webrtc源码走读（七）核心引擎层——Qos模块

1、Qos整体架构与设计哲学

WebRTC 的 QoS（服务质量）机制是一套端到端闭环反馈系统，运行在应用层，不依赖网络设备。其核心目标是：

• 保障实时性：音视频通信对延迟极其敏感（语音 >100ms 就会影响可懂度）
• 最大化可用带宽利用率：在不造成拥塞的前提下尽可能提高画质/音质
• 容忍丢包与抖动：互联网环境无法保证可靠传输，必须具备容错能力

整个系统通过接收端反馈 → 发送端决策 → 媒体调整的闭环，动态适应网络变化。

为什么必须闭环？
因为网络状态瞬息万变（如从 WiFi 切到 4G），只有基于接收端真实反馈的决策才可靠。预测模型（如仅靠丢包率判断拥塞）在真实网络中极易误判。

2、QoS 模块与其他组件的交互关系

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接收端

发送端

原始帧

分层编码

RTP包

带宽估计

层码率建议

重传包

RTP包

解码帧

丢包列表

NACK RTCP

到达时间

抖动值

Transport-CC RTCP

RTCP反馈

Video Encoder

SVC Controller

Pacer

Network

BWE

NACK Resender

Jitter Buffer

Video Decoder

Packet Loss Detector

NACK Generator

Transport Feedback

BWE

Jitter Estimator

关键交互说明：

3、核心机制详解

3.1. NACK：丢包重传（仅视频）

作用

当接收端发现 RTP 包丢失时，主动请求发送端重传。适用于低 RTT（<300ms）且丢包率中等（5%~15%） 的场景。

工作流程与时序图

接收端网络发送端接收端网络发送端#mermaid-svg-kPDX8JylfSJnsUMl{font-family:\”trebuchet ms\”,verdana,arial,sans-serif;font-size:16px;fill:#333;}@keyframes edge-animation-frame{from{stroke-dashoffset:0;}}@keyframes dash{to{stroke-dashoffset:0;}}#mermaid-svg-kPDX8JylfSJnsUMl .edge-animation-slow{stroke-dasharray:9,5!important;stroke-dashoffset:900;animation:dash 50s linear infinite;stroke-linecap:round;}#mermaid-svg-kPDX8JylfSJnsUMl .edge-animation-fast{stroke-dasharray:9,5!important;stroke-dashoffset:900;animation:dash 20s linear infinite;stroke-linecap:round;}#mermaid-svg-kPDX8JylfSJnsUMl .error-icon{fill:#552222;}#mermaid-svg-kPDX8JylfSJnsUMl .error-text{fill:#552222;stroke:#552222;}#mermaid-svg-kPDX8JylfSJnsUMl .edge-thickness-normal{stroke-width:1px;}#mermaid-svg-kPDX8JylfSJnsUMl .edge-thickness-thick{stroke-width:3.5px;}#mermaid-svg-kPDX8JylfSJnsUMl .edge-pattern-solid{stroke-dasharray:0;}#mermaid-svg-kPDX8JylfSJnsUMl .edge-thickness-invisible{stroke-width:0;fill:none;}#mermaid-svg-kPDX8JylfSJnsUMl .edge-pattern-dashed{stroke-dasharray:3;}#mermaid-svg-kPDX8JylfSJnsUMl .edge-pattern-dotted{stroke-dasharray:2;}#mermaid-svg-kPDX8JylfSJnsUMl .marker{fill:#333333;stroke:#333333;}#mermaid-svg-kPDX8JylfSJnsUMl .marker.cross{stroke:#333333;}#mermaid-svg-kPDX8JylfSJnsUMl svg{font-family:\”trebuchet ms\”,verdana,arial,sans-serif;font-size:16px;}#mermaid-svg-kPDX8JylfSJnsUMl p{margin:0;}#mermaid-svg-kPDX8JylfSJnsUMl .actor{stroke:hsl(259.6261682243, 59.7765363128%, 87.9019607843%);fill:#ECECFF;}#mermaid-svg-kPDX8JylfSJnsUMl text.actor>tspan{fill:black;stroke:none;}#mermaid-svg-kPDX8JylfSJnsUMl .actor-line{stroke:hsl(259.6261682243, 59.7765363128%, 