtpStreams())
		{
			packet.reset(new RTC::RTCP::CompoundPacket());

			// 5.生成consumer的SR报文，rtpStream是媒体流。
			consumer->GetRtcp(packet.get(), rtpStream, nowMs);
		}
	}
}

std::vector<RTC::RtpStreamSend*> GetRtpStreams() override
{
	// 2.rtpStreams是媒体流。
	return this->rtpStreams;
}

void SimulcastConsumer::CreateRtpStream()
{
	...
	auto& encoding         = this->rtpParameters.encodings[0];

	RTC::RtpStream::Params params;

	params.ssrc           = encoding.ssrc;

	this->rtpStream = new RTC::RtpStreamSend(this, params, bufferSize);
	// 3.rtpStreams是媒体流。
	this->rtpStreams.push_back(this->rtpStream);

	if (rtxCodec && encoding.hasRtx)
		// 4.RTX重传流在这里。
		this->rtpStream->SetRtx(rtxCodec->payloadType, encoding.rtx.ssrc);
}

void SimulcastConsumer::GetRtcp(RTC::RTCP::CompoundPacket* packet, RTC::RtpStreamSend* rtpStream, uint64_t nowMs)
{
	...
	// 6.生成consumer的SR报文，rtpStream是媒体流。
	auto* report = this->rtpStream->GetRtcpSenderReport(nowMs);
}

RTC::RTCP::SenderReport* RtpStreamSend::GetRtcpSenderReport(uint64_t nowMs)
{
	...
	// GetSsrc()返回的是媒体流SSRC，将媒体流的SSRC设置进SR报文。
	report->SetSsrc(GetSsrc());
}

WebRTC不会向mediasoup发送RTX重传流的RR报文的证据：

void Transport::HandleRtcpPacket(RTC::RTCP::Packet* packet)
{
	...
	switch (packet->GetType())
	{
		case RTC::RTCP::Type::RR:
		{
			auto* consumer = GetConsumerByMediaSsrc(report->GetSsrc());
		}
	}
}

inline RTC::Consumer* Transport::GetConsumerByMediaSsrc(uint32_t ssrc) const
{
	...

	auto mapSsrcConsumerIt = this->mapSsrcConsumer.find(ssrc);

	if (mapSsrcConsumerIt == this->mapSsrcConsumer.end())
		return nullptr;

	auto* consumer = mapSsrcConsumerIt->second;

	return consumer;
}

void Transport::HandleRequest(Channel::Request* request)
{
	...
	case Channel::Request::MethodId::TRANSPORT_CONSUME:
	{
		// 建立ssrc到consumer的映射。
		for (auto ssrc : consumer->GetMediaSsrcs())
		{
			this->mapSsrcConsumer[ssrc] = consumer;
		}

		// 建立rtx ssrc到consumer的映射。
		for (auto ssrc : consumer->GetRtxSsrcs())
		{
			this->mapRtxSsrcConsumer[ssrc] = consumer;
		}
	}
}