Only use 95% of the link capacity if the true link capacity is found by probing.

webrtc/modules/congestion_controller/probe_bitrate_estimator.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_bitrate_estimator.h"
 
 #include <algorithm>
 
 #include "webrtc/base/checks.h"
 #include "webrtc/base/logging.h"
 #include "webrtc/logging/rtc_event_log/rtc_event_log.h"
 
 namespace {
 // The minumum number of probes we need to receive feedback about in percent
 // in order to have a valid estimate.
 constexpr int kMinReceivedProbesPercent = 80;
 
 // The minumum number of bytes we need to receive feedback about in percent
 // in order to have a valid estimate.
 constexpr int kMinReceivedBytesPercent = 80;
 
-// The maximum (receive rate)/(send rate) ratio for a valid estimate.
-constexpr float kValidRatio = 2.0f;
+// The maximum |receive rate| / |send rate| ratio for a valid estimate.
+constexpr float kMaxValidRatio = 2.0f;
+
+// The minimum |receive rate| / |send rate| ratio assuming that the link is
+// not saturated, i.e. we assume that we will receive at least
+// kMinRatioForUnsaturatedLink * |send rate| if |send rate| is less than the
+// link capacity.
+constexpr float kMinRatioForUnsaturatedLink = 0.9f;
+
+// The target utilization of the link. If we know true link capacity
+// we'd like to send at 95% of that rate.
+constexpr float kTargetUtilizationFraction = 0.95f;
 
 // The maximum time period over which the cluster history is retained.
 // This is also the maximum time period beyond which a probing burst is not
 // expected to last.
 constexpr int kMaxClusterHistoryMs = 1000;
 
 // The maximum time interval between first and the last probe on a cluster
 // on the sender side as well as the receive side.
 constexpr int kMaxProbeIntervalMs = 1000;
 }  // namespace
@@ skipping 66 matching lines @@
   float send_bps = send_size / send_interval_ms * 1000;
 
   // Since the |receive_interval_ms| does not include the time it takes to
   // actually receive the first packet the size of the first received packet
   // should not be included when calculating the receive bitrate.
   RTC_DCHECK_GT(cluster->size_total, cluster->size_first_receive);
   float receive_size = cluster->size_total - cluster->size_first_receive;
   float receive_bps = receive_size / receive_interval_ms * 1000;
 
   float ratio = receive_bps / send_bps;
-  if (ratio > kValidRatio) {
+  if (ratio > kMaxValidRatio) {
     LOG(LS_INFO) << "Probing unsuccessful, receive/send ratio too high"
                  << " [cluster id: " << cluster_id << "] [send: " << send_size
                  << " bytes / " << send_interval_ms
                  << " ms = " << send_bps / 1000 << " kb/s]"
                  << " [receive: " << receive_size << " bytes / "
                  << receive_interval_ms << " ms = " << receive_bps / 1000
                  << " kb/s]"
                  << " [ratio: " << receive_bps / 1000 << " / "
-                 << send_bps / 1000 << " = " << ratio << " > kValidRatio ("
-                 << kValidRatio << ")]";
+                 << send_bps / 1000 << " = " << ratio << " > kMaxValidRatio ("
+                 << kMaxValidRatio << ")]";
     if (event_log_)
       event_log_->LogProbeResultFailure(cluster_id, kInvalidSendReceiveRatio);
     return -1;
   }
   LOG(LS_INFO) << "Probing successful"
                << " [cluster id: " << cluster_id << "] [send: " << send_size
                << " bytes / " << send_interval_ms << " ms = " << send_bps / 1000
                << " kb/s]"
                << " [receive: " << receive_size << " bytes / "
                << receive_interval_ms << " ms = " << receive_bps / 1000
                << " kb/s]";
 
   float res = std::min(send_bps, receive_bps);
+  // If we're receiving at significantly lower bitrate than we were sending at,
+  // it suggests that we've found the true capacity of the link. In this case,
+  // set the target bitrate slightly lower to not immediately overuse.
+  if (receive_bps < kMinRatioForUnsaturatedLink * send_bps) {
+    RTC_DCHECK_GT(send_bps, receive_bps);
+    res = kTargetUtilizationFraction * receive_bps;
+  }
   if (event_log_)
     event_log_->LogProbeResultSuccess(cluster_id, res);
   estimated_bitrate_bps_ = rtc::Optional<int>(res);
   return *estimated_bitrate_bps_;
 }
 
 rtc::Optional<int>
 ProbeBitrateEstimator::FetchAndResetLastEstimatedBitrateBps() {
   rtc::Optional<int> estimated_bitrate_bps = estimated_bitrate_bps_;
   estimated_bitrate_bps_.reset();
   return estimated_bitrate_bps;
 }
 
 void ProbeBitrateEstimator::EraseOldClusters(int64_t timestamp_ms) {
   for (auto it = clusters_.begin(); it != clusters_.end();) {
     if (it->second.last_receive_ms < timestamp_ms) {
       it = clusters_.erase(it);
     } else {
       ++it;
     }
   }
 }
 }  // namespace webrtc