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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 <stddef.h>
#include <stdint.h>
#include <map>
#include <vector>
#include "absl/types/optional.h"
#include "modules/include/module_common_types_public.h"
#include "rtc_base/gtest_prod_util.h"
// The NackTracker class keeps track of the lost packets, an estimate of
// time-to-play for each packet is also given.
// Every time a packet is pushed into NetEq, LastReceivedPacket() has to be
// called to update the NACK list.
// Every time 10ms audio is pulled from NetEq LastDecodedPacket() should be
// called, and time-to-play is updated at that moment.
// If packet N is received, any packet prior to N which has not arrived is
// considered lost, and should be labeled as "missing" (the size of
// the list might be limited and older packet eliminated from the list).
// The NackTracker class has to know about the sample rate of the packets to
// compute time-to-play. So sample rate should be set as soon as the first
// packet is received. If there is a change in the receive codec (sender changes
// codec) then NackTracker should be reset. This is because NetEQ would flush
// its buffer and re-transmission is meaning less for old packet. Therefore, in
// that case, after reset the sampling rate has to be updated.
// Thread Safety
// =============
// Please note that this class in not thread safe. The class must be protected
// if different APIs are called from different threads.
namespace webrtc {
class NackTracker {
// A limit for the size of the NACK list.
static const size_t kNackListSizeLimit = 500; // 10 seconds for 20 ms frame
// packets.
// Set a maximum for the size of the NACK list. If the last received packet
// has sequence number of N, then NACK list will not contain any element
// with sequence number earlier than N - `max_nack_list_size`.
// The largest maximum size is defined by `kNackListSizeLimit`
void SetMaxNackListSize(size_t max_nack_list_size);
// Set the sampling rate.
// If associated sampling rate of the received packets is changed, call this
// function to update sampling rate. Note that if there is any change in
// received codec then NetEq will flush its buffer and NACK has to be reset.
// After Reset() is called sampling rate has to be set.
void UpdateSampleRate(int sample_rate_hz);
// Update the sequence number and the timestamp of the last decoded RTP. This
// API should be called every time 10 ms audio is pulled from NetEq.
void UpdateLastDecodedPacket(uint16_t sequence_number, uint32_t timestamp);
// Update the sequence number and the timestamp of the last received RTP. This
// API should be called every time a packet pushed into ACM.
void UpdateLastReceivedPacket(uint16_t sequence_number, uint32_t timestamp);
// Get a list of "missing" packets which have expected time-to-play larger
// than the given round-trip-time (in milliseconds).
// Note: Late packets are not included.
// Calling this method multiple times may give different results, since the
// internal nack list may get flushed if never_nack_multiple_times_ is true.
std::vector<uint16_t> GetNackList(int64_t round_trip_time_ms);
// Reset to default values. The NACK list is cleared.
// `max_nack_list_size_` preserves its value.
void Reset();
// Returns the estimated packet loss rate in Q30, for testing only.
uint32_t GetPacketLossRateForTest() { return packet_loss_rate_; }
// This test need to access the private method GetNackList().
FRIEND_TEST_ALL_PREFIXES(NackTrackerTest, EstimateTimestampAndTimeToPlay);
// Options that can be configured via field trial.
struct Config {
// The exponential decay factor used to estimate the packet loss rate.
double packet_loss_forget_factor = 0.996;
// How many additional ms we are willing to wait (at most) for nacked
// packets for each additional percentage of packet loss.
int ms_per_loss_percent = 20;
// If true, never nack packets more than once.
bool never_nack_multiple_times = false;
struct NackElement {
NackElement(int64_t initial_time_to_play_ms, uint32_t initial_timestamp)
: time_to_play_ms(initial_time_to_play_ms),
estimated_timestamp(initial_timestamp) {}
// Estimated time (ms) left for this packet to be decoded. This estimate is
// updated every time jitter buffer decodes a packet.
int64_t time_to_play_ms;
// A guess about the timestamp of the missing packet, it is used for
// estimation of `time_to_play_ms`. The estimate might be slightly wrong if
// there has been frame-size change since the last received packet and the
// missing packet. However, the risk of this is low, and in case of such
// errors, there will be a minor misestimation in time-to-play of missing
// packets. This will have a very minor effect on NACK performance.
uint32_t estimated_timestamp;
class NackListCompare {
bool operator()(uint16_t sequence_number_old,
uint16_t sequence_number_new) const {
return IsNewerSequenceNumber(sequence_number_new, sequence_number_old);
typedef std::map<uint16_t, NackElement, NackListCompare> NackList;
// This API is used only for testing to assess whether time-to-play is
// computed correctly.
NackList GetNackList() const;
// This function subtracts 10 ms of time-to-play for all packets in NACK list.
// This is called when 10 ms elapsed with no new RTP packet decoded.
void UpdateEstimatedPlayoutTimeBy10ms();
// Returns a valid number of samples per packet given the current received
// sequence number and timestamp or nullopt of none could be computed.
absl::optional<int> GetSamplesPerPacket(
uint16_t sequence_number_current_received_rtp,
uint32_t timestamp_current_received_rtp) const;
// Given the `sequence_number_current_received_rtp` of currently received RTP
// update the list. Packets that are older than the received packet are added
// to the nack list.
void UpdateList(uint16_t sequence_number_current_received_rtp,
uint32_t timestamp_current_received_rtp);
// Packets which have sequence number older that
// `sequence_num_last_received_rtp_` - `max_nack_list_size_` are removed
// from the NACK list.
void LimitNackListSize();
// Estimate timestamp of a missing packet given its sequence number.
uint32_t EstimateTimestamp(uint16_t sequence_number, int samples_per_packet);
// Compute time-to-play given a timestamp.
int64_t TimeToPlay(uint32_t timestamp) const;
// Updates the estimated packet lost rate.
void UpdatePacketLossRate(int packets_lost);
const Config config_;
// Valid if a packet is received.
uint16_t sequence_num_last_received_rtp_;
uint32_t timestamp_last_received_rtp_;
bool any_rtp_received_; // If any packet received.
// Valid if a packet is decoded.
uint16_t sequence_num_last_decoded_rtp_;
uint32_t timestamp_last_decoded_rtp_;
bool any_rtp_decoded_; // If any packet decoded.
int sample_rate_khz_; // Sample rate in kHz.
// A list of missing packets to be retransmitted. Components of the list
// contain the sequence number of missing packets and the estimated time that
// each pack is going to be played out.
NackList nack_list_;
// NACK list will not keep track of missing packets prior to
// `sequence_num_last_received_rtp_` - `max_nack_list_size_`.
size_t max_nack_list_size_;
// Current estimate of the packet loss rate in Q30.
uint32_t packet_loss_rate_ = 0;
} // namespace webrtc