diff --git a/staticlibs/Android.bp b/staticlibs/Android.bp
index 0585c09..4c58228 100644
--- a/staticlibs/Android.bp
+++ b/staticlibs/Android.bp
@@ -139,6 +139,7 @@
     srcs: [
         "device/com/android/net/module/util/HexDump.java",
         "device/com/android/net/module/util/Ipv6Utils.java",
+        "device/com/android/net/module/util/PacketBuilder.java",
         "device/com/android/net/module/util/Struct.java",
         "device/com/android/net/module/util/structs/*.java",
     ],
diff --git a/staticlibs/device/com/android/net/module/util/PacketBuilder.java b/staticlibs/device/com/android/net/module/util/PacketBuilder.java
new file mode 100644
index 0000000..c908528
--- /dev/null
+++ b/staticlibs/device/com/android/net/module/util/PacketBuilder.java
@@ -0,0 +1,257 @@
+/*
+ * Copyright (C) 2021 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;
+
+import static android.system.OsConstants.IPPROTO_IP;
+import static android.system.OsConstants.IPPROTO_TCP;
+import static android.system.OsConstants.IPPROTO_UDP;
+
+import static com.android.net.module.util.IpUtils.ipChecksum;
+import static com.android.net.module.util.IpUtils.tcpChecksum;
+import static com.android.net.module.util.IpUtils.udpChecksum;
+import static com.android.net.module.util.NetworkStackConstants.IPV4_CHECKSUM_OFFSET;
+import static com.android.net.module.util.NetworkStackConstants.IPV4_LENGTH_OFFSET;
+import static com.android.net.module.util.NetworkStackConstants.TCP_CHECKSUM_OFFSET;
+import static com.android.net.module.util.NetworkStackConstants.UDP_CHECKSUM_OFFSET;
+import static com.android.net.module.util.NetworkStackConstants.UDP_LENGTH_OFFSET;
+
+import android.net.MacAddress;
+
+import androidx.annotation.NonNull;
+
+import com.android.net.module.util.structs.EthernetHeader;
+import com.android.net.module.util.structs.Ipv4Header;
+import com.android.net.module.util.structs.TcpHeader;
+import com.android.net.module.util.structs.UdpHeader;
+
+import java.io.IOException;
+import java.net.Inet4Address;
+import java.nio.BufferOverflowException;
+import java.nio.ByteBuffer;
+
+/**
+ * The class is used to build a packet.
+ *
+ * +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+ * |                Layer 2 header (EthernetHeader)                | (optional)
+ * +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+ * |                  Layer 3 header (Ipv4Header)                  |
+ * +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+ * |           Layer 4 header (TcpHeader, UdpHeader)               |
+ * +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+ * |                           Payload                             | (optional)
+ * +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+ *
+ * Below is a sample code to build a packet.
+ *
+ * // Initialize builder
+ * final ByteBuffer buf = ByteBuffer.allocate(...);
+ * final PacketBuilder pb = new PacketBuilder(buf);
+ * // Write headers
+ * pb.writeL2Header(...);
+ * pb.writeIpHeader(...);
+ * pb.writeTcpHeader(...);
+ * // Write payload
+ * buf.putInt(...);
+ * buf.putShort(...);
+ * buf.putByte(...);
+ * // Finalize and use the packet
+ * pb.finalizePacket();
+ * sendPacket(buf);
+ */
+public class PacketBuilder {
+    private final ByteBuffer mBuffer;
+
+    private int mIpv4HeaderOffset = -1;
+    private int mTcpHeaderOffset = -1;
+    private int mUdpHeaderOffset = -1;
+
+    public PacketBuilder(@NonNull ByteBuffer buffer) {
+        mBuffer = buffer;
+    }
+
+    /**
+     * Write an ethernet header.
+     *
+     * @param srcMac source MAC address
+     * @param dstMac destination MAC address
+     * @param etherType ether type
+     */
+    public void writeL2Header(MacAddress srcMac, MacAddress dstMac, short etherType) throws
+            IOException {
+        final EthernetHeader ethv4Header = new EthernetHeader(dstMac, srcMac, etherType);
+        try {
+            ethv4Header.writeToByteBuffer(mBuffer);
+        } catch (IllegalArgumentException | BufferOverflowException e) {
+            throw new IOException("Error writing to buffer: ", e);
+        }
+    }
+
+    /**
+     * Write an IPv4 header.
+     * The IP header length and checksum are calculated and written back in #finalizePacket.
