Handle BW drop in ALR region and initiate probing

webrtc/modules/congestion_controller/probe_controller.cc

 /*
  *  Copyright (c) 2016 The WebRTC project authors. All Rights Reserved.
  *
  *  Use of this source code is governed by a BSD-style license
  *  that can be found in the LICENSE file in the root of the source
  *  tree. An additional intellectual property rights grant can be found
  *  in the file PATENTS.  All contributing project authors may
  *  be found in the AUTHORS file in the root of the source tree.
  */
 
 #include "webrtc/modules/congestion_controller/probe_controller.h"
 
 #include <algorithm>
 #include <initializer_list>
 
 #include "webrtc/base/logging.h"
+#include "webrtc/system_wrappers/include/metrics.h"
 
 namespace webrtc {
 
 namespace {
 
 // Number of deltas between probes per cluster. On the very first cluster,
 // we will need kProbeDeltasPerCluster + 1 probes, but on a cluster following
 // another, we need kProbeDeltasPerCluster probes.
 constexpr int kProbeDeltasPerCluster = 5;
 
 // Maximum waiting time from the time of initiating probing to getting
 // the measured results back.
 constexpr int64_t kMaxWaitingTimeForProbingResultMs = 1000;
 
 // Value of |min_bitrate_to_probe_further_bps_| that indicates
 // further probing is disabled.
 constexpr int kExponentialProbingDisabled = 0;
 
 // A limit to prevent probing at excessive bitrates.
 constexpr int kMaxProbingBitrateBps = 10000000;
 
+// This is a limit on how often probing can be done when there is a BW
+// drop detected in ALR region.
+constexpr int kAlrProbingIntervalLimitMs = 5000;
+
 }  // namespace
 
 ProbeController::ProbeController(PacedSender* pacer, Clock* clock)
     : pacer_(pacer),
       clock_(clock),
       state_(State::kInit),
       min_bitrate_to_probe_further_bps_(kExponentialProbingDisabled),
       time_last_probing_initiated_ms_(0),
       estimated_bitrate_bps_(0),
-      max_bitrate_bps_(0) {}
+      max_bitrate_bps_(0),
+      last_alr_probing_time_(clock_->TimeInMilliseconds()) {}
 
 void ProbeController::SetBitrates(int min_bitrate_bps,
                                   int start_bitrate_bps,
                                   int max_bitrate_bps) {
   rtc::CritScope cs(&critsect_);
   if (state_ == State::kInit) {
     // When probing at 1.8 Mbps ( 6x 300), this represents a threshold of
     // 1.2 Mbps to continue probing.
     InitiateProbing({3 * start_bitrate_bps, 6 * start_bitrate_bps},
                     4 * start_bitrate_bps);
@@ skipping 27 matching lines @@
       LOG(LS_INFO) << "Measured bitrate: " << bitrate_bps
                    << " Minimum to probe further: "
                    << min_bitrate_to_probe_further_bps_;
       if (min_bitrate_to_probe_further_bps_ != kExponentialProbingDisabled &&
           bitrate_bps > min_bitrate_to_probe_further_bps_) {
         // Double the probing bitrate and expect a minimum of 25% gain to
         // continue probing.
         InitiateProbing({2 * bitrate_bps}, 1.25 * bitrate_bps);
       }
     }
+  } else {
+    // A drop in estimated BW when operating in ALR and not already probing.
+    // The current response is to initiate a single probe session at the
+    // previous bitrate and immediately use the reported bitrate as the new
+    // bitrate.
+    //
+    // If the probe session fails, the assumption is that this drop was a
+    // real one from a competing flow or something else on the network and
+    // it ramps up from bitrate_bps.
+    if (pacer_->InApplicationLimitedRegion() &&
+        bitrate_bps < 0.5 * estimated_bitrate_bps_) {
+      int64_t now_ms = clock_->TimeInMilliseconds();
+      if ((now_ms - last_alr_probing_time_) > kAlrProbingIntervalLimitMs) {
+        LOG(LS_INFO) << "Detected big BW drop in ALR, start probe.";
+        // Track how often we probe in response to BW drop in ALR.
+        RTC_HISTOGRAM_COUNTS_10000("WebRTC.BWE.AlrProbingIntervalInS",
+                                   (now_ms - last_alr_probing_time_) / 1000);
+        InitiateProbing({estimated_bitrate_bps_}, kExponentialProbingDisabled);
+        last_alr_probing_time_ = now_ms;
+      }
+    }
+    // TODO(isheriff): May want to track when we did ALR probing in order
+    // to reset |last_alr_probing_time_| if we validate that it was a
+    // drop due to exogenous event.
   }
   estimated_bitrate_bps_ = bitrate_bps;
 }
 
 void ProbeController::InitiateProbing(
     std::initializer_list<int> bitrates_to_probe,
     int min_bitrate_to_probe_further_bps) {
   bool first_cluster = true;
   for (int bitrate : bitrates_to_probe) {
     bitrate = std::min(bitrate, kMaxProbingBitrateBps);
     if (first_cluster) {
       pacer_->CreateProbeCluster(bitrate, kProbeDeltasPerCluster + 1);
       first_cluster = false;
     } else {
       pacer_->CreateProbeCluster(bitrate, kProbeDeltasPerCluster);
     }
   }
   min_bitrate_to_probe_further_bps_ = min_bitrate_to_probe_further_bps;
   time_last_probing_initiated_ms_ = clock_->TimeInMilliseconds();
   if (min_bitrate_to_probe_further_bps == kExponentialProbingDisabled)
     state_ = State::kProbingComplete;
   else
     state_ = State::kWaitingForProbingResult;
 }
 
 }  // namespace webrtc