[feat] update congestion control feedback

src/common/array_view.h

@@ -0,0 +1,257 @@
+/*
+ * @Author: DI JUNKUN
+ * @Date: 2024-12-18
+ * Copyright (c) 2024 by DI JUNKUN, All Rights Reserved.
+ */
+
+#ifndef _ARRAY_VIEW_H_
+#define _ARRAY_VIEW_H_
+
+#include <algorithm>
+#include <array>
+#include <cstddef>
+#include <iterator>
+#include <type_traits>
+
+#include "type_traits.h"
+
+namespace array_view_internal {
+
+// Magic constant for indicating that the size of an ArrayView is variable
+// instead of fixed.
+enum : std::ptrdiff_t { kArrayViewVarSize = -4711 };
+
+// Base class for ArrayViews of fixed nonzero size.
+template <typename T, std::ptrdiff_t Size>
+class ArrayViewBase {
+  static_assert(Size > 0, "ArrayView size must be variable or non-negative");
+
+ public:
+  ArrayViewBase(T* data, size_t /* size */) : data_(data) {}
+
+  static constexpr size_t size() { return Size; }
+  static constexpr bool empty() { return false; }
+  T* data() const { return data_; }
+
+ protected:
+  static constexpr bool fixed_size() { return true; }
+
+ private:
+  T* data_;
+};
+
+// Specialized base class for ArrayViews of fixed zero size.
+template <typename T>
+class ArrayViewBase<T, 0> {
+ public:
+  explicit ArrayViewBase(T* /* data */, size_t /* size */) {}
+
+  static constexpr size_t size() { return 0; }
+  static constexpr bool empty() { return true; }
+  T* data() const { return nullptr; }
+
+ protected:
+  static constexpr bool fixed_size() { return true; }
+};
+
+// Specialized base class for ArrayViews of variable size.
+template <typename T>
+class ArrayViewBase<T, array_view_internal::kArrayViewVarSize> {
+ public:
+  ArrayViewBase(T* data, size_t size)
+      : data_(size == 0 ? nullptr : data), size_(size) {}
+
+  size_t size() const { return size_; }
+  bool empty() const { return size_ == 0; }
+  T* data() const { return data_; }
+
+ protected:
+  static constexpr bool fixed_size() { return false; }
+
+ private:
+  T* data_;
+  size_t size_;
+};
+
+}  // namespace array_view_internal
+
+template <typename T,
+          std::ptrdiff_t Size = array_view_internal::kArrayViewVarSize>
+class ArrayView final : public array_view_internal::ArrayViewBase<T, Size> {
+ public:
+  using value_type = T;
+  using reference = value_type&;
+  using const_reference = const value_type&;
+  using pointer = value_type*;
+  using const_pointer = const value_type*;
+  using const_iterator = const T*;
+
+  // Construct an ArrayView from a pointer and a length.
+  template <typename U>
+  ArrayView(U* data, size_t size)
+      : array_view_internal::ArrayViewBase<T, Size>::ArrayViewBase(data, size) {
+    RTC_DCHECK_EQ(size == 0 ? nullptr : data, this->data());
+    RTC_DCHECK_EQ(size, this->size());
+    RTC_DCHECK_EQ(!this->data(),
+                  this->size() == 0);  // data is null iff size == 0.
+  }
+
+  // Construct an empty ArrayView. Note that fixed-size ArrayViews of size > 0
+  // cannot be empty.
+  ArrayView() : ArrayView(nullptr, 0) {}
+  ArrayView(std::nullptr_t)  // NOLINT
+      : ArrayView() {}
+  ArrayView(std::nullptr_t, size_t size)
+      : ArrayView(static_cast<T*>(nullptr), size) {
+    static_assert(Size == 0 || Size == array_view_internal::kArrayViewVarSize,
+                  "");
+    RTC_DCHECK_EQ(0, size);
+  }
+
+  // Construct an ArrayView from a C-style array.
+  template <typename U, size_t N>
+  ArrayView(U (&array)[N])  // NOLINT
+      : ArrayView(array, N) {
+    static_assert(Size == N || Size == array_view_internal::kArrayViewVarSize,
+                  "Array size must match ArrayView size");
+  }
+
+  // (Only if size is fixed.) Construct a fixed size ArrayView<T, N> from a
+  // non-const std::array instance. For an ArrayView with variable size, the
+  // used ctor is ArrayView(U& u) instead.
+  template <typename U, size_t N,
+            typename std::enable_if<
+                Size == static_cast<std::ptrdiff_t>(N)>::type* = nullptr>
+  ArrayView(std::array<U, N>& u)  // NOLINT
+      : ArrayView(u.data(), u.size()) {}
+
+  // (Only if size is fixed.) Construct a fixed size ArrayView<T, N> where T is
+  // const from a const(expr) std::array instance. For an ArrayView with
+  // variable size, the used ctor is ArrayView(U& u) instead.
+  template <typename U, size_t N,
+            typename std::enable_if<
+                Size == static_cast<std::ptrdiff_t>(N)>::type* = nullptr>
+  ArrayView(const std::array<U, N>& u)  // NOLINT
+      : ArrayView(u.data(), u.size()) {}
+
+  // (Only if size is fixed.) Construct an ArrayView from any type U that has a
+  // static constexpr size() method whose return value is equal to Size, and a
+  // data() method whose return value converts implicitly to T*. In particular,
+  // this means we allow conversion from ArrayView<T, N> to ArrayView<const T,
+  // N>, but not the other way around. We also don't allow conversion from
+  // ArrayView<T> to ArrayView<T, N>, or from ArrayView<T, M> to ArrayView<T,
+  // N> when M != N.
+  template <typename U, typename std::enable_if<
+                            Size != array_view_internal::kArrayViewVarSize &&
+                            HasDataAndSize<U, T>::value>::type* = nullptr>
+  ArrayView(U& u)  // NOLINT
+      : ArrayView(u.data(), u.size()) {
+    static_assert(U::size() == Size, "Sizes must match exactly");
+  }
+  template <typename U, typename std::enable_if<
+                            Size != array_view_internal::kArrayViewVarSize &&
+                            HasDataAndSize<U, T>::value>::type* = nullptr>
+  ArrayView(const U& u)  // NOLINT(runtime/explicit)
+      : ArrayView(u.data(), u.size()) {
+    static_assert(U::size() == Size, "Sizes must match exactly");
+  }
+
+  // (Only if size is variable.) Construct an ArrayView from any type U that
+  // has a size() method whose return value converts implicitly to size_t, and
+  // a data() method whose return value converts implicitly to T*. In
+  // particular, this means we allow conversion from ArrayView<T> to
+  // ArrayView<const T>, but not the other way around. Other allowed
+  // conversions include
+  // ArrayView<T, N> to ArrayView<T> or ArrayView<const T>,
+  // std::vector<T> to ArrayView<T> or ArrayView<const T>,
+  // const std::vector<T> to ArrayView<const T>,
+  // rtc::Buffer to ArrayView<uint8_t> or ArrayView<const uint8_t>, and
+  // const rtc::Buffer to ArrayView<const uint8_t>.
+  template <typename U, typename std::enable_if<
+                            Size == array_view_internal::kArrayViewVarSize &&
+                            HasDataAndSize<U, T>::value>::type* = nullptr>
+  ArrayView(U& u)  // NOLINT
+      : ArrayView(u.data(), u.size()) {}
+  template <typename U, typename std::enable_if<
+                            Size == array_view_internal::kArrayViewVarSize &&
+                            HasDataAndSize<U, T>::value>::type* = nullptr>
+  ArrayView(const U& u)  // NOLINT(runtime/explicit)
+      : ArrayView(u.data(), u.size()) {}
+
+  // Indexing and iteration. These allow mutation even if the ArrayView is
+  // const, because the ArrayView doesn't own the array. (To prevent mutation,
+  // use a const element type.)
+  T& operator[](size_t idx) const {
+    RTC_DCHECK_LT(idx, this->size());
+    RTC_DCHECK(this->data());
+    return this->data()[idx];
+  }
+  T* begin() const { return this->data(); }
+  T* end() const { return this->data() + this->size(); }
+  const T* cbegin() const { return this->data(); }
+  const T* cend() const { return this->data() + this->size(); }
+  std::reverse_iterator<T*> rbegin() const {
+    return std::make_reverse_iterator(end());
+  }
+  std::reverse_iterator<T*> rend() const {
+    return std::make_reverse_iterator(begin());
+  }
+  std::reverse_iterator<const T*> crbegin() const {
+    return std::make_reverse_iterator(cend());
+  }
+  std::reverse_iterator<const T*> crend() const {
+    return std::make_reverse_iterator(cbegin());
+  }
+
+  ArrayView<T> subview(size_t offset, size_t size) const {
+    return offset < this->size()
+               ? ArrayView<T>(this->data() + offset,
+                              std::min(size, this->size() - offset))
+               : ArrayView<T>();
+  }
+  ArrayView<T> subview(size_t offset) const {
+    return subview(offset, this->size());
+  }
+};
+
+// Comparing two ArrayViews compares their (pointer,size) pairs; it does *not*
+// dereference the pointers.
+template <typename T, std::ptrdiff_t Size1, std::ptrdiff_t Size2>
+bool operator==(const ArrayView<T, Size1>& a, const ArrayView<T, Size2>& b) {
+  return a.data() == b.data() && a.size() == b.size();
+}
+template <typename T, std::ptrdiff_t Size1, std::ptrdiff_t Size2>
+bool operator!=(const ArrayView<T, Size1>& a, const ArrayView<T, Size2>& b) {
+  return !(a == b);
+}
+
+// Variable-size ArrayViews are the size of two pointers; fixed-size ArrayViews
+// are the size of one pointer. (And as a special case, fixed-size ArrayViews
+// of size 0 require no storage.)
+static_assert(sizeof(ArrayView<int>) == 2 * sizeof(int*), "");
+static_assert(sizeof(ArrayView<int, 17>) == sizeof(int*), "");
+static_assert(std::is_empty<ArrayView<int, 0>>::value, "");
+
+template <typename T>
+inline ArrayView<T> MakeArrayView(T* data, size_t size) {
+  return ArrayView<T>(data, size);
+}
+
+// Only for primitive types that have the same size and aligment.
+// Allow reinterpret cast of the array view to another primitive type of the
+// same size.
+// Template arguments order is (U, T, Size) to allow deduction of the template
+// arguments in client calls: reinterpret_array_view<target_type>(array_view).
+template <typename U, typename T, std::ptrdiff_t Size>
+inline ArrayView<U, Size> reinterpret_array_view(ArrayView<T, Size> view) {
+  static_assert(sizeof(U) == sizeof(T) && alignof(U) == alignof(T),
+                "ArrayView reinterpret_cast is only supported for casting "
+                "between views that represent the same chunk of memory.");
+  static_assert(
+      std::is_fundamental<T>::value && std::is_fundamental<U>::value,
+      "ArrayView reinterpret_cast is only supported for casting between "
+      "fundamental types.");
+  return ArrayView<U, Size>(reinterpret_cast<U*>(view.data()), view.size());
+}
+
+#endif