+     *
+     * @param tos type of service
+     * @param id the identification
+     * @param flagsAndFragmentOffset flags and fragment offset
+     * @param ttl time to live
+     * @param protocol protocol
+     * @param srcIp source IP address
+     * @param dstIp destination IP address
+     */
+    public void writeIpv4Header(byte tos, short id, short flagsAndFragmentOffset, byte ttl,
+            byte protocol, @NonNull final Inet4Address srcIp, @NonNull final Inet4Address dstIp)
+            throws IOException {
+        mIpv4HeaderOffset = mBuffer.position();
+        final Ipv4Header ipv4Header = new Ipv4Header(tos,
+                (short) 0 /* totalLength, calculate in #finalizePacket */, id,
+                flagsAndFragmentOffset, ttl, protocol,
+                (short) 0 /* checksum, calculate in #finalizePacket */, srcIp, dstIp);
+
+        try {
+            ipv4Header.writeToByteBuffer(mBuffer);
+        } catch (IllegalArgumentException | BufferOverflowException e) {
+            throw new IOException("Error writing to buffer: ", e);
+        }
+    }
+
+    /**
+     * Write a TCP header.
+     * The TCP header checksum is calculated and written back in #finalizePacket.
+     *
+     * @param srcPort source port
+     * @param dstPort destination port
+     * @param seq sequence number
+     * @param ack acknowledgement number
+     * @param tcpFlags tcp flags
+     * @param window window size
+     * @param urgentPointer urgent pointer
+     */
+    public void writeTcpHeader(short srcPort, short dstPort, short seq, short ack,
+            byte tcpFlags, short window, short urgentPointer) throws IOException {
+        mTcpHeaderOffset = mBuffer.position();
+        final TcpHeader tcpHeader = new TcpHeader(srcPort, dstPort, seq, ack,
+                (short) ((short) 0x5000 | ((byte) 0x3f & tcpFlags)) /* dataOffsetAndControlBits,
+                dataOffset is always 5(*4bytes) because options not supported */, window,
+                (short) 0 /* checksum, calculate in #finalizePacket */,
+                urgentPointer);
+
+        try {
+            tcpHeader.writeToByteBuffer(mBuffer);
+        } catch (IllegalArgumentException | BufferOverflowException e) {
+            throw new IOException("Error writing to buffer: ", e);
+        }
+    }
+
+    /**
+     * Write a UDP header.
+     * The UDP header length and checksum are calculated and written back in #finalizePacket.
+     *
+     * @param srcPort source port
+     * @param dstPort destination port
+     */
+    public void writeUdpHeader(short srcPort, short dstPort) throws IOException {
+        mUdpHeaderOffset = mBuffer.position();
+        final UdpHeader udpHeader = new UdpHeader(srcPort, dstPort,
+                (short) 0 /* length, calculate in #finalizePacket */,
+                (short) 0 /* checksum, calculate in #finalizePacket */);
+
+        try {
+            udpHeader.writeToByteBuffer(mBuffer);
+        } catch (IllegalArgumentException | BufferOverflowException e) {
+            throw new IOException("Error writing to buffer: ", e);
+        }
+    }
+
+    /**
+     * Finalize the packet.
+     *
+     * Call after writing L4 header (no payload) or payload to the buffer used by the builder.
+     * L3 header length, L3 header checksum and L4 header checksum are calculated and written back
+     * after finalization.
+     */
+    @NonNull
+    public ByteBuffer finalizePacket() throws IOException {
+        if (mIpv4HeaderOffset < 0) {
+            // TODO: add support for IPv6
+            throw new IOException("Packet is missing IPv4 header");
+        }
+
+        // Populate the IPv4 totalLength field.
+        mBuffer.putShort(mIpv4HeaderOffset + IPV4_LENGTH_OFFSET,
+                (short) (mBuffer.position() - mIpv4HeaderOffset));
+
+        // Populate the IPv4 header checksum field.
+        mBuffer.putShort(mIpv4HeaderOffset + IPV4_CHECKSUM_OFFSET,
+                ipChecksum(mBuffer, mIpv4HeaderOffset /* headerOffset */));
+
+        if (mTcpHeaderOffset > 0) {
+            // Populate the TCP header checksum field.
+            mBuffer.putShort(mTcpHeaderOffset + TCP_CHECKSUM_OFFSET, tcpChecksum(mBuffer,
+                    mIpv4HeaderOffset /* ipOffset */, mTcpHeaderOffset /* transportOffset */,
+                    mBuffer.position() - mTcpHeaderOffset /* transportLen */));
+        } else if (mUdpHeaderOffset > 0) {
+            // Populate the UDP header length field.
+            mBuffer.putShort(mUdpHeaderOffset + UDP_LENGTH_OFFSET,
+                    (short) (mBuffer.position() - mUdpHeaderOffset));
+
+            // Populate the UDP header checksum field.
+            mBuffer.putShort(mUdpHeaderOffset + UDP_CHECKSUM_OFFSET, udpChecksum(mBuffer,
+                    mIpv4HeaderOffset /* ipOffset */, mUdpHeaderOffset /* transportOffset */));
+        } else {
+            throw new IOException("Packet is missing neither TCP nor UDP header");
+        }
+
+        mBuffer.flip();
+        return mBuffer;
+    }
+
+    /**
+     * Allocate bytebuffer for building the packet.
