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/* Copyright (c) 2013 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 "modules/video_coding/codecs/vp8/screenshare_layers.h"
#include <stdlib.h>
#include <algorithm>
#include <memory>
#include "modules/video_coding/include/video_codec_interface.h"
#include "rtc_base/arraysize.h"
#include "rtc_base/checks.h"
#include "rtc_base/logging.h"
#include "rtc_base/time_utils.h"
#include "system_wrappers/include/metrics.h"
namespace webrtc {
namespace {
using BufferFlags = Vp8FrameConfig::BufferFlags;
constexpr BufferFlags kNone = Vp8FrameConfig::BufferFlags::kNone;
constexpr BufferFlags kReference = Vp8FrameConfig::BufferFlags::kReference;
constexpr BufferFlags kUpdate = Vp8FrameConfig::BufferFlags::kUpdate;
constexpr BufferFlags kReferenceAndUpdate =
Vp8FrameConfig::BufferFlags::kReferenceAndUpdate;
constexpr int kOneSecond90Khz = 90000;
constexpr int kMinTimeBetweenSyncs = kOneSecond90Khz * 2;
constexpr int kMaxTimeBetweenSyncs = kOneSecond90Khz * 4;
constexpr int kQpDeltaThresholdForSync = 8;
constexpr int kMinBitrateKbpsForQpBoost = 500;
constexpr auto kSwitch = DecodeTargetIndication::kSwitch;
} // namespace
const double ScreenshareLayers::kMaxTL0FpsReduction = 2.5;
const double ScreenshareLayers::kAcceptableTargetOvershoot = 2.0;
constexpr int ScreenshareLayers::kMaxNumTemporalLayers;
// Always emit a frame with certain interval, even if bitrate targets have
// been exceeded. This prevents needless keyframe requests.
const int ScreenshareLayers::kMaxFrameIntervalMs = 2750;
ScreenshareLayers::ScreenshareLayers(int num_temporal_layers)
: number_of_temporal_layers_(
std::min(kMaxNumTemporalLayers, num_temporal_layers)),
active_layer_(-1),
last_timestamp_(-1),
last_sync_timestamp_(-1),
last_emitted_tl0_timestamp_(-1),
last_frame_time_ms_(-1),
max_debt_bytes_(0),
encode_framerate_(1000.0f, 1000.0f), // 1 second window, second scale.
bitrate_updated_(false),
checker_(TemporalLayersChecker::CreateTemporalLayersChecker(
Vp8TemporalLayersType::kBitrateDynamic,
num_temporal_layers)) {
RTC_CHECK_GT(number_of_temporal_layers_, 0);
RTC_CHECK_LE(number_of_temporal_layers_, kMaxNumTemporalLayers);
}
ScreenshareLayers::~ScreenshareLayers() {
UpdateHistograms();
}
void ScreenshareLayers::SetQpLimits(size_t stream_index,
int min_qp,
int max_qp) {
RTC_DCHECK_LT(stream_index, StreamCount());
// 0 < min_qp <= max_qp
RTC_DCHECK_LT(0, min_qp);
RTC_DCHECK_LE(min_qp, max_qp);
RTC_DCHECK_EQ(min_qp_.has_value(), max_qp_.has_value());
if (!min_qp_.has_value()) {
min_qp_ = min_qp;
max_qp_ = max_qp;
} else {
RTC_DCHECK_EQ(min_qp, min_qp_.value());
RTC_DCHECK_EQ(max_qp, max_qp_.value());
}
}
size_t ScreenshareLayers::StreamCount() const {
return 1;
}
bool ScreenshareLayers::SupportsEncoderFrameDropping(
size_t stream_index) const {
RTC_DCHECK_LT(stream_index, StreamCount());
// Frame dropping is handled internally by this class.
return false;
}
Vp8FrameConfig ScreenshareLayers::NextFrameConfig(size_t stream_index,
uint32_t timestamp) {
RTC_DCHECK_LT(stream_index, StreamCount());
auto it = pending_frame_configs_.find(timestamp);
if (it != pending_frame_configs_.end()) {
// Drop and re-encode, reuse the previous config.
return it->second.frame_config;
}
if (number_of_temporal_layers_ <= 1) {
// No flags needed for 1 layer screenshare.