src/common/type_traits.h

@@ -0,0 +1,141 @@
+/*
+ *  Copyright 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.
+ */
+
+#ifndef RTC_BASE_TYPE_TRAITS_H_
+#define RTC_BASE_TYPE_TRAITS_H_
+
+#include <cstddef>
+#include <string>
+#include <type_traits>
+
+namespace rtc {
+
+// Determines if the given class has zero-argument .data() and .size() methods
+// whose return values are convertible to T* and size_t, respectively.
+template <typename DS, typename T>
+class HasDataAndSize {
+ private:
+  template <
+      typename C,
+      typename std::enable_if<
+          std::is_convertible<decltype(std::declval<C>().data()), T*>::value &&
+          std::is_convertible<decltype(std::declval<C>().size()),
+                              std::size_t>::value>::type* = nullptr>
+  static int Test(int);
+
+  template <typename>
+  static char Test(...);
+
+ public:
+  static constexpr bool value = std::is_same<decltype(Test<DS>(0)), int>::value;
+};
+
+namespace test_has_data_and_size {
+
+template <typename DR, typename SR>
+struct Test1 {
+  DR data();
+  SR size();
+};
+static_assert(HasDataAndSize<Test1<int*, int>, int>::value, "");
+static_assert(HasDataAndSize<Test1<int*, int>, const int>::value, "");
+static_assert(HasDataAndSize<Test1<const int*, int>, const int>::value, "");
+static_assert(!HasDataAndSize<Test1<const int*, int>, int>::value,
+              "implicit cast of const int* to int*");
+static_assert(!HasDataAndSize<Test1<char*, size_t>, int>::value,
+              "implicit cast of char* to int*");
+
+struct Test2 {
+  int* data;
+  size_t size;
+};
+static_assert(!HasDataAndSize<Test2, int>::value,
+              ".data and .size aren't functions");
+
+struct Test3 {
+  int* data();
+};
+static_assert(!HasDataAndSize<Test3, int>::value, ".size() is missing");
+
+class Test4 {
+  int* data();
+  size_t size();
+};
+static_assert(!HasDataAndSize<Test4, int>::value,
+              ".data() and .size() are private");
+
+}  // namespace test_has_data_and_size
+
+namespace type_traits_impl {
+
+// Determines if the given type is an enum that converts implicitly to
+// an integral type.
+template <typename T>
+struct IsIntEnum {
+ private:
+  // This overload is used if the type is an enum, and unary plus
+  // compiles and turns it into an integral type.
+  template <typename X,
+            typename std::enable_if<
+                std::is_enum<X>::value &&
+                std::is_integral<decltype(+std::declval<X>())>::value>::type* =
+                nullptr>
+  static int Test(int);
+
+  // Otherwise, this overload is used.
+  template <typename>
+  static char Test(...);
+
+ public:
+  static constexpr bool value =
+      std::is_same<decltype(Test<typename std::remove_reference<T>::type>(0)),
+                   int>::value;
+};
+
+}  // namespace type_traits_impl
+
+// Determines if the given type is integral, or an enum that
+// converts implicitly to an integral type.
+template <typename T>
+struct IsIntlike {
+ private:
+  using X = typename std::remove_reference<T>::type;
+
+ public:
+  static constexpr bool value =
+      std::is_integral<X>::value || type_traits_impl::IsIntEnum<X>::value;
+};
+
+namespace test_enum_intlike {
+
+enum E1 { e1 };
+enum { e2 };
+enum class E3 { e3 };
+struct S {};
+
+static_assert(type_traits_impl::IsIntEnum<E1>::value, "");
+static_assert(type_traits_impl::IsIntEnum<decltype(e2)>::value, "");
+static_assert(!type_traits_impl::IsIntEnum<E3>::value, "");
+static_assert(!type_traits_impl::IsIntEnum<int>::value, "");
+static_assert(!type_traits_impl::IsIntEnum<float>::value, "");
+static_assert(!type_traits_impl::IsIntEnum<S>::value, "");
+
+static_assert(IsIntlike<E1>::value, "");
+static_assert(IsIntlike<decltype(e2)>::value, "");
+static_assert(!IsIntlike<E3>::value, "");
+static_assert(IsIntlike<int>::value, "");
+static_assert(!IsIntlike<float>::value, "");
+static_assert(!IsIntlike<S>::value, "");
+
+}  // namespace test_enum_intlike
+
+}  // namespace rtc
+
+#endif  // RTC_BASE_TYPE_TRAITS_H_

