Add functionality which limits the number of bytes on the network.

webrtc/modules/congestion_controller/send_side_congestion_controller.cc

Index: webrtc/modules/congestion_controller/send_side_congestion_controller.cc
diff --git a/webrtc/modules/congestion_controller/send_side_congestion_controller.cc b/webrtc/modules/congestion_controller/send_side_congestion_controller.cc
index 60e692ccdfe46c4ea3e04b110a31448af0da7ea0..8ebb7a1a39568e3358f0c3ed66fce0f167296809 100644
--- a/webrtc/modules/congestion_controller/send_side_congestion_controller.cc
+++ b/webrtc/modules/congestion_controller/send_side_congestion_controller.cc
@@ -23,10 +23,20 @@
 #include "webrtc/rtc_base/ptr_util.h"
 #include "webrtc/rtc_base/rate_limiter.h"
 #include "webrtc/rtc_base/socket.h"
+#include "webrtc/system_wrappers/include/field_trial.h"
 
 namespace webrtc {
 namespace {
 
+const char kCwndExperiment[] = "WebRTC-CwndExperiment";
+
+bool CwndExperimentEnabled() {
+  std::string experiment_string =
+      webrtc::field_trial::FindFullName(kCwndExperiment);
+  // The experiment is enabled iff the field trial string begins with "Enabled".
+  return experiment_string.find("Enabled") == 0;
+}
+
 static const int64_t kRetransmitWindowSizeMs = 500;
 
 // Makes sure that the bitrate and the min, max values are in valid range.
@@ -98,8 +108,11 @@ SendSideCongestionController::SendSideCongestionController(
       last_reported_fraction_loss_(0),
       last_reported_rtt_(0),
       network_state_(kNetworkUp),
+      pause_pacer_(false),

[philipel, 2017/08/02 15:35:15]

I assume you duplicate the paced sender pause state to avoid grabbing its lock,
please add a comment about it.

[stefan-webrtc, 2017/08/04 09:50:00]

Done.

+      pacer_paused_(false),
       min_bitrate_bps_(congestion_controller::GetMinBitrateBps()),
-      delay_based_bwe_(new DelayBasedBwe(event_log_, clock_)) {
+      delay_based_bwe_(new DelayBasedBwe(event_log_, clock_)),
+      in_cwnd_experiment_(CwndExperimentEnabled()) {
   delay_based_bwe_->SetMinBitrate(min_bitrate_bps_);
 }
 
@@ -216,13 +229,9 @@ SendSideCongestionController::GetTransportFeedbackObserver() {
 void SendSideCongestionController::SignalNetworkState(NetworkState state) {
   LOG(LS_INFO) << "SignalNetworkState "
                << (state == kNetworkUp ? "Up" : "Down");
-  if (state == kNetworkUp) {
-    pacer_->Resume();
-  } else {
-    pacer_->Pause();
-  }
   {
     rtc::CritScope cs(&network_state_lock_);
+    pause_pacer_ = state == kNetworkDown;
     network_state_ = state;
   }
   probe_controller_->OnNetworkStateChanged(state);
@@ -243,6 +252,7 @@ void SendSideCongestionController::OnSentPacket(
     return;
   transport_feedback_adapter_.OnSentPacket(sent_packet.packet_id,
                                            sent_packet.send_time_ms);
+  LimitOutstandingBytes(transport_feedback_adapter_.GetOutstandingBytes());
 }
 
 void SendSideCongestionController::OnRttUpdate(int64_t avg_rtt_ms,
@@ -256,6 +266,20 @@ int64_t SendSideCongestionController::TimeUntilNextProcess() {
 }
 
 void SendSideCongestionController::Process() {
+  bool pause_pacer;
+  // TODO(holmer): Once this class is running on a task queue we should
+  // replace this with a task instead.
+  {
+    rtc::CritScope lock(&network_state_lock_);
+    pause_pacer = pause_pacer_;
+  }
+  if (pause_pacer && !pacer_paused_) {
+    pacer_->Pause();
+    pacer_paused_ = true;
+  } else if (!pause_pacer && pacer_paused_) {
+    pacer_->Resume();
+    pacer_paused_ = false;
+  }
   bitrate_controller_->Process();
   probe_controller_->Process();
   MaybeTriggerOnNetworkChanged();
@@ -287,6 +311,32 @@ void SendSideCongestionController::OnTransportFeedback(
   }
   if (result.updated)
     bitrate_controller_->OnDelayBasedBweResult(result);
+  LimitOutstandingBytes(transport_feedback_adapter_.GetOutstandingBytes());
+}
+
+void SendSideCongestionController::LimitOutstandingBytes(
+    size_t num_outstanding_bytes) {
+  if (!in_cwnd_experiment_)
+    return;
+  {
+    rtc::CritScope lock(&network_state_lock_);
+    int64_t min_rtt_ms = transport_feedback_adapter_.GetMinFeedbackLoopRtt();
+    // No valid RTT. Could be because send-side BWE isn't used, in which case
+    // we don't try to limit the outstanding packets.
+    if (min_rtt_ms < 0)
+      return;
+    const int64_t kAcceptedQueueMs = 250;

[philipel, 2017/08/02 15:35:15]

Just curios, why do we use a hard coded additional queue length instead of a
factor of the BDP?

[stefan-webrtc, 2017/08/04 09:50:00]

That'd be an option as well. My reasoning is that it's easier to understand what
this constant means.

+    size_t max_outstanding_bytes = std::max<size_t>(
+        (min_rtt_ms + kAcceptedQueueMs) * last_reported_bitrate_bps_ / 1000 / 8,
+        2 * 1500);

[philipel, 2017/08/02 15:35:15]

kMinCwndBytes = 2*1500

[stefan-webrtc, 2017/08/04 09:50:00]

Done.

+    LOG(LS_INFO) << clock_->TimeInMilliseconds()
+                 << " Outstanding bytes: " << num_outstanding_bytes
+                 << " pacer queue: " << pacer_->QueueInMs()
+                 << " max outstanding: " << max_outstanding_bytes;
+    LOG(LS_INFO) << "Feedback rtt: " << min_rtt_ms
+                 << " Bitrate: " << last_reported_bitrate_bps_;
+    pause_pacer_ = num_outstanding_bytes > max_outstanding_bytes;
+  }
 }
 
 std::vector<PacketFeedback>