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diff --git a/third_party/libwebrtc/net/dcsctp/packet/bounded_byte_writer.h b/third_party/libwebrtc/net/dcsctp/packet/bounded_byte_writer.h
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+++ b/third_party/libwebrtc/net/dcsctp/packet/bounded_byte_writer.h
@@ -0,0 +1,103 @@
+/*
+ *  Copyright (c) 2021 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 NET_DCSCTP_PACKET_BOUNDED_BYTE_WRITER_H_
+#define NET_DCSCTP_PACKET_BOUNDED_BYTE_WRITER_H_
+
+#include <algorithm>
+
+#include "api/array_view.h"
+
+namespace dcsctp {
+
+// TODO(boivie): These generic functions - and possibly this entire class -
+// could be a candidate to have added to rtc_base/. They should use compiler
+// intrinsics as well.
+namespace internal {
+// Stores a 8-bit unsigned word at `data`.
+inline void StoreBigEndian8(uint8_t* data, uint8_t val) {
+  data[0] = val;
+}
+
+// Stores a 16-bit unsigned word at `data`.
+inline void StoreBigEndian16(uint8_t* data, uint16_t val) {
+  data[0] = val >> 8;
+  data[1] = val;
+}
+
+// Stores a 32-bit unsigned word at `data`.
+inline void StoreBigEndian32(uint8_t* data, uint32_t val) {
+  data[0] = val >> 24;
+  data[1] = val >> 16;
+  data[2] = val >> 8;
+  data[3] = val;
+}
+}  // namespace internal
+
+// BoundedByteWriter wraps an ArrayView and divides it into two parts; A fixed
+// size - which is the template parameter - and a variable size, which is what
+// remains in `data` after the `FixedSize`.
+//
+// The BoundedByteWriter provides methods to write big endian numbers to the
+// FixedSize portion of the buffer, and these are written with static bounds
+// checking, to avoid out-of-bounds accesses without a run-time penalty.
+//
+// The variable sized portion can either be used to create sub-writers, which
+// themselves would provide compile-time bounds-checking, or data can be copied
+// to it.
+template <int FixedSize>
+class BoundedByteWriter {
+ public:
+  explicit BoundedByteWriter(rtc::ArrayView<uint8_t> data) : data_(data) {
+    RTC_CHECK(data.size() >= FixedSize);
+  }
+
+  template <size_t offset>
+  void Store8(uint8_t value) {
+    static_assert(offset + sizeof(uint8_t) <= FixedSize, "Out-of-bounds");
+    internal::StoreBigEndian8(&data_[offset], value);
+  }
+
+  template <size_t offset>
+  void Store16(uint16_t value) {
+    static_assert(offset + sizeof(uint16_t) <= FixedSize, "Out-of-bounds");
+    static_assert((offset % sizeof(uint16_t)) == 0, "Unaligned access");
+    internal::StoreBigEndian16(&data_[offset], value);
+  }
+
+  template <size_t offset>
+  void Store32(uint32_t value) {
+    static_assert(offset + sizeof(uint32_t) <= FixedSize, "Out-of-bounds");
+    static_assert((offset % sizeof(uint32_t)) == 0, "Unaligned access");
+    internal::StoreBigEndian32(&data_[offset], value);
+  }
+
+  template <size_t SubSize>
+  BoundedByteWriter<SubSize> sub_writer(size_t variable_offset) {
+    RTC_CHECK(FixedSize + variable_offset + SubSize <= data_.size());
+
+    return BoundedByteWriter<SubSize>(
+        data_.subview(FixedSize + variable_offset, SubSize));
+  }
+
+  void CopyToVariableData(rtc::ArrayView<const uint8_t> source) {
+    size_t copy_size = std::min(source.size(), data_.size() - FixedSize);
+    if (source.data() == nullptr || copy_size == 0) {
+      return;
+    }
+    memcpy(data_.data() + FixedSize, source.data(), copy_size);
+  }
+
+ private:
+  rtc::ArrayView<uint8_t> data_;
+};
+}  // namespace dcsctp
+
+#endif  // NET_DCSCTP_PACKET_BOUNDED_BYTE_WRITER_H_