diff --git a/bluetooth/hci/hci_internals.h b/bluetooth/hci/hci_internals.h
new file mode 100644
index 0000000..71b6191
--- /dev/null
+++ b/bluetooth/hci/hci_internals.h
@@ -0,0 +1,55 @@
+/*
+ * Copyright 2022 The Android Open Source Project
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+#pragma once
+
+#include <cstdlib>
+
+namespace android::hardware::bluetooth::hci {
+
+// HCI UART transport packet types (Volume 4, Part A, 2)
+enum class PacketType : uint8_t {
+  UNKNOWN = 0,
+  COMMAND = 1,
+  ACL_DATA = 2,
+  SCO_DATA = 3,
+  EVENT = 4,
+  ISO_DATA = 5,
+};
+
+// 2 bytes for opcode, 1 byte for parameter length (Volume 4, Part E, 5.4.1)
+static constexpr size_t kCommandHeaderSize = 3;
+static constexpr size_t kCommandLengthOffset = 2;
+
+// 2 bytes for handle, 2 bytes for data length (Volume 4, Part E, 5.4.2)
+static constexpr size_t kAclHeaderSize = 4;
+static constexpr size_t kAclLengthOffset = 2;
+
+// 2 bytes for handle, 1 byte for data length (Volume 4, Part E, 5.4.3)
+static constexpr size_t kScoHeaderSize = 3;
+static constexpr size_t kScoLengthOffset = 2;
+
+// 1 byte for event code, 1 byte for parameter length (Volume 4, Part E, 5.4.4)
+static constexpr size_t kEventHeaderSize = 2;
+static constexpr size_t kEventLengthOffset = 1;
+
+// 2 bytes for handle, 2 bytes for data length (Volume 4, Part E, 5.4.5)
+static constexpr size_t kIsoHeaderSize = 4;
+static constexpr size_t kIsoLengthOffset = 2;
+
+static constexpr size_t kMaxHeaderSize = kAclHeaderSize;
+
+}  // namespace android::hardware::bluetooth::hci