87.9019607843%);}#mermaid-svg-kPDX8JylfSJnsUMl .innerArc{stroke-width:1.5;stroke-dasharray:none;}#mermaid-svg-kPDX8JylfSJnsUMl .messageLine0{stroke-width:1.5;stroke-dasharray:none;stroke:#333;}#mermaid-svg-kPDX8JylfSJnsUMl .messageLine1{stroke-width:1.5;stroke-dasharray:2,2;stroke:#333;}#mermaid-svg-kPDX8JylfSJnsUMl #arrowhead path{fill:#333;stroke:#333;}#mermaid-svg-kPDX8JylfSJnsUMl .sequenceNumber{fill:white;}#mermaid-svg-kPDX8JylfSJnsUMl #sequencenumber{fill:#333;}#mermaid-svg-kPDX8JylfSJnsUMl #crosshead path{fill:#333;stroke:#333;}#mermaid-svg-kPDX8JylfSJnsUMl .messageText{fill:#333;stroke:none;}#mermaid-svg-kPDX8JylfSJnsUMl .labelBox{stroke:hsl(259.6261682243, 59.7765363128%, 87.9019607843%);fill:#ECECFF;}#mermaid-svg-kPDX8JylfSJnsUMl .labelText,#mermaid-svg-kPDX8JylfSJnsUMl .labelText>tspan{fill:black;stroke:none;}#mermaid-svg-kPDX8JylfSJnsUMl .loopText,#mermaid-svg-kPDX8JylfSJnsUMl .loopText>tspan{fill:black;stroke:none;}#mermaid-svg-kPDX8JylfSJnsUMl .loopLine{stroke-width:2px;stroke-dasharray:2,2;stroke:hsl(259.6261682243, 59.7765363128%, 87.9019607843%);fill:hsl(259.6261682243, 59.7765363128%, 87.9019607843%);}#mermaid-svg-kPDX8JylfSJnsUMl .note{stroke:#aaaa33;fill:#fff5ad;}#mermaid-svg-kPDX8JylfSJnsUMl .noteText,#mermaid-svg-kPDX8JylfSJnsUMl .noteText>tspan{fill:black;stroke:none;}#mermaid-svg-kPDX8JylfSJnsUMl .activation0{fill:#f4f4f4;stroke:#666;}#mermaid-svg-kPDX8JylfSJnsUMl .activation1{fill:#f4f4f4;stroke:#666;}#mermaid-svg-kPDX8JylfSJnsUMl .activation2{fill:#f4f4f4;stroke:#666;}#mermaid-svg-kPDX8JylfSJnsUMl .actorPopupMenu{position:absolute;}#mermaid-svg-kPDX8JylfSJnsUMl .actorPopupMenuPanel{position:absolute;fill:#ECECFF;box-shadow:0px 8px 16px 0px rgba(0,0,0,0.2);filter:drop-shadow(3px 5px 2px rgb(0 0 0 / 0.4));}#mermaid-svg-kPDX8JylfSJnsUMl .actor-man line{stroke:hsl(259.6261682243, 59.7765363128%, 87.9019607843%);fill:#ECECFF;}#mermaid-svg-kPDX8JylfSJnsUMl .actor-man circle,#mermaid-svg-kPDX8JylfSJnsUMl line{stroke:hsl(259.6261682243, 59.7765363128%, 87.9019607843%);fill:#ECECFF;stroke-width:2px;}#mermaid-svg-kPDX8JylfSJnsUMl :root{–mermaid-font-family:\”trebuchet ms\”,verdana,arial,sans-serif;}包发送 RTP 包收到检测到发送 NACK RTCP (seq=101)从历史缓存查找重传 RTP 包收到重传包完整帧组装 → 解码

源码（接收端生成 NACK）

// 文件: modules/rtp_rtcp/source/rtcp_receiver.cc
void RtcpReceiver::OnReceivedRtcpReportBlock(
const RtcpReportBlock& report_block,
uint32_t ssrc) {

uint16_t last_seq = report_block.last_sequence_number;
std::vector<uint16_t> nack_list;

