SR RR
通过RR包，将丢包率信息回传给发送端，根据丢包率做发送端带宽估计。接收RTCP包，以及对应处理流程基本相同，具体流程如下：
void UdpTransportImpl::IncomingRTCPCallback
void UdpTransportImpl::IncomingRTCPFunction
void VideoChannelTransport::IncomingRTCPPacket
int ViENetworkImpl::ReceivedRTCPPacket
int32_t ViEChannel::ReceivedRTCPPacket
int ViEReceiver::ReceivedRTCPPacket
int ViEReceiver::InsertRTCPPacket
int32_t ModuleRtpRtcpImpl::IncomingRtcpPacket
void RTCPReceiver::TriggerCallbacksFromRTCPPacket
void OnReceivedRtcpReceiverReport
void BitrateControllerImpl::OnReceivedRtcpReceiverReport
void SendSideBandwidthEstimation::UpdateReceiverBlock
void SendSideBandwidthEstimation::UpdateEstimate
bitrate_controller模块接口是bitrate_controller_impl.h文件，通过这个文件就可以建立整个模块。
int ViEBaseImpl::CreateChannel
int ViEBaseImpl::CreateChannel
rtc::scoped_ptr<ChannelGroup> group
ChannelGroup::ChannelGroup
BitrateController* BitrateController::CreateBitrateController
其他的对象也都在此BitrateControllerImpl类中，从而将整个模块联系起来。
有一个线程一直会轮询这个更新带宽评估的函数，因为接收到反馈包之后，会进行评估，这里具体什么用意，后面看。
int32_t BitrateControllerImpl::Process()
void SendSideBandwidthEstimation::UpdateEstimate(int64_t now_ms)
设置保留带宽，用于音频
int Conductor::VideoSetStream
int ViERTP_RTCPImpl::SetReservedTransmitBitrate
bool ViEChannelManager::SetReservedTransmitBitrate
void BitrateControllerImpl::SetReservedBitrate
获取用于编码器的起始码率
int Conductor::VideoSetSendCodec
int ViECodecImpl::SetSendCodec
int32_t ViEEncoder::SetEncoder
bool BitrateControllerImpl::AvailableBandwidth
根据上一步的起始码率以及设置的起始码率，取较大值设置到SendSideBandwidthEstimation
int32_t ViEEncoder::SetEncoder
void BitrateControllerImpl::SetStartBitrate
void SendSideBandwidthEstimation::SetSendBitrate
设置最小最大码率，这个码率怎么来的，应该是上面设置下来的，后面看。
int32_t ViEEncoder::SetEncoder
void BitrateControllerImpl::SetMinMaxBitrate
void SendSideBandwidthEstimation::SetMinMaxBitrate
设置模式，然后决定是否使用RembSuppressor，这个做什么用，后面看。
int32_t ViEEncoder::SetEncoder
void BitrateControllerImpl::SetCodecMode(webrtc::VideoCodecMode mode)
void RembSuppressor::SetEnabled(bool enabled)
计算RTT
照抄下面这篇博客，验证基本正确。
A 发送 SR 包, 并记录SR包的发送时间. 记为send_time
B 接收到 A的SR包后, 记录下最后一次接受到SR包的时间. 记为last_recv_time... (B等待发送rtcp包)B 发送 RR包, 计算从[last_recv_time] 到 当前时间的延时. 记录为delay_since_last_SR. 附加到RR包中. A 收到 B的RR包后, 计算RTT
RTT = send_time - delay_since_last_SR - last_recv_time
A -> 发送SR包.
ModuleRtpRtcpImpl::Process
RTCPSender::SendRTCP
RTCPSender::PrepareRTCP
RTCPSender::BuildSR
PrepareRTCP 中通过_sending(是否是发送端) 状态判定发送SR或RR. SR包中含有发送时的NTP时间戳. BuildSR中_lastSendReport 记录NTP时间的中间32位. 可以标识SR包, 也就是B回应RR包中report block的LSR字段(last SR timestamp ), 通过LSR可以查找_lastRTCPTime._lastRTCPTime记录RTCP_NUMBER_OF_SR个数的SR发送时间.这两个数组是一一对应的.
_lastRTCPTime[0] = Clock::NtpToMs(NTPsec, NTPfrac);
_lastSendReport[0] = (NTPsec << 16) + (NTPfrac >> 16);
最后SendToNetwork.
B -> 接收到SR包.
ModuleRtpRtcpImpl::IncomingRtcpPacket
RTCPReceiver::IncomingRTCPPacket
RTCPReceiver::HandleSenderReceiverReport
在HandleSenderReceiverReport 中保存 SR包中的NTP时间戳
_remoteSenderInfo.NTPseconds = rtcpPacket.SR.NTPMostSignificant;
_remoteSenderInfo.NTPfraction = rtcpPacket.SR.NTPLeastSignificant;
并记录SR包接到时的NTP时间戳
_clock->CurrentNtp(_lastReceivedSRNTPsecs, _lastReceivedSRNTPfrac);
B -> 发送RR包
获取回馈状态, 并发送给A
ModuleRtpRtcpImpl::Process()
if (rtcp_sender_.TimeToSendRTCPReport()) {
rtcp_sender_.SendRTCP(GetFeedbackState(), kRtcpReport);
}
ModuleRtpRtcpImpl::GetFeedbackState()
ModuleRtpRtcpImpl::LastReceivedNTP
state.last_rr_ntp_secs 和state.last_rr_ntp_frac即为上一次接收到SR包时, 记录的_clock->CurrentNtp(_lastReceivedSRNTPsecs, _lastReceivedSRNTPfrac); 时间戳state.remote_sr 通过_remoteSenderInfo.NTPseconds 和 _remoteSenderInfo.NTPfraction, 取中间32位算出.