// TODO(pbos): Consider updating only last, and not all buffers.
DependencyInfo dependency_info{
"S", {kReferenceAndUpdate, kReferenceAndUpdate, kReferenceAndUpdate}};
pending_frame_configs_[timestamp] = dependency_info;
return dependency_info.frame_config;
}
const int64_t now_ms = rtc::TimeMillis();
int64_t unwrapped_timestamp = time_wrap_handler_.Unwrap(timestamp);
int64_t ts_diff;
if (last_timestamp_ == -1) {
ts_diff = kOneSecond90Khz / capture_framerate_.value_or(*target_framerate_);
} else {
ts_diff = unwrapped_timestamp - last_timestamp_;
}
if (target_framerate_) {
// If input frame rate exceeds target frame rate, either over a one second
// averaging window, or if frame interval is below 90% of desired value,
// drop frame.
if (encode_framerate_.Rate(now_ms).value_or(0) > *target_framerate_)
return Vp8FrameConfig(kNone, kNone, kNone);
// Primarily check if frame interval is too short using frame timestamps,
// as if they are correct they won't be affected by queuing in webrtc.
const int64_t expected_frame_interval_90khz =
kOneSecond90Khz / *target_framerate_;
if (last_timestamp_ != -1 && ts_diff > 0) {
if (ts_diff < 85 * expected_frame_interval_90khz / 100) {
return Vp8FrameConfig(kNone, kNone, kNone);
}
} else {
// Timestamps looks off, use realtime clock here instead.
const int64_t expected_frame_interval_ms = 1000 / *target_framerate_;
if (last_frame_time_ms_ != -1 &&
now_ms - last_frame_time_ms_ <
(85 * expected_frame_interval_ms) / 100) {
return Vp8FrameConfig(kNone, kNone, kNone);
}
}
}
if (stats_.first_frame_time_ms_ == -1)
stats_.first_frame_time_ms_ = now_ms;
// Make sure both frame droppers leak out bits.
layers_[0].UpdateDebt(ts_diff / 90);
layers_[1].UpdateDebt(ts_diff / 90);
last_timestamp_ = timestamp;
last_frame_time_ms_ = now_ms;
TemporalLayerState layer_state = TemporalLayerState::kDrop;
if (active_layer_ == -1 ||
layers_[active_layer_].state != TemporalLayer::State::kDropped) {
if (last_emitted_tl0_timestamp_ != -1 &&
(unwrapped_timestamp - last_emitted_tl0_timestamp_) / 90 >
kMaxFrameIntervalMs) {
// Too long time has passed since the last frame was emitted, cancel
// enough debt to allow a single frame.
layers_[0].debt_bytes_ = max_debt_bytes_ - 1;
}
if (layers_[0].debt_bytes_ > max_debt_bytes_) {
// Must drop TL0, encode TL1 instead.
if (layers_[1].debt_bytes_ > max_debt_bytes_) {
// Must drop both TL0 and TL1.
active_layer_ = -1;
} else {
active_layer_ = 1;
}
} else {
active_layer_ = 0;
}
}
switch (active_layer_) {
case 0:
layer_state = TemporalLayerState::kTl0;
last_emitted_tl0_timestamp_ = unwrapped_timestamp;
break;
case 1:
if (layers_[1].state != TemporalLayer::State::kDropped) {
if (TimeToSync(unwrapped_timestamp) ||
layers_[1].state == TemporalLayer::State::kKeyFrame) {
last_sync_timestamp_ = unwrapped_timestamp;
layer_state = TemporalLayerState::kTl1Sync;
} else {
layer_state = TemporalLayerState::kTl1;
}
} else {
layer_state = last_sync_timestamp_ == unwrapped_timestamp
? TemporalLayerState::kTl1Sync
: TemporalLayerState::kTl1;
}
break;
case -1:
layer_state = TemporalLayerState::kDrop;
++stats_.num_dropped_frames_;
break;
default:
RTC_DCHECK_NOTREACHED();
}
DependencyInfo dependency_info;
// TODO(pbos): Consider referencing but not updating the 'alt' buffer for all
// layers.
switch (layer_state) {
case TemporalLayerState::kDrop:
dependency_info = {"", {kNone, kNone, kNone}};
break;
case TemporalLayerState::kTl0:
// TL0 only references and updates 'last'.