src/qos/congestion_control_feedback.cc

@@ -0,0 +1,302 @@
+#include "congestion_control_feedback.h"
+
+#include <algorithm>
+#include <cstddef>
+#include <cstdint>
+#include <utility>
+#include <vector>
+
+#include "array_view.h"
+#include "byte_io.h"
+#include "log.h"
+
+/*
+     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+     |V=2|P| FMT=11  |   PT = 205    |          length               |
+     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+     |                 SSRC of RTCP packet sender                    |
+     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+     |                   SSRC of 1st RTP Stream                      |
+     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+     |          begin_seq            |          num_reports          |
+     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+     |R|ECN|  Arrival time offset    | ...                           .
+     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+     .                                                               .
+     .                                                               .
+     .                                                               .
+     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+     |                   SSRC of nth RTP Stream                      |
+     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+     |          begin_seq            |          num_reports          |
+     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+     |R|ECN|  Arrival time offset    | ...                           |
+     .                                                               .
+     .                                                               .
+     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+     |                 Report Timestamp (32 bits)                    |
+     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+*/
+
+namespace {
+
+constexpr size_t kSenderSsrcLength = 4;
+constexpr size_t kHeaderPerMediaSssrcLength = 8;
+constexpr size_t kTimestampLength = 4;
+
+// RFC-3168, Section 5
+constexpr uint16_t kEcnEct1 = 0x01;
+constexpr uint16_t kEcnEct0 = 0x02;
+constexpr uint16_t kEcnCe = 0x03;
+
+// Arrival time offset (ATO, 13 bits):
+// The arrival time of the RTP packet at the receiver, as an offset before the
+// time represented by the Report Timestamp (RTS) field of this RTCP congestion
+// control feedback report. The ATO field is in units of 1/1024 seconds (this
+// unit is chosen to give exact offsets from the RTS field) so, for example, an
+// ATO value of 512 indicates that the corresponding RTP packet arrived exactly
+// half a second before the time instant represented by the RTS field. If the
+// measured value is greater than 8189/1024 seconds (the value that would be
+// coded as 0x1FFD), the value 0x1FFE MUST be reported to indicate an over-range
+// measurement. If the measurement is unavailable or if the arrival time of the
+// RTP packet is after the time represented by the RTS field, then an ATO value
+// of 0x1FFF MUST be reported for the packet.
+uint16_t To13bitAto(int64_t arrival_time_offset) {
+  if (arrival_time_offset < 0) {
+    return 0x1FFF;
+  }
+  return std::min(static_cast<int64_t>(1024 * (arrival_time_offset / 1000)),
+                  int64_t{0x1FFE});
+}
+
+int64_t AtoToTimeDelta(uint16_t receive_info) {
+  // receive_info
+  // +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+  // |R|ECN|  Arrival time offset    |
+  // +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+
+  // ato -> second
+  const uint16_t ato = receive_info & 0x1FFF;
+  if (ato == 0x1FFE) {
+    return std::numeric_limits<int64_t>::max();
+  }
+  if (ato == 0x1FFF) {
+    return std::numeric_limits<int64_t>::min();
+  }
+  return ato / 1024;
+}
+
+uint16_t To2BitEcn(EcnMarking ecn_marking) {
+  switch (ecn_marking) {
+    case EcnMarking::kNotEct:
+      return 0;
+    case EcnMarking::kEct1:
+      return kEcnEct1 << 13;
+    case EcnMarking::kEct0:
+      return kEcnEct0 << 13;
+    case EcnMarking::kCe:
+      return kEcnCe << 13;
+  }
+}
+
+EcnMarking ToEcnMarking(uint16_t receive_info) {
+  const uint16_t ecn = (receive_info >> 13) & 0b11;
+  if (ecn == kEcnEct1) {
+    return EcnMarking::kEct1;
+  }
+  if (ecn == kEcnEct0) {
+    return EcnMarking::kEct0;
+  }
+  if (ecn == kEcnCe) {
+    return EcnMarking::kCe;
+  }
+  return EcnMarking::kNotEct;
+}
+
+}  // namespace
+
+CongestionControlFeedback ::CongestionControlFeedback(
+    std::vector<PacketInfo> packets, uint32_t compact_ntp_timestamp)
+    : packets_(std::move(packets)),
+      report_timestamp_compact_ntp_(compact_ntp_timestamp) {}
+
+bool CongestionControlFeedback::Create(uint8_t* buffer, size_t* position,
+                                       size_t max_length,
+                                       PacketReadyCallback callback) const {
+  // Ensure there is enough room for this packet.
+  while (*position + BlockLength() > max_length) {
+    if (!OnBufferFull(buffer, position, callback)) return false;
+  }
+  const size_t position_end = *position + BlockLength();
+
+  //    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+  //    |V=2|P| FMT=11  |   PT = 205    |          length               |
+  //    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+  //    |                 SSRC of RTCP packet sender                    |
+  //    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+  CreateHeader(kFeedbackMessageType, kPacketType, HeaderLength(), buffer,
+               position);
+  ByteWriter<uint32_t>::WriteBigEndian(&buffer[*position], sender_ssrc());
+  *position += 4;
+
+  //   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+  //   |                   SSRC of nth RTP Stream                      |
+  //   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+  //   |          begin_seq            |          num_reports          |
+  //   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+  //   |R|ECN|  Arrival time offset    | ...                           .
+  //   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+  //   .                                                               .
+  auto write_report_for_ssrc = [&](ArrayView<const PacketInfo> packets) {
+    // SSRC of nth RTP stream.
+    ByteWriter<uint32_t>::WriteBigEndian(&buffer[*position], packets[0].ssrc);
+    *position += 4;
+
+    // begin_seq
+    ByteWriter<uint16_t>::WriteBigEndian(&buffer[*position],
+                                         packets[0].sequence_number);
+    *position += 2;
+    // num_reports
+    uint16_t num_reports = packets.size();
+    RTC_DCHECK_EQ(static_cast<uint16_t>(
+
+                      packets[packets.size() - 1].sequence_number -
+                      packets[0].sequence_number + 1),
+                  packets.size())
+        << "Expected continous rtp sequence numbers.";
+
+    // Each report block MUST NOT include more than 16384 packet metric
+    // blocks (i.e., it MUST NOT report on more than one quarter of the
+    // sequence number space in a single report).
+    if (num_reports > 16384) {
+      LOG_FATAL("Unexpected number of reports: {}", num_reports);
+      return;
+    }
+    ByteWriter<uint16_t>::WriteBigEndian(&buffer[*position], num_reports);
+    *position += 2;
+
+    for (const PacketInfo& packet : packets) {
+      bool received = packet.arrival_time_offset.IsFinite();
+      uint16_t packet_info = 0;
+      if (received) {
+        packet_info = 0x8000 | To2BitEcn(packet.ecn) |
+                      To13bitAto(packet.arrival_time_offset);
+      }
+      ByteWriter<uint16_t>::WriteBigEndian(&buffer[*position], packet_info);
+      *position += 2;
+    }
+    // 32bit align per SSRC block.
+    if (num_reports % 2 != 0) {
+      ByteWriter<uint16_t>::WriteBigEndian(&buffer[*position], 0);
+      *position += 2;
+    }
+  };
+
+  ArrayView<const PacketInfo> remaining(packets_);
+  while (!remaining.empty()) {
+    int number_of_packets_for_ssrc = 0;
+    uint32_t ssrc = remaining[0].ssrc;
+    for (const PacketInfo& packet_info : remaining) {
+      if (packet_info.ssrc != ssrc) {
+        break;
+      }
+      ++number_of_packets_for_ssrc;
+    }
+    write_report_for_ssrc(remaining.subview(0, number_of_packets_for_ssrc));
+    remaining = remaining.subview(number_of_packets_for_ssrc);
+  }
+
+  //   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+  //   |                 Report Timestamp (32 bits)                    |
+  //   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+  ByteWriter<uint32_t>::WriteBigEndian(&buffer[*position],
+                                       report_timestamp_compact_ntp_);
+  *position += 4;
+
+  RTC_DCHECK_EQ(*position, position_end);
+  return true;
+}
+
+size_t CongestionControlFeedback::BlockLength() const {
+  // Total size of this packet
+  size_t total_size = kSenderSsrcLength + kHeaderLength + kTimestampLength;
+  if (packets_.empty()) {
+    return total_size;
+  }
+
+  auto increase_size_per_ssrc = [](int number_of_packets_for_ssrc) {
+    // Each packet report needs two bytes.
+    size_t packet_block_size = number_of_packets_for_ssrc * 2;
+    // 32 bit aligned.
+    return kHeaderPerMediaSssrcLength + packet_block_size +
+           ((number_of_packets_for_ssrc % 2) != 0 ? 2 : 0);
+  };
+
+  uint32_t ssrc = packets_.front().ssrc;
+  uint16_t first_sequence_number = packets_.front().sequence_number;
+  for (size_t i = 0; i < packets_.size(); ++i) {
+    if (packets_[i].ssrc != ssrc) {
+      uint16_t number_of_packets =
+          packets_[i - 1].sequence_number - first_sequence_number + 1;
+      total_size += increase_size_per_ssrc(number_of_packets);
+      ssrc = packets_[i].ssrc;
+      first_sequence_number = packets_[i].sequence_number;
+    }
+  }
+  uint16_t number_of_packets =
+      packets_.back().sequence_number - first_sequence_number + 1;
+  total_size += increase_size_per_ssrc(number_of_packets);
+
+  return total_size;
+}
+
+bool CongestionControlFeedback::Parse(const rtcp::CommonHeader& packet) {
+  const uint8_t* payload = packet.payload();
+  const uint8_t* payload_end = packet.payload() + packet.payload_size_bytes();
+
+  if (packet.payload_size_bytes() % 4 != 0 ||
+      packet.payload_size_bytes() < kSenderSsrcLength + kTimestampLength) {
+    return false;
+  }
+
+  SetSenderSsrc(ByteReader<uint32_t>::ReadBigEndian(payload));
+  payload += 4;
+
+  report_timestamp_compact_ntp_ =
+      ByteReader<uint32_t>::ReadBigEndian(payload_end - 4);
+  payload_end -= 4;
+
+  while (payload + kHeaderPerMediaSssrcLength < payload_end) {
+    uint32_t ssrc = ByteReader<uint32_t>::ReadBigEndian(payload);
+    payload += 4;
+
+    uint16_t base_seqno = ByteReader<uint16_t>::ReadBigEndian(payload);
+    payload += 2;
+    uint16_t num_reports = ByteReader<uint16_t>::ReadBigEndian(payload);
+    payload += 2;
+
+    constexpr size_t kPerPacketLength = 2;
+    if (payload + kPerPacketLength * num_reports > payload_end) {
+      return false;
+    }
+
+    for (int i = 0; i < num_reports; ++i) {
+      uint16_t packet_info = ByteReader<uint16_t>::ReadBigEndian(payload);
+      payload += 2;
+
+      uint16_t seq_no = base_seqno + i;
+      bool received = (packet_info & 0x8000);
+      packets_.push_back(
+          {.ssrc = ssrc,
+           .sequence_number = seq_no,
+           .arrival_time_offset = received ? AtoToTimeDelta(packet_info)
+                                           : TimeDelta::MinusInfinity(),
+           .ecn = ToEcnMarking(packet_info)});
+    }
+    if (num_reports % 2) {
+      // 2 bytes padding
+      payload += 2;
+    }
+  }
+  return payload == payload_end;
+}

