Reduce the BWE with 50% when feedback is received too late.

webrtc/modules/congestion_controller/delay_based_bwe.cc

Index: webrtc/modules/congestion_controller/delay_based_bwe.cc
diff --git a/webrtc/modules/congestion_controller/delay_based_bwe.cc b/webrtc/modules/congestion_controller/delay_based_bwe.cc
index 00074a4b245dd9feccb0f10a055f308024c53882..1aaf81f65af0527bf64c51c20bf1c496f1a7a9ff 100644
--- a/webrtc/modules/congestion_controller/delay_based_bwe.cc
+++ b/webrtc/modules/congestion_controller/delay_based_bwe.cc
@@ -49,6 +49,8 @@ constexpr double kDefaultTrendlineThresholdGain = 4.0;
 constexpr size_t kDefaultMedianSlopeWindowSize = 20;
 constexpr double kDefaultMedianSlopeThresholdGain = 4.0;
 
+constexpr int kMaxConsecutiveFailedLookups = 5;
+
 const char kBitrateEstimateExperiment[] = "WebRTC-ImprovedBitrateEstimate";
 const char kBweTrendlineFilterExperiment[] = "WebRTC-BweTrendlineFilter";
 const char kBweMedianSlopeFilterExperiment[] = "WebRTC-BweMedianSlopeFilter";
@@ -223,7 +225,8 @@ DelayBasedBwe::DelayBasedBwe(Clock* clock)
       trendline_threshold_gain_(kDefaultTrendlineThresholdGain),
       probing_interval_estimator_(&rate_control_),
       median_slope_window_size_(kDefaultMedianSlopeWindowSize),
-      median_slope_threshold_gain_(kDefaultMedianSlopeThresholdGain) {
+      median_slope_threshold_gain_(kDefaultMedianSlopeThresholdGain),
+      consecutive_delayed_feedbacks_(0) {
   if (in_trendline_experiment_) {
     ReadTrendlineFilterExperimentParameters(&trendline_window_size_,
                                             &trendline_smoothing_coeff_,
@@ -256,14 +259,42 @@ DelayBasedBwe::Result DelayBasedBwe::IncomingPacketFeedbackVector(
     uma_recorded_ = true;
   }
   Result aggregated_result;
+  bool delayed_feedback = true;
   for (const auto& packet_info : packet_feedback_vector) {
+    if (packet_info.send_time_ms < 0)
+      continue;
+    delayed_feedback = false;
     Result result = IncomingPacketInfo(packet_info);
     if (result.updated)
       aggregated_result = result;
   }
+  if (delayed_feedback) {
+    ++consecutive_delayed_feedbacks_;
+  } else {
+    consecutive_delayed_feedbacks_ = 0;
+  }
+  if (consecutive_delayed_feedbacks_ >= kMaxConsecutiveFailedLookups) {
+    aggregated_result =
+        OnLongFeedbackDelay(packet_feedback_vector.back().arrival_time_ms);
+  }
   return aggregated_result;
 }
 
+DelayBasedBwe::Result DelayBasedBwe::OnLongFeedbackDelay(
+    int64_t arrival_time_ms) {
+  RTC_DCHECK(rate_control_.ValidEstimate());

[terelius, 2017/02/10 15:21:12]

Is this DCHECK really guaranteed to pass? There are several code paths that can
set bitrate_is_initialized_, and it is not clear to me which one is supposed to
have executed.

[stefan-webrtc, 2017/02/13 08:22:48]

It is guranteed, since we always set the start bitrate from the Call
constructor. I can probably be convinced that this isn't a good thing to DCHECK
on though.

[nisse-webrtc, 2017/02/13 09:55:34]

What's the alternative? Do nothing if there's no estimate?

[stefan-webrtc, 2017/02/13 14:41:13]

Either that, or to take our estimated acked bitrate, if we have one, and use
that as our estimate.

Another option would be to compute an estimate as we talked about, by making an
assumption of how large the packets sent actually were. That's a bigger change,
so I'd prefer to do that later if we think it's worth it.

[terelius, 2017/02/13 14:46:47]

Acknowledged.

[nisse-webrtc, 2017/02/13 14:49:49]

If we think RTC_DCHECK is good enough for now, can you add a comment pointing
out why it's true (thanks to the Call constructor)?

[stefan-webrtc, 2017/02/13 15:35:48]

Extended the comment. Thanks

+  rate_control_.SetEstimate(rate_control_.LatestEstimate() / 2,
+                            arrival_time_ms);
+  Result result;
+  result.updated = true;
+  result.probe = false;
+  result.target_bitrate_bps = rate_control_.LatestEstimate();
+  consecutive_delayed_feedbacks_ = 0;
+  LOG(LS_WARNING) << "Long feedback delay detected, reducing BWE to "
+                  << result.target_bitrate_bps;
+  return result;
+}
+
 DelayBasedBwe::Result DelayBasedBwe::IncomingPacketInfo(
     const PacketInfo& info) {
   int64_t now_ms = clock_->TimeInMilliseconds();