dependency_info = {"SS", {kReferenceAndUpdate, kNone, kNone}};
dependency_info.frame_config.packetizer_temporal_idx = 0;
break;
case TemporalLayerState::kTl1:
// TL1 references both 'last' and 'golden' but only updates 'golden'.
dependency_info = {"-R", {kReference, kReferenceAndUpdate, kNone}};
dependency_info.frame_config.packetizer_temporal_idx = 1;
break;
case TemporalLayerState::kTl1Sync:
// Predict from only TL0 to allow participants to switch to the high
// bitrate stream. Updates 'golden' so that TL1 can continue to refer to
// and update 'golden' from this point on.
dependency_info = {"-S", {kReference, kUpdate, kNone}};
dependency_info.frame_config.packetizer_temporal_idx = 1;
dependency_info.frame_config.layer_sync = true;
break;
}
pending_frame_configs_[timestamp] = dependency_info;
return dependency_info.frame_config;
}
void ScreenshareLayers::OnRatesUpdated(
size_t stream_index,
const std::vector<uint32_t>& bitrates_bps,
int framerate_fps) {
RTC_DCHECK_LT(stream_index, StreamCount());
RTC_DCHECK_GT(framerate_fps, 0);
RTC_DCHECK_GE(bitrates_bps.size(), 1);
RTC_DCHECK_LE(bitrates_bps.size(), 2);
// `bitrates_bps` uses individual rates per layer, but we want to use the
// accumulated rate here.
uint32_t tl0_kbps = bitrates_bps[0] / 1000;
uint32_t tl1_kbps = tl0_kbps;
if (bitrates_bps.size() > 1) {
tl1_kbps += bitrates_bps[1] / 1000;
}
if (!target_framerate_) {
// First OnRatesUpdated() is called during construction, with the
// configured targets as parameters.
target_framerate_ = framerate_fps;
capture_framerate_ = target_framerate_;
bitrate_updated_ = true;
} else {
if ((capture_framerate_ &&
framerate_fps != static_cast<int>(*capture_framerate_)) ||
(tl0_kbps != layers_[0].target_rate_kbps_) ||
(tl1_kbps != layers_[1].target_rate_kbps_)) {
bitrate_updated_ = true;
}
if (framerate_fps < 0) {
capture_framerate_.reset();
} else {
capture_framerate_ = framerate_fps;
}
}
layers_[0].target_rate_kbps_ = tl0_kbps;
layers_[1].target_rate_kbps_ = tl1_kbps;
}
void ScreenshareLayers::OnEncodeDone(size_t stream_index,
uint32_t rtp_timestamp,
size_t size_bytes,
bool is_keyframe,
int qp,
CodecSpecificInfo* info) {
RTC_DCHECK_LT(stream_index, StreamCount());
if (size_bytes == 0) {
RTC_LOG(LS_WARNING) << "Empty frame; treating as dropped.";
OnFrameDropped(stream_index, rtp_timestamp);
return;
}
absl::optional<DependencyInfo> dependency_info;
auto it = pending_frame_configs_.find(rtp_timestamp);
if (it != pending_frame_configs_.end()) {
dependency_info = it->second;
pending_frame_configs_.erase(it);
if (checker_) {
RTC_DCHECK(checker_->CheckTemporalConfig(is_keyframe,
dependency_info->frame_config));
}
}
CodecSpecificInfoVP8& vp8_info = info->codecSpecific.VP8;
GenericFrameInfo& generic_frame_info = info->generic_frame_info.emplace();
if (number_of_temporal_layers_ == 1) {
vp8_info.temporalIdx = kNoTemporalIdx;
vp8_info.layerSync = false;
generic_frame_info.temporal_id = 0;
generic_frame_info.decode_target_indications = {kSwitch};
generic_frame_info.encoder_buffers.emplace_back(
0, /*referenced=*/!is_keyframe, /*updated=*/true);
} else {
int64_t unwrapped_timestamp = time_wrap_handler_.Unwrap(rtp_timestamp);
if (dependency_info) {
vp8_info.temporalIdx =
dependency_info->frame_config.packetizer_temporal_idx;
vp8_info.layerSync = dependency_info->frame_config.layer_sync;
generic_frame_info.temporal_id = vp8_info.temporalIdx;
generic_frame_info.decode_target_indications =
dependency_info->decode_target_indications;
} else {
RTC_DCHECK(is_keyframe);
}
if (is_keyframe) {
vp8_info.temporalIdx = 0;