src/qos/congestion_control_feedback.h

@@ -0,0 +1,78 @@
+/*
+ * @Author: DI JUNKUN
+ * @Date: 2024-12-18
+ * Copyright (c) 2024 by DI JUNKUN, All Rights Reserved.
+ */
+
+#ifndef _CONGESTION_CONTROL_FEEDBACK_H_
+#define _CONGESTION_CONTROL_FEEDBACK_H_
+
+#include <cstddef>
+#include <cstdint>
+#include <limits>
+#include <vector>
+
+#include "rtp_feedback.h"
+
+// L4S Explicit Congestion Notification (ECN) .
+// https://www.rfc-editor.org/rfc/rfc9331.html ECT stands for ECN-Capable
+// Transport and CE stands for Congestion Experienced.
+
+// RFC-3168, Section 5
+// +-----+-----+
+// | ECN FIELD |
+// +-----+-----+
+//   ECT   CE         [Obsolete] RFC 2481 names for the ECN bits.
+//    0     0         Not-ECT
+//    0     1         ECT(1)
+//    1     0         ECT(0)
+//    1     1         CE
+
+enum class EcnMarking {
+  kNotEct = 0,  // Not ECN-Capable Transport
+  kEct1 = 1,    // ECN-Capable Transport
+  kEct0 = 2,    // Not used by L4s (or webrtc.)
+  kCe = 3,      // Congestion experienced
+};
+
+// Congestion control feedback message as specified in
+// https://www.rfc-editor.org/rfc/rfc8888.html
+class CongestionControlFeedback : public RtpFeedback {
+ public:
+  struct PacketInfo {
+    uint32_t ssrc = 0;
+    uint16_t sequence_number = 0;
+    //  Time offset from report timestamp. Minus infinity if the packet has not
+    //  been received.
+    int64_t arrival_time_offset = std::numeric_limits<int64_t>::min();
+    rtc::EcnMarking ecn = rtc::EcnMarking::kNotEct;
+  };
+
+  static constexpr uint8_t kFeedbackMessageType = 11;
+
+  // `Packets` MUST be sorted in sequence_number order per SSRC. There MUST not
+  // be missing sequence numbers between `Packets`. `Packets` MUST not include
+  // duplicate sequence numbers.
+  CongestionControlFeedback(std::vector<PacketInfo> packets,
+                            uint32_t report_timestamp_compact_ntp);
+  CongestionControlFeedback() = default;
+
+  bool Parse(const CommonHeader& packet);
+
+  rtc::ArrayView<const PacketInfo> packets() const { return packets_; }
+
+  uint32_t report_timestamp_compact_ntp() const {
+    return report_timestamp_compact_ntp_;
+  }
+
+  // Serialize the packet.
+  bool Create(uint8_t* packet, size_t* position, size_t max_length,
+              PacketReadyCallback callback) const override;
+  size_t BlockLength() const override;
+
+ private:
+  std::vector<PacketInfo> packets_;
+  uint32_t report_timestamp_compact_ntp_ = 0;
+};
+
+#endif

