WebRTC研究：rrt 时间计算 之 接收 SR / RR 包

一、当 sender / receiver 收到一个 RTCP 包时，会根据包类型调用分别调用不同的处理函数。对于 SR 和 RR 包，都会交由 HandleSenderReceiverReport() 处理：

int32_t ModuleRtpRtcpImpl::IncomingRtcpPacket(const uint8_t* rtcp_packet, const size_t length) --------- rtp_rtcp_impl.cc

int32_t RTCPReceiver::IncomingRTCPPacket(RTCPPacketInformation& rtcpPacketInformation, RTCPUtility::RTCPParserV2* rtcpParser) --------- rtp_rtcp_impl.cc

int32_t
RTCPReceiver::IncomingRTCPPacket(RTCPPacketInformation& rtcpPacketInformation,
                                 RTCPUtility::RTCPParserV2* rtcpParser)
{
   
  ...
  ...
  ...
  while (pktType != RTCPPacketTypes::kInvalid) 
  {
   
        // Each "case" is responsible for iterate the parser to the
        // next top level packet.
        switch (pktType)
        {
   
        case RTCPPacketTypes::kSr:
        case RTCPPacketTypes::kRr:
            HandleSenderReceiverReport(*rtcpParser, rtcpPacketInformation);
            break;
        ...
        ...
        ...
        }
        pktType = rtcpParser->PacketType();
  }
  ...
  ...
  ...
}

二、对于 SR 包，receiver 会保留包内所携带的 NTP 时间戳，并记录当前 NTP 时间戳：

void
RTCPReceiver::HandleSenderReceiverReport(RTCPUtility::RTCPParserV2& rtcpParser,
                                         RTCPPacketInformation& rtcpPacketInformation)
{
   
    RTCPUtility::RTCPPacketTypes rtcpPacketType = rtcpParser.PacketType(