last_sync_timestamp_ = unwrapped_timestamp;
vp8_info.layerSync = true;
layers_[0].state = TemporalLayer::State::kKeyFrame;
layers_[1].state = TemporalLayer::State::kKeyFrame;
active_layer_ = 1;
info->template_structure =
GetTemplateStructure(number_of_temporal_layers_);
generic_frame_info.temporal_id = vp8_info.temporalIdx;
generic_frame_info.decode_target_indications = {kSwitch, kSwitch};
} else if (active_layer_ >= 0 && layers_[active_layer_].state ==
TemporalLayer::State::kKeyFrame) {
layers_[active_layer_].state = TemporalLayer::State::kNormal;
}
vp8_info.useExplicitDependencies = true;
RTC_DCHECK_EQ(vp8_info.referencedBuffersCount, 0u);
RTC_DCHECK_EQ(vp8_info.updatedBuffersCount, 0u);
// Note that `frame_config` is not derefernced if `is_keyframe`,
// meaning it's never dereferenced if the optional may be unset.
for (int i = 0; i < static_cast<int>(Vp8FrameConfig::Buffer::kCount); ++i) {
bool references = false;
bool updates = is_keyframe;
if (!is_keyframe && dependency_info->frame_config.References(
static_cast<Vp8FrameConfig::Buffer>(i))) {
RTC_DCHECK_LT(vp8_info.referencedBuffersCount,
arraysize(CodecSpecificInfoVP8::referencedBuffers));
references = true;
vp8_info.referencedBuffers[vp8_info.referencedBuffersCount++] = i;
}
if (is_keyframe || dependency_info->frame_config.Updates(
static_cast<Vp8FrameConfig::Buffer>(i))) {
RTC_DCHECK_LT(vp8_info.updatedBuffersCount,
arraysize(CodecSpecificInfoVP8::updatedBuffers));
updates = true;
vp8_info.updatedBuffers[vp8_info.updatedBuffersCount++] = i;
}
if (references || updates)
generic_frame_info.encoder_buffers.emplace_back(i, references, updates);
}
}
encode_framerate_.Update(1, rtc::TimeMillis());
if (number_of_temporal_layers_ == 1)
return;
RTC_DCHECK_NE(-1, active_layer_);
if (layers_[active_layer_].state == TemporalLayer::State::kDropped) {
layers_[active_layer_].state = TemporalLayer::State::kQualityBoost;
}
if (qp != -1)
layers_[active_layer_].last_qp = qp;
if (active_layer_ == 0) {
layers_[0].debt_bytes_ += size_bytes;
layers_[1].debt_bytes_ += size_bytes;
++stats_.num_tl0_frames_;
stats_.tl0_target_bitrate_sum_ += layers_[0].target_rate_kbps_;
stats_.tl0_qp_sum_ += qp;
} else if (active_layer_ == 1) {
layers_[1].debt_bytes_ += size_bytes;
++stats_.num_tl1_frames_;
stats_.tl1_target_bitrate_sum_ += layers_[1].target_rate_kbps_;
stats_.tl1_qp_sum_ += qp;
}
}
void ScreenshareLayers::OnFrameDropped(size_t stream_index,
uint32_t rtp_timestamp) {
layers_[active_layer_].state = TemporalLayer::State::kDropped;
++stats_.num_overshoots_;
}
void ScreenshareLayers::OnPacketLossRateUpdate(float packet_loss_rate) {}
void ScreenshareLayers::OnRttUpdate(int64_t rtt_ms) {}
void ScreenshareLayers::OnLossNotification(
const VideoEncoder::LossNotification& loss_notification) {}
FrameDependencyStructure ScreenshareLayers::GetTemplateStructure(
int num_layers) const {
RTC_CHECK_LT(num_layers, 3);
RTC_CHECK_GT(num_layers, 0);
FrameDependencyStructure template_structure;
template_structure.num_decode_targets = num_layers;
switch (num_layers) {
case 1: {
template_structure.templates.resize(2);
template_structure.templates[0].T(0).Dtis("S");
template_structure.templates[1].T(0).Dtis("S").FrameDiffs({1});
return template_structure;
}
case 2: {
template_structure.templates.resize(3);
template_structure.templates[0].T(0).Dtis("SS");
template_structure.templates[1].T(0).Dtis("SS").FrameDiffs({1});
template_structure.templates[2].T(1).Dtis("-S").FrameDiffs({1});
return template_structure;
}
default:
RTC_DCHECK_NOTREACHED();
// To make the compiler happy!
