Piggybacking simulcast id and ALR experiment id into video content type extension.

webrtc/modules/pacing/alr_detector.h

 /*
  *  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.
  */
 
@@ skipping 29 matching lines @@
   // Returns time in milliseconds when the current application-limited region
   // started or empty result if the sender is currently not application-limited.
   rtc::Optional<int64_t> GetApplicationLimitedRegionStartTime() const;
 
   struct AlrExperimentSettings {
     float pacing_factor = PacedSender::kDefaultPaceMultiplier;
     int64_t max_paced_queue_time = PacedSender::kMaxQueueLengthMs;
     int alr_bandwidth_usage_percent = kDefaultAlrBandwidthUsagePercent;
     int alr_start_budget_level_percent = kDefaultAlrStartBudgetLevelPercent;
     int alr_stop_budget_level_percent = kDefaultAlrStopBudgetLevelPercent;
+    // Will be sent to the receive side for stats slicing.
+    // Can be 0..6, because 3 bits are used and one value is reserved for N/A.
+    int group_id = 0;

[sprang_webrtc, 2017/08/24 09:13:16]

Which value means N/A? Maybe easier if 0 is undefined? Could also create a
constant for that value.

[ilnik, 2017/08/25 12:35:07]

Rephrased the comment. It doesn't matter which value will be used as N/A. All
maters is that we have 3 bits and one reserved value. It leaves 7 possible
groups here.

   };
   static rtc::Optional<AlrExperimentSettings> ParseAlrSettingsFromFieldTrial(
       const char* experiment_name);
 
   // Sent traffic percentage as a function of network capacity used to determine
   // application-limited region. ALR region start when bandwidth usage drops
   // below kAlrStartUsagePercent and ends when it raises above
   // kAlrEndUsagePercent. NOTE: This is intentionally conservative at the moment
   // until BW adjustments of application limited region is fine tuned.
   static constexpr int kDefaultAlrBandwidthUsagePercent = 65;
@@ skipping 10 matching lines @@
   int alr_start_budget_level_percent_;
   int alr_stop_budget_level_percent_;
 
   IntervalBudget alr_budget_;
   rtc::Optional<int64_t> alr_started_time_ms_;
 };
 
 }  // namespace webrtc
 
 #endif  // WEBRTC_MODULES_PACING_ALR_DETECTOR_H_