+     *
+     * @param hasEther has ethernet header. Set this flag to indicate that the packet has an
+     *        ethernet header.
+     * @param l3proto the layer 3 protocol. Only {@code IPPROTO_IP} currently supported.
+     * @param l4proto the layer 4 protocol. Only {@code IPPROTO_TCP} and {@code IPPROTO_UDP}
+     *        currently supported.
+     * @param payloadLen length of the payload.
+     */
+    @NonNull
+    public static ByteBuffer allocate(boolean hasEther, int l3proto, int l4proto, int payloadLen) {
+        if (l3proto != IPPROTO_IP) {
+            // TODO: add support for IPv6
+            throw new IllegalArgumentException("Unsupported layer 3 protocol " + l3proto);
+        }
+
+        if (l4proto != IPPROTO_TCP && l4proto != IPPROTO_UDP) {
+            throw new IllegalArgumentException("Unsupported layer 4 protocol " + l4proto);
+        }
+
+        if (payloadLen < 0) {
+            throw new IllegalArgumentException("Invalid payload length " + payloadLen);
+        }
+
+        int packetLen = 0;
+        if (hasEther) packetLen += Struct.getSize(EthernetHeader.class);
+        packetLen += Struct.getSize(Ipv4Header.class);
+        packetLen += (l4proto == IPPROTO_TCP) ? Struct.getSize(TcpHeader.class)
+                : Struct.getSize(UdpHeader.class);
+        packetLen += payloadLen;
+
+        return ByteBuffer.allocate(packetLen);
+    }
+}
diff --git a/staticlibs/framework/com/android/net/module/util/NetworkStackConstants.java b/staticlibs/framework/com/android/net/module/util/NetworkStackConstants.java
index f7151d7..353fe69 100644
--- a/staticlibs/framework/com/android/net/module/util/NetworkStackConstants.java
+++ b/staticlibs/framework/com/android/net/module/util/NetworkStackConstants.java
@@ -89,9 +89,11 @@
     public static final int IPV4_MAX_MTU = 65_535;
     public static final int IPV4_HEADER_MIN_LEN = 20;
     public static final int IPV4_IHL_MASK = 0xf;
+    public static final int IPV4_LENGTH_OFFSET = 2;
     public static final int IPV4_FLAGS_OFFSET = 6;
     public static final int IPV4_FRAGMENT_MASK = 0x1fff;
     public static final int IPV4_PROTOCOL_OFFSET = 9;
+    public static final int IPV4_CHECKSUM_OFFSET = 10;
     public static final int IPV4_SRC_ADDR_OFFSET = 12;
     public static final int IPV4_DST_ADDR_OFFSET = 16;
     public static final int IPV4_ADDR_LEN = 4;
@@ -164,13 +166,29 @@
     public static final byte PIO_FLAG_AUTONOMOUS = (byte) (1 << 6);
 
     /**
+     * TCP constants.
+     *
+     * See also:
+     *     - https://tools.ietf.org/html/rfc793
+     */
+    public static final int TCP_HEADER_MIN_LEN = 20;
+    public static final int TCP_CHECKSUM_OFFSET = 16;
+    public static final byte TCPHDR_FIN = (byte) (1 << 0);
+    public static final byte TCPHDR_SYN = (byte) (1 << 1);
+    public static final byte TCPHDR_RST = (byte) (1 << 2);
+    public static final byte TCPHDR_PSH = (byte) (1 << 3);
+    public static final byte TCPHDR_ACK = (byte) (1 << 4);
+    public static final byte TCPHDR_URG = (byte) (1 << 5);
+
+    /**
      * UDP constants.
      *
      * See also:
      *     - https://tools.ietf.org/html/rfc768
      */
     public static final int UDP_HEADER_LEN = 8;
-
+    public static final int UDP_LENGTH_OFFSET = 4;
+    public static final int UDP_CHECKSUM_OFFSET = 6;
 
     /**
      * DHCP constants.