return template_structure;
}
}
bool ScreenshareLayers::TimeToSync(int64_t timestamp) const {
RTC_DCHECK_EQ(1, active_layer_);
RTC_DCHECK_NE(-1, layers_[0].last_qp);
if (layers_[1].last_qp == -1) {
// First frame in TL1 should only depend on TL0 since there are no
// previous frames in TL1.
return true;
}
RTC_DCHECK_NE(-1, last_sync_timestamp_);
int64_t timestamp_diff = timestamp - last_sync_timestamp_;
if (timestamp_diff > kMaxTimeBetweenSyncs) {
// After a certain time, force a sync frame.
return true;
} else if (timestamp_diff < kMinTimeBetweenSyncs) {
// If too soon from previous sync frame, don't issue a new one.
return false;
}
// Issue a sync frame if difference in quality between TL0 and TL1 isn't too
// large.
if (layers_[0].last_qp - layers_[1].last_qp < kQpDeltaThresholdForSync)
return true;
return false;
}
uint32_t ScreenshareLayers::GetCodecTargetBitrateKbps() const {
uint32_t target_bitrate_kbps = layers_[0].target_rate_kbps_;
if (number_of_temporal_layers_ > 1) {
// Calculate a codec target bitrate. This may be higher than TL0, gaining
// quality at the expense of frame rate at TL0. Constraints:
// - TL0 frame rate no less than framerate / kMaxTL0FpsReduction.
// - Target rate * kAcceptableTargetOvershoot should not exceed TL1 rate.
target_bitrate_kbps =
std::min(layers_[0].target_rate_kbps_ * kMaxTL0FpsReduction,
layers_[1].target_rate_kbps_ / kAcceptableTargetOvershoot);
}
return std::max(layers_[0].target_rate_kbps_, target_bitrate_kbps);
}
Vp8EncoderConfig ScreenshareLayers::UpdateConfiguration(size_t stream_index) {
RTC_DCHECK_LT(stream_index, StreamCount());
RTC_DCHECK(min_qp_.has_value());
RTC_DCHECK(max_qp_.has_value());
const uint32_t target_bitrate_kbps = GetCodecTargetBitrateKbps();
// TODO(sprang): We _really_ need to make an overhaul of this class. :(
// If we're dropping frames in order to meet a target framerate, adjust the
// bitrate assigned to the encoder so the total average bitrate is correct.
float encoder_config_bitrate_kbps = target_bitrate_kbps;
if (target_framerate_ && capture_framerate_ &&
*target_framerate_ < *capture_framerate_) {
encoder_config_bitrate_kbps *=
static_cast<float>(*capture_framerate_) / *target_framerate_;
}
if (bitrate_updated_ ||
encoder_config_.rc_target_bitrate !=
absl::make_optional(encoder_config_bitrate_kbps)) {
encoder_config_.rc_target_bitrate = encoder_config_bitrate_kbps;
// Don't reconfigure qp limits during quality boost frames.
if (active_layer_ == -1 ||
layers_[active_layer_].state != TemporalLayer::State::kQualityBoost) {
const int min_qp = min_qp_.value();
const int max_qp = max_qp_.value();
// After a dropped frame, a frame with max qp will be encoded and the
// quality will then ramp up from there. To boost the speed of recovery,
// encode the next frame with lower max qp, if there is sufficient
// bandwidth to do so without causing excessive delay.
// TL0 is the most important to improve since the errors in this layer
// will propagate to TL1.
