[nat64] add functions for processing IPv4 packets (#7824)

With NAT64, we need to handle the IPv4 packets inside the border
router functions in OpenThread core.

This commit introduces a few new classes for NAT64:
- Add `Ip4::Cidr` (for specifying CIDR block of translated packets)
- Updated Ip4 address format -- use a union of (m8[4], m16[2], m32)
  instead of 4 bytes.
- Extend `Checksum` for supporting ICMP in Ip4 (and TCP4 / UDP4) and
  IPv4 header.

include/Makefile.am

@@ -70,6 +70,7 @@ openthread_headers                      = \
     openthread/logging.h                  \
     openthread/message.h                  \
     openthread/multi_radio.h              \
+    openthread/nat64.h                    \
     openthread/ncp.h                      \
     openthread/netdata.h                  \
     openthread/netdata_publisher.h        \

include/openthread/BUILD.gn

@@ -74,6 +74,7 @@ source_set("openthread") {
     "logging.h",
     "message.h",
     "multi_radio.h",
+    "nat64.h",
     "ncp.h",
     "netdata.h",
     "netdata_publisher.h",

include/openthread/instance.h

@@ -53,7 +53,7 @@ extern "C" {
  * @note This number versions both OpenThread platform and user APIs.
  *
  */
-#define OPENTHREAD_API_VERSION (226)
+#define OPENTHREAD_API_VERSION (227)
 
 /**
  * @addtogroup api-instance

include/openthread/nat64.h

@@ -0,0 +1,99 @@
+/*
+ *  Copyright (c) 2022, The OpenThread Authors.
+ *  All rights reserved.
+ *
+ *  Redistribution and use in source and binary forms, with or without
+ *  modification, are permitted provided that the following conditions are met:
+ *  1. Redistributions of source code must retain the above copyright
+ *     notice, this list of conditions and the following disclaimer.
+ *  2. Redistributions in binary form must reproduce the above copyright
+ *     notice, this list of conditions and the following disclaimer in the
+ *     documentation and/or other materials provided with the distribution.
+ *  3. Neither the name of the copyright holder nor the
+ *     names of its contributors may be used to endorse or promote products
+ *     derived from this software without specific prior written permission.
+ *
+ *  THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
+ *  AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
+ *  IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
+ *  ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDER OR CONTRIBUTORS BE
+ *  LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
+ *  CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
+ *  SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
+ *  INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN
+ *  CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
+ *  ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
+ *  POSSIBILITY OF SUCH DAMAGE.
+ */
+
+/**
+ * @file
+ * @brief
+ *  This file defines the OpenThread API for NAT64 on a border router.
+ */
+
+#ifndef OPENTHREAD_NAT64_H_
+#define OPENTHREAD_NAT64_H_
+
+#include <openthread/message.h>
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+/**
+ * @addtogroup api-nat64
+ *
+ * @brief This module includes functions and structs for the NAT64 function on the border router. These functions are
+ * only available when `OPENTHREAD_CONFIG_BORDER_ROUTING_NAT64_ENABLE` is enabled.
+ *
+ * @{
+ *
+ */
+
+#define OT_IP4_ADDRESS_SIZE 4 ///< Size of an IPv4 address (bytes)
+
+/**
+ * @struct otIp4Address
+ *
+ * This structure represents an IPv4 address.
+ *
+ */
+OT_TOOL_PACKED_BEGIN
+struct otIp4Address
+{
+    union OT_TOOL_PACKED_FIELD
+    {
+        uint8_t  m8[OT_IP4_ADDRESS_SIZE]; ///< 8-bit fields
+        uint32_t m32;                     ///< 32-bit representation
+    } mFields;
+} OT_TOOL_PACKED_END;
+
+/**
+ * This structure represents an IPv4 address.
+ *
+ */
+typedef struct otIp4Address otIp4Address;
+
+/**
+ * @struct otIp4Cidr
+ *
+ * This structure represents an IPv4 CIDR block.
+ *
+ */
+typedef struct otIp4Cidr
+{
+    otIp4Address mAddress;
+    uint8_t      mLength;
+} otIp4Cidr;
+
+/**
+ * @}
+ *
+ */
+
+#ifdef __cplusplus
+}
+#endif
+
+#endif // OPENTHREAD_NAT64_H_