diff --git a/staticlibs/tests/unit/src/com/android/net/module/util/PacketBuilderTest.java b/staticlibs/tests/unit/src/com/android/net/module/util/PacketBuilderTest.java
new file mode 100644
index 0000000..8f9a1f9
--- /dev/null
+++ b/staticlibs/tests/unit/src/com/android/net/module/util/PacketBuilderTest.java
@@ -0,0 +1,532 @@
+/*
+ * Copyright (C) 2021 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;
+
+import static android.system.OsConstants.IPPROTO_IP;
+import static android.system.OsConstants.IPPROTO_TCP;
+import static android.system.OsConstants.IPPROTO_UDP;
+
+import static com.android.net.module.util.NetworkStackConstants.ETHER_TYPE_IPV4;
+import static com.android.net.module.util.NetworkStackConstants.IPV4_HEADER_MIN_LEN;
+import static com.android.net.module.util.NetworkStackConstants.TCPHDR_ACK;
+import static com.android.net.module.util.NetworkStackConstants.TCP_HEADER_MIN_LEN;
+import static com.android.net.module.util.NetworkStackConstants.UDP_HEADER_LEN;
+import static com.android.testutils.MiscAsserts.assertThrows;
+
+import static org.junit.Assert.assertArrayEquals;
+import static org.junit.Assert.assertEquals;
+import static org.junit.Assert.fail;
+
+import android.net.InetAddresses;
+import android.net.MacAddress;
+
+import androidx.annotation.NonNull;
+import androidx.annotation.Nullable;
+import androidx.test.filters.SmallTest;
+import androidx.test.runner.AndroidJUnit4;
+
+import com.android.net.module.util.structs.EthernetHeader;
+import com.android.net.module.util.structs.Ipv4Header;
+import com.android.net.module.util.structs.TcpHeader;
+import com.android.net.module.util.structs.UdpHeader;
+
+import org.junit.Test;
+import org.junit.runner.RunWith;
+
+import java.io.IOException;
+import java.net.Inet4Address;
+import java.nio.ByteBuffer;
+
+@RunWith(AndroidJUnit4.class)
+@SmallTest
+public class PacketBuilderTest {
+    private static final MacAddress SRC_MAC = MacAddress.fromString("11:22:33:44:55:66");
+    private static final MacAddress DST_MAC = MacAddress.fromString("aa:bb:cc:dd:ee:ff");
+    private static final Inet4Address IPV4_SRC_ADDR = addr("192.0.2.1");
+    private static final Inet4Address IPV4_DST_ADDR = addr("198.51.100.1");
+    private static final short SRC_PORT = 9876;
+    private static final short DST_PORT = 433;
+    private static final short SEQ_NO = 13579;
+    private static final short ACK_NO = 24680;
+    private static final byte TYPE_OF_SERVICE = 0;
+    private static final short ID = 27149;
+    private static final short FLAGS_AND_FRAGMENT_OFFSET = (short) 0x4000; // flags=DF, offset=0
+    private static final byte TIME_TO_LIVE = (byte) 0x40;
+    private static final short WINDOW = (short) 0x2000;
+    private static final short URGENT_POINTER = 0;
+    private static final ByteBuffer DATA = ByteBuffer.wrap(new byte[] {
+            (byte) 0xde, (byte) 0xad, (byte) 0xbe, (byte) 0xef
+    });
+
+    private static final byte[] TEST_PACKET_ETHERHDR_IPV4HDR_TCPHDR =
+            new byte[] {
+                // packet = (scapy.Ether(src="11:22:33:44:55:66", dst="aa:bb:cc:dd:ee:ff",
+                //                       type='IPv4') /
+                //           scapy.IP(src="192.0.2.1", dst="198.51.100.1",
+                //                    tos=0, id=27149, flags='DF') /
+                //           scapy.TCP(sport=9876, dport=433, seq=13579, ack=24680,
+                //                     flags='A', window=8192, urgptr=0))
+                // Ether header
+                (byte) 0xaa, (byte) 0xbb, (byte) 0xcc, (byte) 0xdd,
+                (byte) 0xee, (byte) 0xff, (byte) 0x11, (byte) 0x22,
+                (byte) 0x33, (byte) 0x44, (byte) 0x55, (byte) 0x66,
+                (byte) 0x08, (byte) 0x00,
+                // IPv4 header
+                (byte) 0x45, (byte) 0x00, (byte) 0x00, (byte) 0x28,
+                (byte) 0x6a, (byte) 0x0d, (byte) 0x40, (byte) 0x00,
+                (byte) 0x40, (byte) 0x06, (byte) 0xe4, (byte) 0x8c,
+                (byte) 0xc0, (byte) 0x00, (byte) 0x02, (byte) 0x01,
+                (byte) 0xc6, (byte) 0x33, (byte) 0x64, (byte) 0x01,
+                // TCP header
+                (byte) 0x26, (byte) 0x94, (byte) 0x01, (byte) 0xb1,
+                (byte) 0x00, (byte) 0x00, (byte) 0x35, (byte) 0x0b,
+                (byte) 0x00, (byte) 0x00, (byte) 0x60, (byte) 0x68,
+                (byte) 0x50, (byte) 0x10, (byte) 0x20, (byte) 0x00,
+                (byte) 0xe5, (byte) 0xe5, (byte) 0x00, (byte) 0x00