// 回溯最多 2000 毫秒内的包（默认窗口）
for (uint16_t seq = last_seq – 1;
seq != last_seq && nack_list.size() < kMaxNackListSize;
—seq) {

if (packet_receiver_->IsPacketLost(seq)) {
nack_list.push_back(seq);
}

// 防止无限循环（序列号回绕）
if (nack_list.size() >= 2000) break;
}

if (!nack_list.empty()) {
rtp_rtcp_->SendNack(ssrc, nack_list); // 发送 NACK RTCP
}
}

源码（发送端重传）

// 文件: modules/rtp_rtcp/source/rtp_packet_history.cc
bool RtpPacketHistory::GetPacket(uint16_t sequence_number,
RtpPacketToSend* packet) {
// 在历史缓存中查找对应序列号的包
for (const auto& item : packet_history_) {
if (item.packet.sequence_number() == sequence_number) {
*packet = item.packet;
return true;
}
}
return false; // 包已过期（超过 2000ms），无法重传
}

3.2. RTT：往返延迟测量

作用

RTT 是判断是否启用 NACK 的关键阈值（通常 <100ms 才有效），也用于 Jitter Buffer 的目标延迟计算。

工作流程

ReceiverSenderReceiverSender#mermaid-svg-UcFCmd26IvF3ljsp{font-family:\”trebuchet ms\”,verdana,arial,sans-serif;font-size:16px;fill:#333;}@keyframes edge-animation-frame{from{stroke-dashoffset:0;}}@keyframes dash{to{stroke-dashoffset:0;}}#mermaid-svg-UcFCmd26IvF3ljsp .edge-animation-slow{stroke-dasharray:9,5!important;stroke-dashoffset:900;animation:dash 50s linear infinite;stroke-linecap:round;}#mermaid-svg-UcFCmd26IvF3ljsp .edge-animation-fast{stroke-dasharray:9,5!important;stroke-dashoffset:900;animation:dash 20s linear infinite;stroke-linecap:round;}#mermaid-svg-UcFCmd26IvF3ljsp .error-icon{fill:#552222;}#mermaid-svg-UcFCmd26IvF3ljsp .error-text{fill:#552222;stroke:#552222;}#mermaid-svg-UcFCmd26IvF3ljsp .edge-thickness-normal{stroke-width:1px;}#mermaid-svg-UcFCmd26IvF3ljsp .edge-thickness-thick{stroke-width:3.5px;}#mermaid-svg-UcFCmd26IvF3ljsp .edge-pattern-solid{stroke-dasharray:0;}#mermaid-svg-UcFCmd26IvF3ljsp .edge-thickness-invisible{stroke-width:0;fill:none;}#mermaid-svg-UcFCmd26IvF3ljsp .edge-pattern-dashed{stroke-dasharray:3;}#mermaid-svg-UcFCmd26IvF3ljsp .edge-pattern-dotted{stroke-dasharray:2;}#mermaid-svg-UcFCmd26IvF3ljsp .marker{fill:#333333;stroke:#333333;}#mermaid-svg-UcFCmd26IvF3ljsp .marker.cross{stroke:#333333;}#mermaid-svg-UcFCmd26IvF3ljsp svg{font-family:\”trebuchet ms\”,verdana,arial,sans-serif;font-size:16px;}#mermaid-svg-UcFCmd26IvF3ljsp p{margin:0;}#mermaid-svg-UcFCmd26IvF3ljsp .actor{stroke:hsl(259.6261682243, 59.7765363128%, 87.9019607843%);fill:#ECECFF;}#mermaid-svg-UcFCmd26IvF3ljsp