Add IPv6 FragmentHeader struct support

Bug: 285124667
Test: atest NetworkStaticLibTests:com.android.net.moduletests.util.structs.FragmentHeaderTest
Change-Id: I90c8a52daa35589977333494355ac239ea057204

staticlibs/device/com/android/net/module/util/structs/FragmentHeader.java

@@ -0,0 +1,56 @@
+/*
+ * Copyright (C) 2024 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.
+ */
+
+package com.android.net.module.util.structs;
+
+import com.android.net.module.util.Struct;
+import com.android.net.module.util.Struct.Field;
+import com.android.net.module.util.Struct.Type;
+
+/**
+ * IPv6 Fragment Extension header, as per https://tools.ietf.org/html/rfc2460.
+ *
+ * 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
+ * +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+ * |  Next Header  |   Reserved    |      Fragment Offset    |Res|M|
+ * +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+ * |                         Identification                        |
+ * +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+ */
+public class FragmentHeader extends Struct {
+    @Field(order = 0, type = Type.U8)
+    public final short nextHeader;
+    @Field(order = 1, type = Type.S8)
+    public final byte reserved;
+    @Field(order = 2, type = Type.U16)
+    public final int fragmentOffset;
+    @Field(order = 3, type = Type.S32)
+    public final int identification;
+
+    public FragmentHeader(final short nextHeader, final byte reserved, final int fragmentOffset,
+            final int identification) {
+        this.nextHeader = nextHeader;
+        this.reserved = reserved;
+        this.fragmentOffset = fragmentOffset;
+        this.identification = identification;
+    }
+
+    public FragmentHeader(final short nextHeader, final int fragmentOffset,
+            final int identification) {
+        this(nextHeader, (byte) 0, fragmentOffset, identification);
+    }
+}

staticlibs/tests/unit/src/com/android/net/module/util/structs/FragmentHeaderTest.java

@@ -0,0 +1,76 @@
+/*
+ * Copyright (C) 2024 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.
+ */
+
+package com.android.net.module.util.structs;
+
+import static org.junit.Assert.assertArrayEquals;
+import static org.junit.Assert.assertEquals;
+
+import androidx.test.runner.AndroidJUnit4;
+
+import org.junit.Test;
+import org.junit.runner.RunWith;
+
+import java.nio.ByteBuffer;
+import java.nio.ByteOrder;
+import java.util.Arrays;
+
+@RunWith(AndroidJUnit4.class)
+public class FragmentHeaderTest {
+    private static final byte[] HEADER_BYTES = new byte[] {
+        17, /* nextHeader  */
+        0, /* reserved */
+        15, 1, /* fragmentOffset */
+        1, 2, 3, 4 /* identification */
+    };
+
+    @Test
+    public void testConstructor() {
+        FragmentHeader fragHdr = new FragmentHeader((short) 10 /* nextHeader */,
+                (byte) 11 /* reserved */,
+                12 /* fragmentOffset */,
+                13 /* identification */);
+
+        assertEquals(10, fragHdr.nextHeader);
+        assertEquals(11, fragHdr.reserved);
+        assertEquals(12, fragHdr.fragmentOffset);
+        assertEquals(13, fragHdr.identification);
+    }
+
+    @Test
+    public void testParseFragmentHeader() {
+        final ByteBuffer buf = ByteBuffer.wrap(HEADER_BYTES);
+        buf.order(ByteOrder.BIG_ENDIAN);
+        FragmentHeader fragHdr = FragmentHeader.parse(FragmentHeader.class, buf);
+
+        assertEquals(17, fragHdr.nextHeader);
+        assertEquals(0, fragHdr.reserved);
+        assertEquals(0xF01, fragHdr.fragmentOffset);
+        assertEquals(0x1020304, fragHdr.identification);
+    }
+
+    @Test
+    public void testWriteToBytes() {
+        FragmentHeader fragHdr = new FragmentHeader((short) 17 /* nextHeader */,
+                (byte) 0 /* reserved */,
+                0xF01 /* fragmentOffset */,
+                0x1020304 /* identification */);
+
+        byte[] bytes = fragHdr.writeToBytes(ByteOrder.BIG_ENDIAN);
+
+        assertArrayEquals("bytes = " + Arrays.toString(bytes), HEADER_BYTES, bytes);
+    }
+}