+            };
+
+    private static final byte[] TEST_PACKET_ETHERHDR_IPV4HDR_TCPHDR_DATA =
+            new byte[] {
+                // packet = (scapy.Ether(src="11:22:33:44:55:66", dst="aa:bb:cc:dd:ee:ff",
+                //                       type='IPv4') /
+                //           scapy.IP(src="192.0.2.1", dst="198.51.100.1",
+                //                    tos=0, id=27149, flags='DF') /
+                //           scapy.TCP(sport=9876, dport=433, seq=13579, ack=24680,
+                //                     flags='A', window=8192, urgptr=0) /
+                //           b'\xde\xad\xbe\xef')
+                // Ether header
+                (byte) 0xaa, (byte) 0xbb, (byte) 0xcc, (byte) 0xdd,
+                (byte) 0xee, (byte) 0xff, (byte) 0x11, (byte) 0x22,
+                (byte) 0x33, (byte) 0x44, (byte) 0x55, (byte) 0x66,
+                (byte) 0x08, (byte) 0x00,
+                // IPv4 header
+                (byte) 0x45, (byte) 0x00, (byte) 0x00, (byte) 0x2c,
+                (byte) 0x6a, (byte) 0x0d, (byte) 0x40, (byte) 0x00,
+                (byte) 0x40, (byte) 0x06, (byte) 0xe4, (byte) 0x88,
+                (byte) 0xc0, (byte) 0x00, (byte) 0x02, (byte) 0x01,
+                (byte) 0xc6, (byte) 0x33, (byte) 0x64, (byte) 0x01,
+                // TCP header
+                (byte) 0x26, (byte) 0x94, (byte) 0x01, (byte) 0xb1,
+                (byte) 0x00, (byte) 0x00, (byte) 0x35, (byte) 0x0b,
+                (byte) 0x00, (byte) 0x00, (byte) 0x60, (byte) 0x68,
+                (byte) 0x50, (byte) 0x10, (byte) 0x20, (byte) 0x00,
+                (byte) 0x48, (byte) 0x44, (byte) 0x00, (byte) 0x00,
+                // Data
+                (byte) 0xde, (byte) 0xad, (byte) 0xbe, (byte) 0xef
+            };
+
+    private static final byte[] TEST_PACKET_IPV4HDR_TCPHDR =
+            new byte[] {
+                // packet = (scapy.IP(src="192.0.2.1", dst="198.51.100.1",
+                //                    tos=0, id=27149, flags='DF') /
+                //           scapy.TCP(sport=9876, dport=433, seq=13579, ack=24680,
+                //                     flags='A', window=8192, urgptr=0))
+                // IPv4 header
+                (byte) 0x45, (byte) 0x00, (byte) 0x00, (byte) 0x28,
+                (byte) 0x6a, (byte) 0x0d, (byte) 0x40, (byte) 0x00,
+                (byte) 0x40, (byte) 0x06, (byte) 0xe4, (byte) 0x8c,
+                (byte) 0xc0, (byte) 0x00, (byte) 0x02, (byte) 0x01,
+                (byte) 0xc6, (byte) 0x33, (byte) 0x64, (byte) 0x01,
+                // TCP header
+                (byte) 0x26, (byte) 0x94, (byte) 0x01, (byte) 0xb1,
+                (byte) 0x00, (byte) 0x00, (byte) 0x35, (byte) 0x0b,
+                (byte) 0x00, (byte) 0x00, (byte) 0x60, (byte) 0x68,
+                (byte) 0x50, (byte) 0x10, (byte) 0x20, (byte) 0x00,
+                (byte) 0xe5, (byte) 0xe5, (byte) 0x00, (byte) 0x00
+            };
+
+    private static final byte[] TEST_PACKET_IPV4HDR_TCPHDR_DATA =
+            new byte[] {
+                // packet = (scapy.IP(src="192.0.2.1", dst="198.51.100.1",
+                //                    tos=0, id=27149, flags='DF') /
+                //           scapy.TCP(sport=9876, dport=433, seq=13579, ack=24680,
+                //                     flags='A', window=8192, urgptr=0) /
+                //           b'\xde\xad\xbe\xef')
+                // IPv4 header
+                (byte) 0x45, (byte) 0x00, (byte) 0x00, (byte) 0x2c,
+                (byte) 0x6a, (byte) 0x0d, (byte) 0x40, (byte) 0x00,
+                (byte) 0x40, (byte) 0x06, (byte) 0xe4, (byte) 0x88,
+                (byte) 0xc0, (byte) 0x00, (byte) 0x02, (byte) 0x01,
+                (byte) 0xc6, (byte) 0x33, (byte) 0x64, (byte) 0x01,
+                // TCP header
+                (byte) 0x26, (byte) 0x94, (byte) 0x01, (byte) 0xb1,
+                (byte) 0x00, (byte) 0x00, (byte) 0x35, (byte) 0x0b,
+                (byte) 0x00, (byte) 0x00, (byte) 0x60, (byte) 0x68,
+                (byte) 0x50, (byte) 0x10, (byte) 0x20, (byte) 0x00,
+                (byte) 0x48, (byte) 0x44, (byte) 0x00, (byte) 0x00,
+                // Data
+                (byte) 0xde, (byte) 0xad, (byte) 0xbe, (byte) 0xef
+            };
+
+    private static final byte[] TEST_PACKET_ETHERHDR_IPV4HDR_UDPHDR =
+            new byte[] {
+                // packet = (scapy.Ether(src="11:22:33:44:55:66", dst="aa:bb:cc:dd:ee:ff",