src/qos/congestion_control_feedback_generator.cc

@@ -0,0 +1,108 @@
+#include "congestion_control_feedback_generator.h"
+
+#include <algorithm>
+#include <cstdint>
+#include <memory>
+#include <optional>
+#include <utility>
+#include <vector>
+
+uint32_t ConvertToCompactNtp(int64_t now_ms) {
+  int64_t seconds = now_ms / 1000;
+  int64_t milliseconds = now_ms % 1000;
+  uint16_t ntp_seconds = static_cast<uint16_t>(seconds & 0xFFFF);
+  uint16_t ntp_fraction = static_cast<uint16_t>((milliseconds * 65536) / 1000);
+  uint32_t compact_ntp = (ntp_seconds << 16) | ntp_fraction;
+  return compact_ntp;
+}
+
+CongestionControlFeedbackGenerator::CongestionControlFeedbackGenerator(
+    RtcpSender rtcp_sender)
+    : rtcp_sender_(std::move(rtcp_sender)) {}
+
+void CongestionControlFeedbackGenerator::OnReceivedPacket(
+    const RtpPacketReceived& packet) {
+  marker_bit_seen_ |= packet.Marker();
+  if (!first_arrival_time_since_feedback_) {
+    first_arrival_time_since_feedback_ = packet.arrival_time();
+  }
+  feedback_trackers_[packet.Ssrc()].ReceivedPacket(packet);
+  if (NextFeedbackTime() < packet.arrival_time()) {
+    auto now = std::chrono::system_clock::now();
+    auto now_ms = std::chrono::duration_cast<std::chrono::milliseconds>(
+                      now.time_since_epoch())
+                      .count();
+    SendFeedback(now_ms);
+  }
+}
+
+int64_t CongestionControlFeedbackGenerator::NextFeedbackTime() const {
+  auto now = std::chrono::system_clock::now();
+  auto now_ms = std::chrono::duration_cast<std::chrono::milliseconds>(
+                    now.time_since_epoch())
+                    .count();
+
+  if (!first_arrival_time_since_feedback_) {
+    return std::max(now_ms + min_time_between_feedback_,
+                    next_possible_feedback_send_time_);
+  }
+
+  if (!marker_bit_seen_) {
+    return std::max(next_possible_feedback_send_time_,
+                    *first_arrival_time_since_feedback_ +
+                        max_time_to_wait_for_packet_with_marker_);
+  }
+  return next_possible_feedback_send_time_;
+}
+
+int64_t CongestionControlFeedbackGenerator::Process(int64_t now_ms) {
+  if (NextFeedbackTime() <= now_ms) {
+    SendFeedback(now_ms);
+  }
+  return NextFeedbackTime() - now_ms;
+}
+
+void CongestionControlFeedbackGenerator::OnSendBandwidthEstimateChanged(
+    DataRate estimate) {
+  // Feedback reports should max occupy 5% of total bandwidth.
+  max_feedback_rate_ = estimate * 0.05;
+}
+
+void CongestionControlFeedbackGenerator::SetTransportOverhead(
+    DataSize overhead_per_packet) {
+  packet_overhead_ = overhead_per_packet;
+}
+
+void CongestionControlFeedbackGenerator::SendFeedback(int64_t now_ms) {
+  uint32_t compact_ntp = ConvertToCompactNtp(now_ms);
+  std::vector<rtcp::CongestionControlFeedback::PacketInfo> rtcp_packet_info;
+  for (auto& [unused, tracker] : feedback_trackers_) {
+    tracker.AddPacketsToFeedback(now_ms, rtcp_packet_info);
+  }
+
+  marker_bit_seen_ = false;
+  first_arrival_time_since_feedback_ = std::nullopt;
+
+  auto feedback = std::make_unique<rtcp::CongestionControlFeedback>(
+      std::move(rtcp_packet_info), compact_ntp);
+  CalculateNextPossibleSendTime(feedback->BlockLength(), now_ms);
+
+  std::vector<std::unique_ptr<rtcp::RtcpPacket>> rtcp_packets;
+  rtcp_packets.push_back(std::move(feedback));
+  rtcp_sender_(std::move(rtcp_packets));
+}
+
+void CongestionControlFeedbackGenerator::CalculateNextPossibleSendTime(
+    int64_t feedback_size, int64_t now_ms) {
+  int64_t time_since_last_sent = now - last_feedback_sent_time_;
+  size_t debt_payed = time_since_last_sent * max_feedback_rate_;
+  send_rate_debt_ =
+      debt_payed > send_rate_debt_ ? 0 : send_rate_debt_ - debt_payed;
+  send_rate_debt_ += feedback_size + packet_overhead_;
+  last_feedback_sent_time_ = now_ms;
+  next_possible_feedback_send_time_ =
+      now_ms +
+      std::clamp(max_feedback_rate_ == 0 ? std::numeric_limits<int64_t>::max()
+                                         : send_rate_debt_ / max_feedback_rate_,
+                 min_time_between_feedback_, max_time_between_feedback_);
+}

src/qos/congestion_control_feedback_generator.h

@@ -0,0 +1,59 @@
+/*
+ * @Author: DI JUNKUN
+ * @Date: 2024-12-18
+ * Copyright (c) 2024 by DI JUNKUN, All Rights Reserved.
+ */
+
+#ifndef _CONGESTION_CONTROL_FEEDBACK_GENERATOR_H_
+#define _CONGESTION_CONTROL_FEEDBACK_GENERATOR_H_
+
+#include "rtp_packet_received.h"
+
+class CongestionControlFeedbackGenerator
+    : public RtpTransportFeedbackGenerator {
+ public:
+  CongestionControlFeedbackGenerator(
+      RtpTransportFeedbackGenerator::RtcpSender feedback_sender);
+  ~CongestionControlFeedbackGenerator() = default;
+
+  void OnReceivedPacket(const RtpPacketReceived& packet) override;
+
+  void OnSendBandwidthEstimateChanged(DataRate estimate) override;
+
+  int64_t Process(int64_t now_ms) override;
+
+  void SetTransportOverhead(DataSize overhead_per_packet) override;
+
+ private:
+  int64_t NextFeedbackTime() const;
+
+  void SendFeedback(int64_t now_ms);
+
+  void CalculateNextPossibleSendTime(int64_t feedback_size, int64_t now_ms);
+
+ private:
+  // Feedback should not use more than 5% of the configured send bandwidth
+  // estimate. Min and max duration between feedback is configurable using field
+  // trials, but per default, min is 25ms and max is 250ms.
+  // If possible, given the other constraints, feedback will be sent when a
+  // packet with marker bit is received in order to provide feedback as soon as
+  // possible after receiving a complete video frame. If no packet with marker
+  // bit is received, feedback can be delayed up to 25ms after the first packet
+  // since the last sent feedback. On good networks, this means that a sender
+  // may receive feedback for every sent frame.
+  int64_t min_time_between_feedback_ = 25;
+  int64_t max_time_between_feedback_ = 250;
+  int64_t max_time_to_wait_for_packet_with_marker_ = 25;
+
+  int64_t max_feedback_rate_ = 1000;  // kbps
+  int64_t packet_overhead_ = 0;
+  int64_t send_rate_debt_ = 0;
+
+  std::optional<int64_t> first_arrival_time_since_feedback_;
+  int64_t next_possible_feedback_send_time_ = 0;
+  int64_t last_feedback_sent_time_ = 0;
+
+  bool marker_bit_seen_ = false;
+};
+
+#endif