text.actor>tspan{fill:black;stroke:none;}#mermaid-svg-UcFCmd26IvF3ljsp .actor-line{stroke:hsl(259.6261682243, 59.7765363128%, 87.9019607843%);}#mermaid-svg-UcFCmd26IvF3ljsp .innerArc{stroke-width:1.5;stroke-dasharray:none;}#mermaid-svg-UcFCmd26IvF3ljsp .messageLine0{stroke-width:1.5;stroke-dasharray:none;stroke:#333;}#mermaid-svg-UcFCmd26IvF3ljsp .messageLine1{stroke-width:1.5;stroke-dasharray:2,2;stroke:#333;}#mermaid-svg-UcFCmd26IvF3ljsp #arrowhead path{fill:#333;stroke:#333;}#mermaid-svg-UcFCmd26IvF3ljsp .sequenceNumber{fill:white;}#mermaid-svg-UcFCmd26IvF3ljsp #sequencenumber{fill:#333;}#mermaid-svg-UcFCmd26IvF3ljsp #crosshead path{fill:#333;stroke:#333;}#mermaid-svg-UcFCmd26IvF3ljsp .messageText{fill:#333;stroke:none;}#mermaid-svg-UcFCmd26IvF3ljsp .labelBox{stroke:hsl(259.6261682243, 59.7765363128%, 87.9019607843%);fill:#ECECFF;}#mermaid-svg-UcFCmd26IvF3ljsp .labelText,#mermaid-svg-UcFCmd26IvF3ljsp .labelText>tspan{fill:black;stroke:none;}#mermaid-svg-UcFCmd26IvF3ljsp .loopText,#mermaid-svg-UcFCmd26IvF3ljsp .loopText>tspan{fill:black;stroke:none;}#mermaid-svg-UcFCmd26IvF3ljsp .loopLine{stroke-width:2px;stroke-dasharray:2,2;stroke:hsl(259.6261682243, 59.7765363128%, 87.9019607843%);fill:hsl(259.6261682243, 59.7765363128%, 87.9019607843%);}#mermaid-svg-UcFCmd26IvF3ljsp .note{stroke:#aaaa33;fill:#fff5ad;}#mermaid-svg-UcFCmd26IvF3ljsp .noteText,#mermaid-svg-UcFCmd26IvF3ljsp .noteText>tspan{fill:black;stroke:none;}#mermaid-svg-UcFCmd26IvF3ljsp .activation0{fill:#f4f4f4;stroke:#666;}#mermaid-svg-UcFCmd26IvF3ljsp .activation1{fill:#f4f4f4;stroke:#666;}#mermaid-svg-UcFCmd26IvF3ljsp .activation2{fill:#f4f4f4;stroke:#666;}#mermaid-svg-UcFCmd26IvF3ljsp .actorPopupMenu{position:absolute;}#mermaid-svg-UcFCmd26IvF3ljsp .actorPopupMenuPanel{position:absolute;fill:#ECECFF;box-shadow:0px 8px 16px 0px rgba(0,0,0,0.2);filter:drop-shadow(3px 5px 2px rgb(0 0 0 / 0.4));}#mermaid-svg-UcFCmd26IvF3ljsp .actor-man line{stroke:hsl(259.6261682243, 59.7765363128%, 87.9019607843%);fill:#ECECFF;}#mermaid-svg-UcFCmd26IvF3ljsp .actor-man circle,#mermaid-svg-UcFCmd26IvF3ljsp line{stroke:hsl(259.6261682243, 59.7765363128%, 87.9019607843%);fill:#ECECFF;stroke-width:2px;}#mermaid-svg-UcFCmd26IvF3ljsp :root{–mermaid-font-family:\”trebuchet ms\”,verdana,arial,sans-serif;}发送 Sender Report (SR)，含 LSR 时间戳记录 LSR，并等待一段时间（DLSR）发送 Receiver Report (RR)，含 DLSR计算 RTT = 当前时间 – LSR – DLSR