+                //                 type='IPv4') /
+                //           scapy.IP(src="192.0.2.1", dst="198.51.100.1",
+                //                 tos=0, id=27149, flags='DF') /
+                //           scapy.UDP(sport=9876, dport=433))
+                // Ether header
+                (byte) 0xaa, (byte) 0xbb, (byte) 0xcc, (byte) 0xdd,
+                (byte) 0xee, (byte) 0xff, (byte) 0x11, (byte) 0x22,
+                (byte) 0x33, (byte) 0x44, (byte) 0x55, (byte) 0x66,
+                (byte) 0x08, (byte) 0x00,
+                // IP header
+                (byte) 0x45, (byte) 0x00, (byte) 0x00, (byte) 0x1c,
+                (byte) 0x6a, (byte) 0x0d, (byte) 0x40, (byte) 0x00,
+                (byte) 0x40, (byte) 0x11, (byte) 0xe4, (byte) 0x8d,
+                (byte) 0xc0, (byte) 0x00, (byte) 0x02, (byte) 0x01,
+                (byte) 0xc6, (byte) 0x33, (byte) 0x64, (byte) 0x01,
+                // UDP header
+                (byte) 0x26, (byte) 0x94, (byte) 0x01, (byte) 0xb1,
+                (byte) 0x00, (byte) 0x08, (byte) 0xeb, (byte) 0x62
+            };
+
+    private static final byte[] TEST_PACKET_ETHERHDR_IPV4HDR_UDPHDR_DATA =
+            new byte[] {
+                // packet = (scapy.Ether(src="11:22:33:44:55:66", dst="aa:bb:cc:dd:ee:ff",
+                //                 type='IPv4') /
+                //           scapy.IP(src="192.0.2.1", dst="198.51.100.1",
+                //                 tos=0, id=27149, flags='DF') /
+                //           scapy.UDP(sport=9876, dport=433) /
+                //           b'\xde\xad\xbe\xef')
+                // Ether header
+                (byte) 0xaa, (byte) 0xbb, (byte) 0xcc, (byte) 0xdd,
+                (byte) 0xee, (byte) 0xff, (byte) 0x11, (byte) 0x22,
+                (byte) 0x33, (byte) 0x44, (byte) 0x55, (byte) 0x66,
+                (byte) 0x08, (byte) 0x00,
+                // IP header
+                (byte) 0x45, (byte) 0x00, (byte) 0x00, (byte) 0x20,
+                (byte) 0x6a, (byte) 0x0d, (byte) 0x40, (byte) 0x00,
+                (byte) 0x40, (byte) 0x11, (byte) 0xe4, (byte) 0x89,
+                (byte) 0xc0, (byte) 0x00, (byte) 0x02, (byte) 0x01,
+                (byte) 0xc6, (byte) 0x33, (byte) 0x64, (byte) 0x01,
+                // UDP header
+                (byte) 0x26, (byte) 0x94, (byte) 0x01, (byte) 0xb1,
+                (byte) 0x00, (byte) 0x0c, (byte) 0x4d, (byte) 0xbd,
+                // Data
+                (byte) 0xde, (byte) 0xad, (byte) 0xbe, (byte) 0xef
+            };
+
+    private static final byte[] TEST_PACKET_IPV4HDR_UDPHDR =
+            new byte[] {
+                // packet = (scapy.IP(src="192.0.2.1", dst="198.51.100.1",
+                //                 tos=0, id=27149, flags='DF') /
+                //           scapy.UDP(sport=9876, dport=433))
+                // IP header
+                (byte) 0x45, (byte) 0x00, (byte) 0x00, (byte) 0x1c,
+                (byte) 0x6a, (byte) 0x0d, (byte) 0x40, (byte) 0x00,
+                (byte) 0x40, (byte) 0x11, (byte) 0xe4, (byte) 0x8d,
+                (byte) 0xc0, (byte) 0x00, (byte) 0x02, (byte) 0x01,
+                (byte) 0xc6, (byte) 0x33, (byte) 0x64, (byte) 0x01,
+                // UDP header
+                (byte) 0x26, (byte) 0x94, (byte) 0x01, (byte) 0xb1,
+                (byte) 0x00, (byte) 0x08, (byte) 0xeb, (byte) 0x62
+            };
+
+    private static final byte[] TEST_PACKET_IPV4HDR_UDPHDR_DATA =
+            new byte[] {
+                // packet = (scapy.IP(src="192.0.2.1", dst="198.51.100.1",
+                //                 tos=0, id=27149, flags='DF') /
+                //           scapy.UDP(sport=9876, dport=433) /
+                //           b'\xde\xad\xbe\xef')
+                // IP header
+                (byte) 0x45, (byte) 0x00, (byte) 0x00, (byte) 0x20,
+                (byte) 0x6a, (byte) 0x0d, (byte) 0x40, (byte) 0x00,
+                (byte) 0x40, (byte) 0x11, (byte) 0xe4, (byte) 0x89,
+                (byte) 0xc0, (byte) 0x00, (byte) 0x02, (byte) 0x01,
+                (byte) 0xc6, (byte) 0x33, (byte) 0x64, (byte) 0x01,
+                // UDP header
+                (byte) 0x26, (byte) 0x94, (byte) 0x01, (byte) 0xb1,
+                (byte) 0x00, (byte) 0x0c, (byte) 0x4d, (byte) 0xbd,
+                // Data
+                (byte) 0xde, (byte) 0xad, (byte) 0xbe, (byte) 0xef
+            };
+
+    /**
+     * Build an IPv4 packet which has ether header, IPv4 header, TCP/UDP header and data.