src/qos/congestion_control_feedback_tracker.cc

@@ -0,0 +1,87 @@
+#include "congestion_control_feedback_tracker.h"
+
+#include <cstdint>
+#include <tuple>
+#include <vector>
+
+void CongestionControlFeedbackTracker::ReceivedPacket(
+    const RtpPacketReceived& packet) {
+  int64_t unwrapped_sequence_number =
+      unwrapper_.Unwrap(packet.SequenceNumber());
+  if (last_sequence_number_in_feedback_ &&
+      unwrapped_sequence_number < *last_sequence_number_in_feedback_ + 1) {
+    RTC_LOG(LS_WARNING)
+        << "Received packet unorderered between feeedback. SSRC: "
+        << packet.Ssrc() << " Seq: " << packet.SequenceNumber()
+        << " last feedback: "
+        << static_cast<uint16_t>(*last_sequence_number_in_feedback_);
+    // TODO: bugs.webrtc.org/374550342 - According to spec, the old packets
+    // should be reported again. But at the moment, we dont store history of
+    // packet we already reported and thus, they will be reported as lost. Note
+    // that this is likely not a problem in webrtc since the packets will also
+    // be removed from the send history when they are first reported as
+    // received.
+    last_sequence_number_in_feedback_ = unwrapped_sequence_number - 1;
+  }
+  packets_.push_back({.ssrc = packet.Ssrc(),
+                      .unwrapped_sequence_number = unwrapped_sequence_number,
+                      .arrival_time = packet.arrival_time(),
+                      .ecn = packet.ecn()});
+}
+
+void CongestionControlFeedbackTracker::AddPacketsToFeedback(
+    int64_t feedback_time,
+    std::vector<CongestionControlFeedback::PacketInfo>& packet_feedback) {
+  if (packets_.empty()) {
+    return;
+  }
+  absl::c_sort(packets_, [](const PacketInfo& a, const PacketInfo& b) {
+    return std::tie(a.unwrapped_sequence_number, a.arrival_time) <
+           std::tie(b.unwrapped_sequence_number, b.arrival_time);
+  });
+  if (!last_sequence_number_in_feedback_) {
+    last_sequence_number_in_feedback_ =
+        packets_.front().unwrapped_sequence_number - 1;
+  }
+
+  auto packet_it = packets_.begin();
+  uint32_t ssrc = packet_it->ssrc;
+  for (int64_t sequence_number = *last_sequence_number_in_feedback_ + 1;
+       sequence_number <= packets_.back().unwrapped_sequence_number;
+       ++sequence_number) {
+    RTC_DCHECK(packet_it != packets_.end());
+    RTC_DCHECK_EQ(ssrc, packet_it->ssrc);
+
+    rtc::EcnMarking ecn = rtc::EcnMarking::kNotEct;
+    TimeDelta arrival_time_offset = TimeDelta::MinusInfinity();
+
+    if (sequence_number == packet_it->unwrapped_sequence_number) {
+      arrival_time_offset = feedback_time - packet_it->arrival_time;
+      ecn = packet_it->ecn;
+      ++packet_it;
+      while (packet_it != packets_.end() &&
+             packet_it->unwrapped_sequence_number == sequence_number) {
+        // According to RFC 8888:
+        // If duplicate copies of a particular RTP packet are received, then the
+        // arrival time of the first copy to arrive MUST be reported. If any of
+        // the copies of the duplicated packet are ECN-CE marked, then an ECN-CE
+        // mark MUST be reported for that packet; otherwise, the ECN mark of the
+        // first copy to arrive is reported.
+        if (packet_it->ecn == rtc::EcnMarking::kCe) {
+          ecn = rtc::EcnMarking::kCe;
+        }
+        RTC_LOG(LS_WARNING) << "Received duplicate packet ssrc:" << ssrc
+                            << " seq:" << static_cast<uint16_t>(sequence_number)
+                            << " ecn: " << static_cast<int>(ecn);
+        ++packet_it;
+      }
+    }  // else - the packet has not been received yet.
+    packet_feedback.push_back(
+        {.ssrc = ssrc,
+         .sequence_number = static_cast<uint16_t>(sequence_number),
+         .arrival_time_offset = arrival_time_offset,
+         .ecn = ecn});
+  }
+  last_sequence_number_in_feedback_ = packets_.back().unwrapped_sequence_number;
+  packets_.clear();
+}

src/qos/congestion_control_feedback_tracker.h

@@ -0,0 +1,43 @@
+/*
+ * @Author: DI JUNKUN
+ * @Date: 2024-12-18
+ * Copyright (c) 2024 by DI JUNKUN, All Rights Reserved.
+ */
+
+#ifndef _CONGESTION_CONTROL_FEEDBACK_TRACKER_H_
+#define _CONGESTION_CONTROL_FEEDBACK_TRACKER_H_
+
+#include <optional>
+#include <vector>
+
+#include "rtp_packet_received.h"
+
+class CongestionControlFeedbackTracker {
+ public:
+  CongestionControlFeedbackTracker() = default;
+
+  void ReceivedPacket(const RtpPacketReceived& packet);
+
+  // Adds received packets to `packet_feedback`
+  // RTP sequence numbers are continous from the last created feedback unless
+  // reordering has occured between feedback packets. If so, the sequence
+  // number range may overlap with previousely sent feedback.
+  void AddPacketsToFeedback(
+      int64_t feedback_time,
+      std::vector<CongestionControlFeedback::PacketInfo>& packet_feedback);
+
+ private:
+  struct PacketInfo {
+    uint32_t ssrc;
+    int64_t unwrapped_sequence_number = 0;
+    int64_t arrival_time;
+    rtc::EcnMarking ecn = rtc::EcnMarking::kNotEct;
+  };
+
+  std::optional<int64_t> last_sequence_number_in_feedback_;
+  SeqNumUnwrapper<uint16_t> unwrapper_;
+
+  std::vector<PacketInfo> packets_;
+};
+
+#endif