// Currently, reduce max qp by 20% for TL0 and 15% for TL1.
if (layers_[1].target_rate_kbps_ >= kMinBitrateKbpsForQpBoost) {
layers_[0].enhanced_max_qp = min_qp + (((max_qp - min_qp) * 80) / 100);
layers_[1].enhanced_max_qp = min_qp + (((max_qp - min_qp) * 85) / 100);
} else {
layers_[0].enhanced_max_qp = -1;
layers_[1].enhanced_max_qp = -1;
}
}
if (capture_framerate_) {
int avg_frame_size =
(target_bitrate_kbps * 1000) / (8 * *capture_framerate_);
// Allow max debt to be the size of a single optimal frame.
// TODO(sprang): Determine if this needs to be adjusted by some factor.
// (Lower values may cause more frame drops, higher may lead to queuing
// delays.)
max_debt_bytes_ = avg_frame_size;
}
bitrate_updated_ = false;
}
// Don't try to update boosts state if not active yet.
if (active_layer_ == -1)
return encoder_config_;
if (number_of_temporal_layers_ <= 1)
return encoder_config_;
// If layer is in the quality boost state (following a dropped frame), update
// the configuration with the adjusted (lower) qp and set the state back to
// normal.
unsigned int adjusted_max_qp = max_qp_.value(); // Set the normal max qp.
if (layers_[active_layer_].state == TemporalLayer::State::kQualityBoost) {
if (layers_[active_layer_].enhanced_max_qp != -1) {
// Bitrate is high enough for quality boost, update max qp.
adjusted_max_qp = layers_[active_layer_].enhanced_max_qp;
}
// Regardless of qp, reset the boost state for the next frame.
layers_[active_layer_].state = TemporalLayer::State::kNormal;
}
encoder_config_.rc_max_quantizer = adjusted_max_qp;
return encoder_config_;
}
void ScreenshareLayers::TemporalLayer::UpdateDebt(int64_t delta_ms) {
uint32_t debt_reduction_bytes = target_rate_kbps_ * delta_ms / 8;
if (debt_reduction_bytes >= debt_bytes_) {
debt_bytes_ = 0;
} else {
debt_bytes_ -= debt_reduction_bytes;
}
}
void ScreenshareLayers::UpdateHistograms() {
if (stats_.first_frame_time_ms_ == -1)
return;
int64_t duration_sec =
(rtc::TimeMillis() - stats_.first_frame_time_ms_ + 500) / 1000;
if (duration_sec >= metrics::kMinRunTimeInSeconds) {
RTC_HISTOGRAM_COUNTS_10000(
"WebRTC.Video.Screenshare.Layer0.FrameRate",
(stats_.num_tl0_frames_ + (duration_sec / 2)) / duration_sec);
RTC_HISTOGRAM_COUNTS_10000(
"WebRTC.Video.Screenshare.Layer1.FrameRate",
(stats_.num_tl1_frames_ + (duration_sec / 2)) / duration_sec);
int total_frames = stats_.num_tl0_frames_ + stats_.num_tl1_frames_;
RTC_HISTOGRAM_COUNTS_10000(
"WebRTC.Video.Screenshare.FramesPerDrop",
(stats_.num_dropped_frames_ == 0
? 0
: total_frames / stats_.num_dropped_frames_));
RTC_HISTOGRAM_COUNTS_10000(
"WebRTC.Video.Screenshare.FramesPerOvershoot",
(stats_.num_overshoots_ == 0 ? 0
: total_frames / stats_.num_overshoots_));
if (stats_.num_tl0_frames_ > 0) {
RTC_HISTOGRAM_COUNTS_10000("WebRTC.Video.Screenshare.Layer0.Qp",
stats_.tl0_qp_sum_ / stats_.num_tl0_frames_);
RTC_HISTOGRAM_COUNTS_10000(
"WebRTC.Video.Screenshare.Layer0.TargetBitrate",
stats_.tl0_target_bitrate_sum_ / stats_.num_tl0_frames_);
}
if (stats_.num_tl1_frames_ > 0) {
RTC_HISTOGRAM_COUNTS_10000("WebRTC.Video.Screenshare.Layer1.Qp",
stats_.tl1_qp_sum_ / stats_.num_tl1_frames_);
RTC_HISTOGRAM_COUNTS_10000(
"WebRTC.Video.Screenshare.Layer1.TargetBitrate",
stats_.tl1_target_bitrate_sum_ / stats_.num_tl1_frames_);
}
}
}
} // namespace webrtc