Enables configuration of transmission max bitrate multiplier and fec protection level.

Bug: webrtc:8963
Change-Id: I5e323f3947f84f87791a42024a4168f721ac6094
Reviewed-on: https://webrtc-review.googlesource.com/59142
Commit-Queue: Ying Wang <yinwa@webrtc.org>
Reviewed-by: Stefan Holmer <stefan@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#22273}

call/BUILD.gn

@@ -126,6 +126,7 @@ rtc_source_set("bitrate_allocator") {
     "../rtc_base:rtc_base_approved",
     "../rtc_base:sequenced_task_checker",
     "../system_wrappers",
+    "../system_wrappers:field_trial_api",
     "../system_wrappers:metrics_api",
   ]
   if (!build_with_chromium && is_clang) {

call/bitrate_allocator.cc

@@ -20,16 +20,14 @@
 #include "rtc_base/checks.h"
 #include "rtc_base/logging.h"
 #include "system_wrappers/include/clock.h"
+#include "system_wrappers/include/field_trial.h"
 #include "system_wrappers/include/metrics.h"
 
 namespace webrtc {
 
 // Allow packets to be transmitted in up to 2 times max video bitrate if the
 // bandwidth estimate allows it.
-// TODO(bugs.webrtc.org/8541): May be worth to refactor to keep this logic in
+const uint8_t kTransmissionMaxBitrateMultiplier = 2;
-// video send stream. Similar logic is implemented in
-// AudioPriorityBitrateAllocationStrategy.
-const int kTransmissionMaxBitrateMultiplier = 2;
 const int kDefaultBitrateBps = 300000;
 
 // Require a bitrate increase of max(10%, 20kbps) to resume paused streams.
@@ -61,7 +59,9 @@ BitrateAllocator::BitrateAllocator(LimitObserver* limit_observer)
       last_bwe_log_time_(0),
       total_requested_padding_bitrate_(0),
       total_requested_min_bitrate_(0),
-      bitrate_allocation_strategy_(nullptr) {
+      bitrate_allocation_strategy_(nullptr),
+      transmission_max_bitrate_multiplier_(
+          GetTransmissionMaxBitrateMultiplier()) {
   sequenced_checker_.Detach();
 }
 
@@ -70,6 +70,17 @@ BitrateAllocator::~BitrateAllocator() {
                            num_pause_events_);
 }
 
+uint8_t BitrateAllocator::GetTransmissionMaxBitrateMultiplier() {
+  uint64_t multiplier = strtoul(webrtc::field_trial::FindFullName(
+                                    "WebRTC-TransmissionMaxBitrateMultiplier")
+                                    .c_str(),
+                                nullptr, 10);
+  if (multiplier > 0 && multiplier <= kTransmissionMaxBitrateMultiplier) {
+    return static_cast<uint8_t>(multiplier);
+  }
+  return kTransmissionMaxBitrateMultiplier;
+}
+
 void BitrateAllocator::OnNetworkChanged(uint32_t target_bitrate_bps,
                                         uint8_t fraction_loss,
                                         int64_t rtt,
@@ -302,7 +313,7 @@ BitrateAllocator::ObserverAllocation BitrateAllocator::AllocateBitrates(
   if (bitrate <= sum_max_bitrates)
     return NormalRateAllocation(bitrate, sum_min_bitrates);
 
-  // All observers will get up to kTransmissionMaxBitrateMultiplier x max.
+  // All observers will get up to transmission_max_bitrate_multiplier_ x max.
   return MaxRateAllocation(bitrate, sum_max_bitrates);
 }
 
@@ -406,7 +417,7 @@ BitrateAllocator::ObserverAllocation BitrateAllocator::MaxRateAllocation(
     allocation[observer_config.observer] = observer_config.max_bitrate_bps;
     bitrate -= observer_config.max_bitrate_bps;
   }
-  DistributeBitrateEvenly(bitrate, true, kTransmissionMaxBitrateMultiplier,
+  DistributeBitrateEvenly(bitrate, true, transmission_max_bitrate_multiplier_,
                           &allocation);
   return allocation;
 }

call/bitrate_allocator.h

@@ -188,6 +188,13 @@ class BitrateAllocator {
       const ObserverAllocation& observers_capacities,
       ObserverAllocation* allocation);
 
+  // Allow packets to be transmitted in up to 2 times max video bitrate if the
+  // bandwidth estimate allows it.
+  // TODO(bugs.webrtc.org/8541): May be worth to refactor to keep this logic in
+  // video send stream. Similar logic is implemented in
+  // AudioPriorityBitrateAllocationStrategy.
+  uint8_t GetTransmissionMaxBitrateMultiplier();
+
   rtc::SequencedTaskChecker sequenced_checker_;
   LimitObserver* const limit_observer_ RTC_GUARDED_BY(&sequenced_checker_);
   // Stored in a list to keep track of the insertion order.
@@ -205,6 +212,7 @@ class BitrateAllocator {
   uint32_t total_requested_min_bitrate_ RTC_GUARDED_BY(&sequenced_checker_);
   std::unique_ptr<rtc::BitrateAllocationStrategy> bitrate_allocation_strategy_
       RTC_GUARDED_BY(&sequenced_checker_);
+  uint8_t transmission_max_bitrate_multiplier_;
 };
 
 }  // namespace webrtc