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diff --git a/dom/media/platforms/agnostic/bytestreams/AnnexB.h b/dom/media/platforms/agnostic/bytestreams/AnnexB.h
new file mode 100644
index 0000000000..dbb8a7c3e1
--- /dev/null
+++ b/dom/media/platforms/agnostic/bytestreams/AnnexB.h
@@ -0,0 +1,66 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#ifndef DOM_MEDIA_PLATFORMS_AGNOSTIC_BYTESTREAMS_ANNEX_B_H_
+#define DOM_MEDIA_PLATFORMS_AGNOSTIC_BYTESTREAMS_ANNEX_B_H_
+
+#include "ErrorList.h"
+#include "mozilla/RefPtr.h"
+#include "mozilla/Result.h"
+
+template <class>
+class nsTArray;
+
+namespace mozilla {
+class BufferReader;
+class MediaRawData;
+class MediaByteBuffer;
+
+class AnnexB {
+ public:
+  struct NALEntry {
+    NALEntry(int64_t aOffset, int64_t aSize) : mOffset(aOffset), mSize(aSize) {
+      MOZ_ASSERT(mOffset >= 0);
+      MOZ_ASSERT(mSize >= 0);
+    }
+    // They should be non-negative, so we use int64_t to assert their value when
+    // assigning value to them.
+    int64_t mOffset;
+    int64_t mSize;
+  };
+  // All conversions assume size of NAL length field is 4 bytes.
+  // Convert a sample from AVCC format to Annex B.
+  static mozilla::Result<mozilla::Ok, nsresult> ConvertSampleToAnnexB(
+      mozilla::MediaRawData* aSample, bool aAddSPS = true);
+  // Convert a sample from Annex B to AVCC.
+  // an AVCC extradata must not be set.
+  static bool ConvertSampleToAVCC(
+      mozilla::MediaRawData* aSample,
+      const RefPtr<mozilla::MediaByteBuffer>& aAVCCHeader = nullptr);
+  static mozilla::Result<mozilla::Ok, nsresult> ConvertSampleTo4BytesAVCC(
+      mozilla::MediaRawData* aSample);
+
+  // Parse an AVCC extradata and construct the Annex B sample header.
+  static already_AddRefed<mozilla::MediaByteBuffer> ConvertExtraDataToAnnexB(
+      const mozilla::MediaByteBuffer* aExtraData);
+  // Returns true if format is AVCC and sample has valid extradata.
+  static bool IsAVCC(const mozilla::MediaRawData* aSample);
+  // Returns true if format is AnnexB.
+  static bool IsAnnexB(const mozilla::MediaRawData* aSample);
+
+  // Parse NAL entries from the bytes stream to know the offset and the size of
+  // each NAL in the bytes stream.
+  static void ParseNALEntries(const Span<const uint8_t>& aSpan,
+                              nsTArray<AnnexB::NALEntry>& aEntries);
+
+ private:
+  // AVCC box parser helper.
+  static mozilla::Result<mozilla::Ok, nsresult> ConvertSPSOrPPS(
+      mozilla::BufferReader& aReader, uint8_t aCount,
+      mozilla::MediaByteBuffer* aAnnexB);
+};
+
+}  // namespace mozilla
+
+#endif  // DOM_MEDIA_PLATFORMS_AGNOSTIC_BYTESTREAMS_ANNEX_B_H_