源码（RTT 计算）

// 文件: modules/rtp_rtcp/source/rtcp_receiver.cc
void RtcpReceiver::OnRtcpReceived(const RtcpPacket* packet) {
if (packet->type() == kRtcpRr) {
const RtcpRrPacket* rr = static_cast<const RtcpRrPacket*>(packet);

// 提取 LSR（Sender Report 时间戳）和 DLSR（Delay since LSR）
uint32_t lsr = rr->lsr();
uint32_t dlsr = rr->dlsr();

// 转换为毫秒（高16位是秒，低16位是小数）
int64_t lsr_ms = (lsr >> 16) * 1000;
int64_t dlsr_ms = (dlsr >> 16) * 1000;

// RTT = 当前时间 – LSR – DLSR
int64_t rtt_ms = clock_->TimeInMilliseconds() – lsr_ms – dlsr_ms;

// 指数加权平均平滑（α=0.1）
if (rtt_ms > 0) {
estimated_rtt_ms_ = 0.1 * rtt_ms + 0.9 * estimated_rtt_ms_;
}
}
}

3.3. FEC：前向纠错（仅视频）

作用

通过冗余包恢复丢失数据，适用于高丢包率（>15%）或高 RTT（>300ms） 场景，避免 NACK 延迟过高。

与 NACK 的协同策略

源码（FEC 生成）

// 文件: modules/rtp_rtcp/source/fec_generator.cc
void FecGenerator::GenerateFecPackets(
const std::vector<RtpPacketToSend>& packets,
std::vector<RtpPacketToSend>* fec_packets) {

if (packets.empty()) return;

// 计算 FEC 包数量（例如冗余度 20%）
size_t num_fec = (packets.size() * fec_rate_ + 99) / 100;

for (size_t i = 0; i < num_fec; ++i) {
RtpPacketToSend fec;
fec.SetPayloadType(kFlexfecPayloadType);
fec.SetSequenceNumber(packets.back().sequence_number() + i + 1);

// 简化版：对所有包 payload 异或生成 FEC
std::vector<uint8_t> fec_payload;
for (const auto& pkt : packets) {
const uint8_t* p = pkt.payload();
size_t sz = pkt.payload_size();
if (fec_payload.empty()) {
fec_payload.assign(p, p + sz);
} else {
for (size_t j = 0; j < std::min(fec_payload.size(), sz); ++j) {
fec_payload[j] ^= p[j];
}
}
}
fec.SetPayload(fec_payload.data(), fec_payload.size());
fec_packets->push_back(std::move(fec));
}
}

3.4. BWE：带宽估计（Transport-CC）

作用

实时估算可用上行带宽，为 Pacer 和 SVC 提供决策依据。

工作流程与时序图

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源码（BWE 更新）

// 文件: modules/congestion_controller/goog_cc/goog_cc_bandwidth_estimator.cc
void GoogCcBandwidthEstimator::OnTransportFeedback(
const TransportFeedback& feedback) {

for (const auto& packet : feedback.packets) {
int64_t delay_ms = packet.arrival_time_ms – packet.send_time_ms;

// 卡尔曼滤波器平滑延迟
double predicted = kalman_filter_.Predict();
double residual = delay_ms – predicted;
kalman_filter_.Update(residual);

// 简化带宽计算（实际使用 AIMD 算法）
if (delay_ms > 0) {
int bitrate = (packet.size_bytes * 8000) / delay_ms;
current_estimate_ = bitrate;
}
}

// 通知 Pacer 和 SVC
observer_->OnBandwidthEstimate(current_estimate_);
}

3.5. SVC：可伸缩视频编码

作用

将视频分为基础层（BL）和增强层（EL），根据带宽动态调整质量。

层级结构示例（VP9 S1T3 模式）

源码（SVC 控制）

// 文件: modules/video_coding/svc/scalable_video_controller.cc
void ScalableVideoController::OnBandwidthEstimate(int bitrate_bps) {
const float weights[3] = {0.3f, 0.4f, 0.3f}; // BL, EL1, EL2
int layer_bitrates[3] = {0};

for (int i = 0; i < num_layers_; ++i) {
layer_bitrates[i] = static_cast<int>(bitrate_bps * weights[i]);
// 不超过该层上限
if (layer_bitrates[i] > max_bitrate_[i]) {
layer_bitrates[i] = max_bitrate_[i];
}
}

encoder_->SetLayerBitrates(layer_bitrates); // 通知编码器
}

3.6. Pacer：发送调度器

作用

将 BWE 限制的总带宽均匀分配到时间片，避免突发流量导致拥塞。

工作流程

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BWE: 1000kbps

Pacer

每5ms发送 625 字节

网络

源码（Pacer 调度）

// 文件: modules/pacing/paced_sender.cc
void PacedSender::Process() {
int target_bps = congestion_controller_->GetTargetBitrate();
// 5ms 时间片应发送的字节数
int bytes_to_send = (target_bps * 5) / (1000 * 8);

int sent = 0;
while (sent < bytes_to_send && !packet_queue_.empty()) {
RtpPacketToSend pkt = packet_queue_.Pop();
network_->SendPacket(pkt);
sent += pkt.size();
}

// 5ms 后再次调度
task_queue_.PostDelayedTask([this]() { Process(); }, 5);
}

4、全链路工作流程示例（网络恶化场景）

场景：用户从 WiFi 切换到 4G，丢包率从 2% 升至 15%，RTT 从 50ms 升至 250ms。

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5、不同场景关键参数调整