+     * The ethernet header and data are optional. Note that both source mac address and
+     * destination mac address are required for ethernet header.
+     *
+     * +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+     * |                Layer 2 header (EthernetHeader)                | (optional)
+     * +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+     * |                  Layer 3 header (Ipv4Header)                  |
+     * +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+     * |           Layer 4 header (TcpHeader, UdpHeader)               |
+     * +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+     * |                          Payload                              | (optional)
+     * +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+     *
+     * @param srcMac source MAC address. used by L2 ether header.
+     * @param dstMac destination MAC address. used by L2 ether header.
+     * @param l4proto the layer 4 protocol. support either IPPROTO_TCP or IPPROTO_UDP.
+     * @param payload the payload.
+     */
+    @NonNull
+    private ByteBuffer buildIpv4Packet(@Nullable final MacAddress srcMac,
+            @Nullable final MacAddress dstMac, final int l4proto,
+            @Nullable final ByteBuffer payload)
+            throws Exception {
+        if (l4proto != IPPROTO_TCP && l4proto != IPPROTO_UDP) {
+            fail("Unsupported layer 4 protocol " + l4proto);
+        }
+
+        final boolean hasEther = (srcMac != null && dstMac != null);
+        final int payloadLen = (payload == null) ? 0 : payload.limit();
+        final ByteBuffer buffer = PacketBuilder.allocate(hasEther, IPPROTO_IP, l4proto,
+                payloadLen);
+        final PacketBuilder packetBuilder = new PacketBuilder(buffer);
+
+        if (hasEther) packetBuilder.writeL2Header(srcMac, dstMac, (short) ETHER_TYPE_IPV4);
+        packetBuilder.writeIpv4Header(TYPE_OF_SERVICE, ID, FLAGS_AND_FRAGMENT_OFFSET,
+                TIME_TO_LIVE, (byte) l4proto, IPV4_SRC_ADDR, IPV4_DST_ADDR);
+        if (l4proto == IPPROTO_TCP) {
+            packetBuilder.writeTcpHeader(SRC_PORT, DST_PORT, SEQ_NO, ACK_NO,
+                    TCPHDR_ACK, WINDOW, URGENT_POINTER);
+        } else if (l4proto == IPPROTO_UDP) {
+            packetBuilder.writeUdpHeader(SRC_PORT, DST_PORT);
+        }
+        if (payload != null) {
+            buffer.put(payload);
+            // in case data might be reused by caller, restore the position and
+            // limit of bytebuffer.
+            payload.clear();
+        }
+
+        return packetBuilder.finalizePacket();
+    }
+
+    /**
+     * Check ethernet header.
+     *
+     * @param actual the packet to check.
+     */
+    private void checkEtherHeader(final ByteBuffer actual) {
+        final EthernetHeader eth = Struct.parse(EthernetHeader.class, actual);
+        assertEquals(SRC_MAC, eth.srcMac);
+        assertEquals(DST_MAC, eth.dstMac);
+        assertEquals(ETHER_TYPE_IPV4, eth.etherType);
+    }
+
+    /**
+     * Check IPv4 header.
+     *
+     * @param l4proto the layer 4 protocol. support either IPPROTO_TCP or IPPROTO_UDP.
+     * @param hasData true if the packet has data payload; false otherwise.
+     * @param actual the packet to check.