src/qos/receive_side_congestion_controller.cc

@@ -0,0 +1,156 @@
+/*
+ *  Copyright (c) 2017 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 "receive_side_congestion_controller.h"
+
+#include <algorithm>
+#include <memory>
+#include <utility>
+
+namespace {
+static const uint32_t kTimeOffsetSwitchThreshold = 30;
+}  // namespace
+
+void ReceiveSideCongestionController::OnRttUpdate(int64_t avg_rtt_ms,
+                                                  int64_t max_rtt_ms) {
+  MutexLock lock(&mutex_);
+  rbe_->OnRttUpdate(avg_rtt_ms, max_rtt_ms);
+}
+
+void ReceiveSideCongestionController::RemoveStream(uint32_t ssrc) {
+  MutexLock lock(&mutex_);
+  rbe_->RemoveStream(ssrc);
+}
+
+DataRate ReceiveSideCongestionController::LatestReceiveSideEstimate() const {
+  MutexLock lock(&mutex_);
+  return rbe_->LatestEstimate();
+}
+
+void ReceiveSideCongestionController::PickEstimator(
+    bool has_absolute_send_time) {
+  if (has_absolute_send_time) {
+    // If we see AST in header, switch RBE strategy immediately.
+    if (!using_absolute_send_time_) {
+      RTC_LOG(LS_INFO)
+          << "WrappingBitrateEstimator: Switching to absolute send time RBE.";
+      using_absolute_send_time_ = true;
+      rbe_ = std::make_unique<RemoteBitrateEstimatorAbsSendTime>(
+          env_, &remb_throttler_);
+    }
+    packets_since_absolute_send_time_ = 0;
+  } else {
+    // When we don't see AST, wait for a few packets before going back to TOF.
+    if (using_absolute_send_time_) {
+      ++packets_since_absolute_send_time_;
+      if (packets_since_absolute_send_time_ >= kTimeOffsetSwitchThreshold) {
+        RTC_LOG(LS_INFO)
+            << "WrappingBitrateEstimator: Switching to transmission "
+               "time offset RBE.";
+        using_absolute_send_time_ = false;
+        rbe_ = std::make_unique<RemoteBitrateEstimatorSingleStream>(
+            env_, &remb_throttler_);
+      }
+    }
+  }
+}
+
+ReceiveSideCongestionController::ReceiveSideCongestionController(
+    const Environment& env,
+    TransportSequenceNumberFeedbackGenenerator::RtcpSender feedback_sender,
+    RembThrottler::RembSender remb_sender,
+    absl::Nullable<NetworkStateEstimator*> network_state_estimator)
+    : env_(env),
+      remb_throttler_(std::move(remb_sender), &env_.clock()),
+      transport_sequence_number_feedback_generator_(feedback_sender,
+                                                    network_state_estimator),
+      congestion_control_feedback_generator_(env, feedback_sender),
+      rbe_(std::make_unique<RemoteBitrateEstimatorSingleStream>(
+          env_, &remb_throttler_)),
+      using_absolute_send_time_(false),
+      packets_since_absolute_send_time_(0) {
+  FieldTrialParameter<bool> force_send_rfc8888_feedback("force_send", false);
+  ParseFieldTrial(
+      {&force_send_rfc8888_feedback},
+      env.field_trials().Lookup("WebRTC-RFC8888CongestionControlFeedback"));
+  if (force_send_rfc8888_feedback) {
+    EnablSendCongestionControlFeedbackAccordingToRfc8888();
+  }
+}
+
+void ReceiveSideCongestionController::
+    EnablSendCongestionControlFeedbackAccordingToRfc8888() {
+  RTC_DCHECK_RUN_ON(&sequence_checker_);
+  send_rfc8888_congestion_feedback_ = true;
+}
+
+void ReceiveSideCongestionController::OnReceivedPacket(
+    const RtpPacketReceived& packet, MediaType media_type) {
+  if (send_rfc8888_congestion_feedback_) {
+    RTC_DCHECK_RUN_ON(&sequence_checker_);
+    congestion_control_feedback_generator_.OnReceivedPacket(packet);
+    return;
+  }
+  bool has_transport_sequence_number =
+      packet.HasExtension<TransportSequenceNumber>() ||
+      packet.HasExtension<TransportSequenceNumberV2>();
+  if (media_type == MediaType::AUDIO && !has_transport_sequence_number) {
+    // For audio, we only support send side BWE.
+    return;
+  }
+
+  if (has_transport_sequence_number) {
+    // Send-side BWE.
+    transport_sequence_number_feedback_generator_.OnReceivedPacket(packet);
+  } else {
+    // Receive-side BWE.
+    MutexLock lock(&mutex_);
+    PickEstimator(packet.HasExtension<AbsoluteSendTime>());
+    rbe_->IncomingPacket(packet);
+  }
+}
+
+void ReceiveSideCongestionController::OnBitrateChanged(int bitrate_bps) {
+  RTC_DCHECK_RUN_ON(&sequence_checker_);
+  DataRate send_bandwidth_estimate = DataRate::BitsPerSec(bitrate_bps);
+  transport_sequence_number_feedback_generator_.OnSendBandwidthEstimateChanged(
+      send_bandwidth_estimate);
+  congestion_control_feedback_generator_.OnSendBandwidthEstimateChanged(
+      send_bandwidth_estimate);
+}
+
+TimeDelta ReceiveSideCongestionController::MaybeProcess() {
+  Timestamp now = env_.clock().CurrentTime();
+  if (send_rfc8888_congestion_feedback_) {
+    RTC_DCHECK_RUN_ON(&sequence_checker_);
+    return congestion_control_feedback_generator_.Process(now);
+  }
+  mutex_.Lock();
+  TimeDelta time_until_rbe = rbe_->Process();
+  mutex_.Unlock();
+  TimeDelta time_until_rep =
+      transport_sequence_number_feedback_generator_.Process(now);
+  TimeDelta time_until = std::min(time_until_rbe, time_until_rep);
+  return std::max(time_until, TimeDelta::Zero());
+}
+
+void ReceiveSideCongestionController::SetMaxDesiredReceiveBitrate(
+    DataRate bitrate) {
+  remb_throttler_.SetMaxDesiredReceiveBitrate(bitrate);
+}
+
+void ReceiveSideCongestionController::SetTransportOverhead(
+    DataSize overhead_per_packet) {
+  RTC_DCHECK_RUN_ON(&sequence_checker_);
+  transport_sequence_number_feedback_generator_.SetTransportOverhead(
+      overhead_per_packet);
+  congestion_control_feedback_generator_.SetTransportOverhead(
+      overhead_per_packet);
+}

