WebRTC Bandwidth Probe

Abstract	WebRTC Bandwidth Probe
Authors	Walter Fan
Category	LearningNote
Status	WIP
Updated	2024-08-21

Overview

在会话刚建立时需要确定远端的一个初始带宽，congestion_controller 通过

• ProbeController 控制何时进行启动探测

• ProbeBitrateEstimator 基于探测包进行带宽估算

带宽探测的基本思路是以 cluster 为单位按照一定的速度来发送RTP包，然后在收到RTP包的反馈消息时计算发送速度和接收端的接收速度，取这两个速度的最小值为远端带宽速度。

一次探测为一个cluster, 在同一个cluster内RTP包的cluster_id都相同。ProbeController类用来控制探测行为，例如设定开始探测比特率、分配cluster_id等。

用于探测带宽的RTP包其实就是音视频的RTP包，如果没有发送过音视频RTP包那么探测行为不会发生。音视频编码出生的RTP数量有限，在探测带宽时为满足以一定的速度发送数据的要求，很可能会对已经发送过的RTP包进行填充发送。

和所有的RTP、RTCP包的发送一样，探测包的发送也是通过pacing模块来进行的，所有发送的RTP包都会被保存到congestion_controller模块

在收到feedback消息时，如果此RTP包是用来探测带宽的，那么就会调用到ProbeBitrateEstimator::HandleProbeAndEstimateBitrate函数进行处理。

会话刚建立时会探测两次，以及编码器配置改变时会探测两次，一个cluster为一次探测，

会话刚建立时进行两次探测的bps分别为900000、1800000，一般连续的两次探测，第二次的bps为第一次的两倍。

触发探测的条件

1）network available at startup 2）enable periodic alr probing 3）large drop in estimated bandwidth 4) probing results indicate channel has greater capacity.

Flow

handle TransportPacketsFeedback

• GoogCcNetworkController::OnTransportPacketsFeedback

structures

 struct SentPacket {
     Timestamp send_time = Timestamp::PlusInfinity();
     // Size of packet with overhead up to IP layer.
     DataSize size = DataSize::Zero();
     // Size of preceeding packets that are not part of feedback.
     DataSize prior_unacked_data = DataSize::Zero();
     // Probe cluster id and parameters including bitrate, number of packets and
     // number of bytes.
     PacedPacketInfo pacing_info;
     // True if the packet is an audio packet, false for video, padding, RTX etc.
     bool audio = false;
     // Transport independent sequence number, any tracked packet should have a
     // sequence number that is unique over the whole call and increasing by 1 for
     // each packet.
     int64_t sequence_number;
     // Tracked data in flight when the packet was sent, excluding unacked data.
     DataSize data_in_flight = DataSize::Zero();
 };

 struct PacedPacketInfo {
     PacedPacketInfo();
     PacedPacketInfo(int probe_cluster_id,
                     int probe_cluster_min_probes,
                     int probe_cluster_min_bytes);

     bool operator==(const PacedPacketInfo& rhs) const;

     // TODO(srte): Move probing info to a separate, optional struct.
     static constexpr int kNotAProbe = -1;
     int send_bitrate_bps = -1;
     int probe_cluster_id = kNotAProbe;
     int probe_cluster_min_probes = -1;
     int probe_cluster_min_bytes = -1;
     int probe_cluster_bytes_sent = 0;
 };


 struct PacketResult {
     class ReceiveTimeOrder {
     public:
         bool operator()(const PacketResult& lhs, const PacketResult& rhs);
     };

     PacketResult();
     PacketResult(const PacketResult&);
     ~PacketResult();

     inline bool IsReceived() const { return !receive_time.IsPlusInfinity(); }

     SentPacket sent_packet;
     Timestamp receive_time = Timestamp::PlusInfinity();
 };

 struct AggregatedCluster {
     int num_probes = 0;
     Timestamp first_send = Timestamp::PlusInfinity();
     Timestamp last_send = Timestamp::MinusInfinity();
     Timestamp first_receive = Timestamp::PlusInfinity();
     Timestamp last_receive = Timestamp::MinusInfinity();
     DataSize size_last_send = DataSize::Zero();
     DataSize size_first_receive = DataSize::Zero();
     DataSize size_total = DataSize::Zero();
};

HandleProbeAndEstimateBitrate

• HandleProbeAndEstimateBitrate

absl::optional<DataRate> ProbeBitrateEstimator::HandleProbeAndEstimateBitrate
(const PacketResult& packet_feedback) {
    //...

    DataSize send_size = cluster->size_total - cluster->size_last_send;
    DataRate send_rate = send_size / send_interval;

    DataSize receive_size = cluster->size_total - cluster->size_first_receive;
    DataRate receive_rate = receive_size / receive_interval;

    DataRate res = std::min(send_rate, receive_rate);
    //...
    estimated_data_rate_ = res;
    return estimated_data_rate_;
}