音视频编解码与拥塞控制

const pc = new RTCPeerConnection();
const transceiver = pc.addTransceiver("audio", { direction: "sendrecv" });
const params = transceiver.sender.getParameters();

// 通过 codec 参数影响 Opus 行为（浏览器暴露程度有限，多数靠 SDP fmtp）
console.log(params.codecs); // 查看 opus 的 fmtp 参数

a=rtpmap:111 opus/48000/2
a=fmtp:111 minptime=10;useinbandfec=1;usedtx=1;maxaveragebitrate=32000

// 接收方主动请求关键帧
const receivers = pc.getReceivers();
for (const receiver of receivers) {
  if (receiver.track.kind === "video") {
    // 某些实现提供 requestKeyFrame，或通过 getStats 判断后触发
    console.log(receiver.track.id);
  }
}

// 手工解析一个 RTP 包的头部
function parseRtpHeader(buf) {
  const version = buf[0] >> 6;
  const padding = (buf[0] >> 5) & 0x1;
  const extension = (buf[0] >> 4) & 0x1;
  const csrcCount = buf[0] & 0x0f;
  const marker = (buf[1] >> 7) & 0x1;
  const payloadType = buf[1] & 0x7f;
  const seq = buf.readUInt16BE(2);
  const timestamp = buf.readUInt32BE(4);
  const ssrc = buf.readUInt32BE(8);
  return { version, padding, extension, csrcCount, marker, payloadType, seq, timestamp, ssrc };
}

const RTCP_TYPES = {
  200: "SR",
  201: "RR",
  205: "NACK",
  206: "PSFB", // PLI / FIR / REMB 都属于 PSFB
  205.5: "TWCC",
};

function parseRtcp(buf) {
  const results = [];
  let offset = 0;

  while (offset + 4 <= buf.length) {
    const firstByte = buf[offset];
    const version = firstByte >> 6;
    const packetType = buf[offset + 1];
    const lengthWords = buf.readUInt16BE(offset + 2);
    const totalBytes = (lengthWords + 1) * 4;

    if (version !== 2) break;

    if (packetType === 205) {
      // NACK：统计被请求重传的序列号
      const pid = buf.readUInt16BE(offset + 8);
      const blp = buf.readUInt16BE(offset + 10);
      results.push({ type: "NACK", pid, blp });
    } else if (packetType === 206) {
      const fmt = firstByte & 0x1f;
      if (fmt === 1) results.push({ type: "PLI" });
      if (fmt === 4) {
        // REMB：解析指数与尾数得到带宽估计
        const exp = buf[offset + 12] >> 2;
        const mantissa = ((buf[offset + 12] & 0x03) << 16) |
          (buf[offset + 13] << 8) | buf[offset + 14];
        const bitrate = mantissa * Math.pow(2, exp);
        results.push({ type: "REMB", bitrate });
      }
    } else if (packetType === 200 || packetType === 201) {
      results.push({ type: packetType === 200 ? "SR" : "RR", lengthWords });
    }

    offset += totalBytes;
  }
  return results;
}

setInterval(async () => {
  const stats = await pc.getStats();
  stats.forEach((report) => {
    if (report.type === "candidate-pair" && report.state === "succeeded") {
      console.log("可用上行带宽估计:", report.availableOutgoingBitrate);
      console.log("当前 RTT:", report.currentRoundTripTime * 1000, "ms");
    }
    if (report.type === "outbound-rtp" && report.kind === "video") {
      console.log("实际发送码率:", report.bytesSent, "帧数:", report.framesEncoded);
    }
  });
}, 1000);

// 通过 getStats 观察两条流的时间基准
async function inspectSync(pc) {
  const stats = await pc.getStats();
  const tracks = new Map();
  stats.forEach((r) => {
    if (r.type === "inbound-rtp") {
      tracks.set(r.kind, {
        jitter: r.jitter,
        packetsLost: r.packetsLost,
        framesDecoded: r.framesDecoded,
      });
    }
  });
  console.log("音频:", tracks.get("audio"));
  console.log("视频:", tracks.get("video"));
}