Change frame length on very low bandwidth.

webrtc/modules/audio_coding/audio_network_adaptor/frame_length_controller.cc

Index: webrtc/modules/audio_coding/audio_network_adaptor/frame_length_controller.cc
diff --git a/webrtc/modules/audio_coding/audio_network_adaptor/frame_length_controller.cc b/webrtc/modules/audio_coding/audio_network_adaptor/frame_length_controller.cc
index d684e956a8a95c6ea36cfe61ede19022993f09da..580d08087b3e57d3ba3739d1ac31e4a50061ebd5 100644
--- a/webrtc/modules/audio_coding/audio_network_adaptor/frame_length_controller.cc
+++ b/webrtc/modules/audio_coding/audio_network_adaptor/frame_length_controller.cc
@@ -17,14 +17,25 @@
 
 namespace webrtc {
 
+namespace {
+constexpr int kPreventOveruseMarginBps = 5000;
+
+int OverheadRateBps(size_t overhead_bytes_per_packet, int frame_length_ms) {
+  return static_cast<int>(overhead_bytes_per_packet * 8 * 1000 /
+                          frame_length_ms);
+}
+}
+
 FrameLengthController::Config::Config(
     const std::vector<int>& encoder_frame_lengths_ms,
     int initial_frame_length_ms,
+    int min_encoder_bitrate_bps,
     float fl_increasing_packet_loss_fraction,
     float fl_decreasing_packet_loss_fraction,
     std::map<FrameLengthChange, int> fl_changing_bandwidths_bps)
     : encoder_frame_lengths_ms(encoder_frame_lengths_ms),
       initial_frame_length_ms(initial_frame_length_ms),
+      min_encoder_bitrate_bps(min_encoder_bitrate_bps),
       fl_increasing_packet_loss_fraction(fl_increasing_packet_loss_fraction),
       fl_decreasing_packet_loss_fraction(fl_decreasing_packet_loss_fraction),
       fl_changing_bandwidths_bps(std::move(fl_changing_bandwidths_bps)) {}
@@ -50,6 +61,8 @@ void FrameLengthController::UpdateNetworkMetrics(
     uplink_bandwidth_bps_ = network_metrics.uplink_bandwidth_bps;
   if (network_metrics.uplink_packet_loss_fraction)
     uplink_packet_loss_fraction_ = network_metrics.uplink_packet_loss_fraction;
+  if (network_metrics.overhead_bytes_per_packet)
+    overhead_bytes_per_packet_ = network_metrics.overhead_bytes_per_packet;
 }
 
 void FrameLengthController::MakeDecision(
@@ -81,11 +94,15 @@ bool FrameLengthController::Config::FrameLengthChange::operator<(
 bool FrameLengthController::FrameLengthIncreasingDecision(
     const AudioNetworkAdaptor::EncoderRuntimeConfig& config) const {
   // Increase frame length if
-  // 1. longer frame length is available AND
-  // 2. |uplink_bandwidth_bps| is known to be smaller than a threshold AND
-  // 3. |uplink_packet_loss_fraction| is known to be smaller than a threshold
+  // 1. |uplink_bandwidth_bps| is known to be smaller or equal than
+  //    |min_encoder_bitrate_bps| plus |prevent_overuse_margin_bps| plus the
+  //    current overhead rate OR all the following:
+  // 2. longer frame length is available AND
+  // 3. |uplink_bandwidth_bps| is known to be smaller than a threshold AND
+  // 4. |uplink_packet_loss_fraction| is known to be smaller than a threshold
   //    AND
-  // 4. FEC is not decided or is OFF.
+  // 5. FEC is not decided or is OFF.
+
   auto longer_frame_length_ms = std::next(frame_length_ms_);
   if (longer_frame_length_ms == config_.encoder_frame_lengths_ms.end())
     return false;
@@ -96,6 +113,13 @@ bool FrameLengthController::FrameLengthIncreasingDecision(
   if (increase_threshold == config_.fl_changing_bandwidths_bps.end())
     return false;
 
+  if (uplink_bandwidth_bps_ && overhead_bytes_per_packet_ &&
+      *uplink_bandwidth_bps_ <=
+          config_.min_encoder_bitrate_bps + kPreventOveruseMarginBps +
+              OverheadRateBps(*overhead_bytes_per_packet_, *frame_length_ms_)) {
+    return true;
+  }
+
   return (uplink_bandwidth_bps_ &&
           *uplink_bandwidth_bps_ <= increase_threshold->second) &&
          (uplink_packet_loss_fraction_ &&
@@ -107,10 +131,14 @@ bool FrameLengthController::FrameLengthIncreasingDecision(
 bool FrameLengthController::FrameLengthDecreasingDecision(
     const AudioNetworkAdaptor::EncoderRuntimeConfig& config) const {
   // Decrease frame length if
-  // 1. shorter frame length is available AND one or more of the followings:
-  // 2. |uplink_bandwidth_bps| is known to be larger than a threshold,
-  // 3. |uplink_packet_loss_fraction| is known to be larger than a threshold,
-  // 4. FEC is decided ON.
+  // 1. shorter frame length is available AND
+  // 2. |uplink_bandwidth_bps| is known to be bigger than
+  // |min_encoder_bitrate_bps| plus |prevent_overuse_margin_bps| plus the
+  // overhead which would be produced with the shorter frame length AND
+  // one or more of the followings:
+  // 3. |uplink_bandwidth_bps| is known to be larger than a threshold,
+  // 4. |uplink_packet_loss_fraction| is known to be larger than a threshold,
+  // 5. FEC is decided ON.
   if (frame_length_ms_ == config_.encoder_frame_lengths_ms.begin())
     return false;
 
@@ -121,6 +149,14 @@ bool FrameLengthController::FrameLengthDecreasingDecision(
   if (decrease_threshold == config_.fl_changing_bandwidths_bps.end())
     return false;
 
+  if (uplink_bandwidth_bps_ && overhead_bytes_per_packet_ &&
+      *uplink_bandwidth_bps_ <= config_.min_encoder_bitrate_bps +
+                                    kPreventOveruseMarginBps +
+                                    OverheadRateBps(*overhead_bytes_per_packet_,
+                                                    *shorter_frame_length_ms)) {
+    return false;
+  }
+
   return (uplink_bandwidth_bps_ &&
           *uplink_bandwidth_bps_ >= decrease_threshold->second) ||
          (uplink_packet_loss_fraction_ &&