OnReceiveNack

实现数据包的重传。

ReSendPacket

这里是先将要发送的packet添加到队列中，然后后面再经过网络发送出去。

int32_t RTPSender::ReSendPacket(uint16_t packet_id) {
  // Try to find packet in RTP packet history. Also verify RTT here, so that we
  // don't retransmit too often.
  absl::optional<RtpPacketHistory::PacketState> stored_packet =
      packet_history_->GetPacketState(packet_id);
  if (!stored_packet || stored_packet->pending_transmission) {
    // Packet not found or already queued for retransmission, ignore.
    return 0;
  }
  const int32_t packet_size = static_cast<int32_t>(stored_packet->packet_size);
  const bool rtx = (RtxStatus() & kRtxRetransmitted) > 0;
  std::unique_ptr<RtpPacketToSend> packet =
      packet_history_->GetPacketAndMarkAsPending(
          packet_id, [&](const RtpPacketToSend& stored_packet) {
            // Check if we're overusing retransmission bitrate.
            // TODO(sprang): Add histograms for nack success or failure
            // reasons.
            std::unique_ptr<RtpPacketToSend> retransmit_packet;
            if (retransmission_rate_limiter_ &&
                !retransmission_rate_limiter_->TryUseRate(packet_size)) {
              return retransmit_packet;
            }
              // 如果开启rtx，则构建rtx包
            if (rtx) {
              retransmit_packet = BuildRtxPacket(stored_packet);
            } else {
              retransmit_packet =
                  std::make_unique<RtpPacketToSend>(stored_packet);
            }
            if (retransmit_packet) {
              retransmit_packet->set_retransmitted_sequence_number(
                  stored_packet.SequenceNumber());
            }
            return retransmit_packet;
          });
  if (!packet) {
    return -1;
  }
  packet->set_packet_type(RtpPacketMediaType::kRetransmission);
  packet->set_fec_protect_packet(false);
  std::vector<std::unique_ptr<RtpPacketToSend>> packets;
  packets.emplace_back(std::move(packet));
  paced_sender_->EnqueuePackets(std::move(packets));
  return packet_size;
}

这里调用了GetPacketAndMarkAsPending，里面用了匿名函数。

GetPacketAndMarkAsPending

std::unique_ptr<RtpPacketToSend> RtpPacketHistory::GetPacketAndMarkAsPending(
    uint16_t sequence_number,
    rtc::FunctionView<std::unique_ptr<RtpPacketToSend>(const RtpPacketToSend&)>
        encapsulate) {
  MutexLock lock(&lock_);
  if (mode_ == StorageMode::kDisabled) {
    return nullptr;
  }
  StoredPacket* packet = GetStoredPacket(sequence_number);
  if (packet == nullptr) {
    return nullptr;
  }
  if (packet->pending_transmission_) {
    // Packet already in pacer queue, ignore this request.
    return nullptr;
  }
  if (!VerifyRtt(*packet, clock_->TimeInMilliseconds())) {
    // Packet already resent within too short a time window, ignore.
    return nullptr;
  }
  // Copy and/or encapsulate packet.
  std::unique_ptr<RtpPacketToSend> encapsulated_packet =
      encapsulate(*packet->packet_);
  if (encapsulated_packet) {
    packet->pending_transmission_ = true;
  }
  return encapsulated_packet;
}

SetRtxStatus调用栈