RTCPSender::PrepareReport
在这里计算延时, 填充到report block中.
// get our NTP as late as possible to avoid a race
_clock->CurrentNtp(*ntp_secs, *ntp_frac);
// Delay since last received report
uint32_t delaySinceLastReceivedSR = 0;
if ((feedback_state.last_rr_ntp_secs != 0) ||
(feedback_state.last_rr_ntp_frac != 0)) {
// get the 16 lowest bits of seconds and the 16 higest bits of fractions
uint32_t now=*ntp_secs&0x0000FFFF;
now <<=16;
now += (*ntp_frac&0xffff0000)>>16;
uint32_t receiveTime = feedback_state.last_rr_ntp_secs&0x0000FFFF;
receiveTime <<=16;
receiveTime += (feedback_state.last_rr_ntp_frac&0xffff0000)>>16;
delaySinceLastReceivedSR = now-receiveTime;
}
report_block->delaySinceLastSR = delaySinceLastReceivedSR;
report_block->lastSR = feedback_state.remote_sr;
report_block->delaySinceLastSR 即为 从接到SR包到发送RR包之间的延时.
report_block->lastSR 即SR包中NTP时间戳的中间32位. (在A端_lastSendReport数组中记录).
A 收到 B的RR包
ModuleRtpRtcpImpl::IncomingRtcpPacket
RTCPReceiver::IncomingRTCPPacket
RTCPReceiver::HandleSenderReceiverReport
RTCPReceiver::HandleReportBlock
通过 lastSR 到sender模块中取出SR包的发送时间.
uint32_t sendTimeMS =
_rtpRtcp.SendTimeOfSendReport(rtcpPacket.ReportBlockItem.LastSR);
计算RTT .
uint32_t delaySinceLastSendReport =
rtcpPacket.ReportBlockItem.DelayLastSR;
// local NTP time when we received this
uint32_t lastReceivedRRNTPsecs = 0;
uint32_t lastReceivedRRNTPfrac = 0;
_clock->CurrentNtp(lastReceivedRRNTPsecs, lastReceivedRRNTPfrac);
// time when we received this in MS
uint32_t receiveTimeMS = Clock::NtpToMs(lastReceivedRRNTPsecs,
lastReceivedRRNTPfrac);
// Estimate RTT
uint32_t d = (delaySinceLastSendReport & 0x0000ffff) * 1000;
d /= 65536;
d += ((delaySinceLastSendReport & 0xffff0000) >> 16) * 1000;
int32_t RTT = 0;
if (sendTimeMS > 0) {
RTT = receiveTimeMS - d - sendTimeMS;
....
}
注意：
delay since last SR (DLSR) 的单位是1/65536秒.
为什么在获取本地时间不直接获取，而要先得到NTP再转为毫秒，应该是要时间统一都用NTP时间，而不是本地时间。
NTP相关计算
通过_clock->CurrentNtp()方法来获得当前时刻的ntp,其中lastReceivedRRNTPsecs 为秒，lastReceivedRRNTPfrac 为小数点后面部分。
// local NTP time when we received this
uint32_t lastReceivedRRNTPsecs = 0;
uint32_t lastReceivedRRNTPfrac = 0;
_clock->CurrentNtp(lastReceivedRRNTPsecs, lastReceivedRRNTPfrac);
// time when we received this in MS
void CurrentNtp(uint32_t& seconds, uint32_t& fractions) const override {
timeval tv = CurrentTimeVal();
double microseconds_in_seconds;
Adjust(tv, &seconds, µseconds_in_seconds);
fractions = static_cast<uint32_t>(
microseconds_in_seconds * kMagicNtpFractionalUnit + 0.5);
}
通过CurrentTimeVal()方法的复杂计算之后，最后得到秒和微妙。
timeval CurrentTimeVal() const override {
const uint64_t FILETIME_1970 = 0x019db1ded53e8000;
FILETIME StartTime;
uint64_t Time;
struct timeval tv;
// We can't use query performance counter since they can change depending on
// speed stepping.
GetTime(&StartTime);
Time = (((uint64_t) StartTime.dwHighDateTime) << 32) +
(uint64_t) StartTime.dwLowDateTime;
// Convert the hecto-nano second time to tv format.
Time -= FILETIME_1970;
tv.tv_sec = (uint32_t)(Time / (uint64_t)10000000);
tv.tv_usec = (uint32_t)((Time % (uint64_t)10000000) / 10);
return tv;
}
Adjust()对秒和微秒做微调。
static void Adjust(const timeval& tv, uint32_t* adjusted_s,
double* adjusted_us_in_s) {
*adjusted_s = tv.tv_sec + kNtpJan1970;
*adjusted_us_in_s = tv.tv_usec / 1e6;
if (*adjusted_us_in_s >= 1) {
*adjusted_us_in_s -= 1;
++*adjusted_s;
} else if (*adjusted_us_in_s < -1) {
*adjusted_us_in_s += 1;
--*adjusted_s;
}
}
};
通过NtpToMs()将NTP时间转为毫秒
int64_t receiveTimeMS = Clock::NtpToMs(lastReceivedRRNTPsecs,
lastReceivedRRNTPfrac);
int64_t Clock::NtpToMs(uint32_t ntp_secs, uint32_t ntp_frac) {
const double ntp_frac_ms = static_cast<double>(ntp_frac) / kNtpFracPerMs;
return 1000 * static_cast<int64_t>(ntp_secs) +
static_cast<int64_t>(ntp_frac_ms + 0.5);
}
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