src/qos/receive_side_congestion_controller.h

@@ -0,0 +1,69 @@
+/*
+ * @Author: DI JUNKUN
+ * @Date: 2024-12-12
+ * Copyright (c) 2024 by DI JUNKUN, All Rights Reserved.
+ */
+
+#ifndef _RECEIVE_SIDE_CONGESTION_CONTROLLER_H_
+#define _RECEIVE_SIDE_CONGESTION_CONTROLLER_H_
+
+class ReceiveSideCongestionController {
+ public:
+  ReceiveSideCongestionController();
+  ~ReceiveSideCongestionController() override = default;
+
+ public:
+  void EnablSendCongestionControlFeedbackAccordingToRfc8888();
+
+  void OnReceivedPacket(const RtpPacketReceived& packet, MediaType media_type);
+
+  // Implements CallStatsObserver.
+  void OnRttUpdate(int64_t avg_rtt_ms, int64_t max_rtt_ms) override;
+
+  // This is send bitrate, used to control the rate of feedback messages.
+  void OnBitrateChanged(int bitrate_bps);
+
+  // Ensures the remote party is notified of the receive bitrate no larger than
+  // `bitrate` using RTCP REMB.
+  void SetMaxDesiredReceiveBitrate(DataRate bitrate);
+
+  void SetTransportOverhead(DataSize overhead_per_packet);
+
+  // Returns latest receive side bandwidth estimation.
+  // Returns zero if receive side bandwidth estimation is unavailable.
+  DataRate LatestReceiveSideEstimate() const;
+
+  // Removes stream from receive side bandwidth estimation.
+  // Noop if receive side bwe is not used or stream doesn't participate in it.
+  void RemoveStream(uint32_t ssrc);
+
+  // Runs periodic tasks if it is time to run them, returns time until next
+  // call to `MaybeProcess` should be non idle.
+  TimeDelta MaybeProcess();
+
+ private:
+  void PickEstimator(bool has_absolute_send_time)
+      RTC_EXCLUSIVE_LOCKS_REQUIRED(mutex_);
+
+  const Environment env_;
+  RembThrottler remb_throttler_;
+
+  // TODO: bugs.webrtc.org/42224904 - Use sequence checker for all usage of
+  // ReceiveSideCongestionController. At the time of
+  // writing OnReceivedPacket and MaybeProcess can unfortunately be called on an
+  // arbitrary thread by external projects.
+  SequenceChecker sequence_checker_;
+
+  bool send_rfc8888_congestion_feedback_ = false;
+  TransportSequenceNumberFeedbackGenenerator
+      transport_sequence_number_feedback_generator_;
+  CongestionControlFeedbackGenerator congestion_control_feedback_generator_
+      RTC_GUARDED_BY(sequence_checker_);
+
+  mutable Mutex mutex_;
+  std::unique_ptr<RemoteBitrateEstimator> rbe_ RTC_GUARDED_BY(mutex_);
+  bool using_absolute_send_time_ RTC_GUARDED_BY(mutex_);
+  uint32_t packets_since_absolute_send_time_ RTC_GUARDED_BY(mutex_);
+};
+
+#endif

src/rtcp/rtp_feedback/rtp_feedback.cpp

@@ -0,0 +1,29 @@
+#include "rtp_feedback.h"
+
+#include "byte_io.h"
+
+// RFC 4585, Section 6.1: Feedback format.
+//
+// Common packet format:
+//
+//    0                   1                   2                   3
+//    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+//   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+//   |V=2|P|   FMT   |       PT      |          length               |
+//   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+// 0 |                  SSRC of packet sender                        |
+//   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+// 4 |                  SSRC of media source                         |
+//   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+//   :            Feedback Control Information (FCI)                 :
+//   :                                                               :
+
+void RtpFeedback::ParseCommonFeedback(const uint8_t* payload) {
+  SetSenderSsrc(ByteReader<uint32_t>::ReadBigEndian(&payload[0]));
+  SetMediaSsrc(ByteReader<uint32_t>::ReadBigEndian(&payload[4]));
+}
+
+void RtpFeedback::CreateCommonFeedback(uint8_t* payload) const {
+  ByteWriter<uint32_t>::WriteBigEndian(&payload[0], sender_ssrc());
+  ByteWriter<uint32_t>::WriteBigEndian(&payload[4], media_ssrc());
+}

src/rtcp/rtp_feedback/rtp_feedback.h

@@ -0,0 +1,37 @@
+/*
+ * @Author: DI JUNKUN
+ * @Date: 2024-12-18
+ * Copyright (c) 2024 by DI JUNKUN, All Rights Reserved.
+ */
+
+#ifndef _RTP_FEEDBACK_H_
+#define _RTP_FEEDBACK_H_
+
+#include <stddef.h>
+#include <stdint.h>
+
+#include "rtcp_packet.h"
+
+// RTPFB: Transport layer feedback message.
+// RFC4585, Section 6.2
+class RtpFeedback : public RtcpPacket {
+ public:
+  static constexpr uint8_t kPacketType = 205;
+
+  RtpFeedback() = default;
+  ~RtpFeedback() override = default;
+
+  void SetMediaSsrc(uint32_t ssrc) { media_ssrc_ = ssrc; }
+
+  uint32_t media_ssrc() const { return media_ssrc_; }
+
+ protected:
+  static constexpr size_t kCommonFeedbackLength = 8;
+  void ParseCommonFeedback(const uint8_t* payload);
+  void CreateCommonFeedback(uint8_t* payload) const;
+
+ private:
+  uint32_t media_ssrc_ = 0;
+};
+
+#endif

src/rtp/rtp_packet/rtp_packet_received.cpp

@@ -0,0 +1,14 @@
+#include "rtp_packet_received.h"
+
+RtpPacketReceived::RtpPacketReceived() = default;
+RtpPacketReceived::RtpPacketReceived(int64_t arrival_time)
+    : arrival_time_(arrival_time) {}
+RtpPacketReceived::RtpPacketReceived(const RtpPacketReceived& packet) = default;
+RtpPacketReceived::RtpPacketReceived(RtpPacketReceived&& packet) = default;
+
+RtpPacketReceived& RtpPacketReceived::operator=(
+    const RtpPacketReceived& packet) = default;
+RtpPacketReceived& RtpPacketReceived::operator=(RtpPacketReceived&& packet) =
+    default;
+
+RtpPacketReceived::~RtpPacketReceived() {}

src/rtp/rtp_packet/rtp_packet_received.h

@@ -0,0 +1,31 @@
+/*
+ * @Author: DI JUNKUN
+ * @Date: 2024-12-18
+ * Copyright (c) 2024 by DI JUNKUN, All Rights Reserved.
+ */
+
+#ifndef _RTP_PACKET_RECEIVED_H_
+#define _RTP_PACKET_RECEIVED_H_
+
+#include <limits>
+
+#include "rtp_packet.h"
+
+class RtpPacketReceived : public RtpPacket {
+ public:
+  RtpPacketReceived();
+  explicit RtpPacketReceived(
+      int64_t arrival_time = std::numeric_limits<int64_t>::min());
+  RtpPacketReceived(const RtpPacketReceived& packet);
+  RtpPacketReceived(RtpPacketReceived&& packet);
+
+  RtpPacketReceived& operator=(const RtpPacketReceived& packet);
+  RtpPacketReceived& operator=(RtpPacketReceived&& packet);
+
+  ~RtpPacketReceived();
+
+ private:
+  int64_t arrival_time_ = std::numeric_limits<int64_t>::min();
+};
+
+#endif

xmake.lua

@@ -79,8 +79,12 @@ target("statistics")
 target("rtcp")
     set_kind("object")
     add_deps("log", "common")
-    add_files("src/rtcp/*.cpp")
-    add_includedirs("src/rtcp", {public = true})
+    add_files("src/rtcp/*.cpp",
+    "src/rtcp/rtcp_packet/*.cpp",
+    "src/rtcp/rtp_feedback/*.cpp")
+    add_includedirs("src/rtcp",
+    "src/rtcp/rtcp_packet",
+    "src/rtcp/rtp_feedback", {public = true})
 
 target("rtp")
     set_kind("object")