+     */
+    private void checkIpv4Header(final int l4proto, final boolean hasData,
+            final ByteBuffer actual) {
+        if (l4proto != IPPROTO_TCP && l4proto != IPPROTO_UDP) {
+            fail("Unsupported layer 4 protocol " + l4proto);
+        }
+
+        final Ipv4Header ipv4Header = Struct.parse(Ipv4Header.class, actual);
+        assertEquals(Ipv4Header.IPHDR_VERSION_IHL, ipv4Header.vi);
+        assertEquals(TYPE_OF_SERVICE, ipv4Header.tos);
+        assertEquals(ID, ipv4Header.id);
+        assertEquals(FLAGS_AND_FRAGMENT_OFFSET, ipv4Header.flagsAndFragmentOffset);
+        assertEquals(TIME_TO_LIVE, ipv4Header.ttl);
+        assertEquals(IPV4_SRC_ADDR, ipv4Header.srcIp);
+        assertEquals(IPV4_DST_ADDR, ipv4Header.dstIp);
+
+        final int dataLength = hasData ? DATA.limit() : 0;
+        if (l4proto == IPPROTO_TCP) {
+            assertEquals(IPV4_HEADER_MIN_LEN + TCP_HEADER_MIN_LEN + dataLength,
+                    ipv4Header.totalLength);
+            assertEquals((byte) IPPROTO_TCP, ipv4Header.protocol);
+            assertEquals(hasData ? (short) 0xe488 : (short) 0xe48c, ipv4Header.checksum);
+        } else if (l4proto == IPPROTO_UDP) {
+            assertEquals(IPV4_HEADER_MIN_LEN + UDP_HEADER_LEN + dataLength,
+                    ipv4Header.totalLength);
+            assertEquals((byte) IPPROTO_UDP, ipv4Header.protocol);
+            assertEquals(hasData ? (short) 0xe489 : (short) 0xe48d, ipv4Header.checksum);
+        }
+    }
+
+    /**
+     * Check TCPv4 packet.
+     *
+     * @param hasEther true if the packet has ether header; false otherwise.
+     * @param hasData true if the packet has data payload; false otherwise.
+     * @param actual the packet to check.
+     */
+    private void checkTcpv4Packet(final boolean hasEther, final boolean hasData,
+            final ByteBuffer actual) {
+        if (hasEther) {
+            checkEtherHeader(actual);
+        }
+        checkIpv4Header(IPPROTO_TCP, hasData, actual);
+
+        final TcpHeader tcpHeader = Struct.parse(TcpHeader.class, actual);
+        assertEquals(SRC_PORT, tcpHeader.srcPort);
+        assertEquals(DST_PORT, tcpHeader.dstPort);
+        assertEquals(SEQ_NO, tcpHeader.seq);
+        assertEquals(ACK_NO, tcpHeader.ack);
+        assertEquals((short) 0x5010 /* offset=5(*4bytes), control bits=ACK */,
+                tcpHeader.dataOffsetAndControlBits);
+        assertEquals(WINDOW, tcpHeader.window);
+        assertEquals(hasData ? (short) 0x4844 : (short) 0xe5e5, tcpHeader.checksum);
+        assertEquals(URGENT_POINTER, tcpHeader.urgentPointer);
+
+        if (hasData) {
+            assertEquals(0xdeadbeef, actual.getInt());
+        }
+    }
+
+    /**
+     * Check UDPv4 packet.
+     *
+     * @param hasEther true if the packet has ether header; false otherwise.
+     * @param hasData true if the packet has data payload; false otherwise.
+     * @param actual the packet to check.
+     */
+    private void checkUdpv4Packet(final boolean hasEther, final boolean hasData,
+            final ByteBuffer actual) {
+        if (hasEther) {
+            checkEtherHeader(actual);
+        }
+        checkIpv4Header(IPPROTO_UDP, hasData, actual);
+
+        final UdpHeader udpHeader = Struct.parse(UdpHeader.class, actual);
+        assertEquals(SRC_PORT, udpHeader.srcPort);
+        assertEquals(DST_PORT, udpHeader.dstPort);
+        final int dataLength = hasData ? DATA.limit() : 0;
+        assertEquals(UDP_HEADER_LEN + dataLength, udpHeader.length);
+        assertEquals(hasData ? (short) 0x4dbd : (short) 0xeb62, udpHeader.checksum);
+
+        if (hasData) {
+            assertEquals(0xdeadbeef, actual.getInt());
+        }
+    }
+
+    @Test
+    public void testBuildPacketEtherIPv4Tcp() throws Exception {
+        final ByteBuffer packet = buildIpv4Packet(SRC_MAC, DST_MAC, IPPROTO_TCP, null /* data */);
+        checkTcpv4Packet(true /* hasEther */, false /* hasData */, packet);
+        assertArrayEquals(TEST_PACKET_ETHERHDR_IPV4HDR_TCPHDR, packet.array());
+    }
+
+    @Test
+    public void testBuildPacketEtherIPv4TcpData() throws Exception {
+        final ByteBuffer packet = buildIpv4Packet(SRC_MAC, DST_MAC, IPPROTO_TCP, DATA);
+        checkTcpv4Packet(true /* hasEther */, true /* hasData */, packet);
+        assertArrayEquals(TEST_PACKET_ETHERHDR_IPV4HDR_TCPHDR_DATA,
+                packet.array());
+    }
+
+    @Test
+    public void testBuildPacketIPv4Tcp() throws Exception {
+        final ByteBuffer packet = buildIpv4Packet(null /* srcMac */, null /* dstMac */,
+                IPPROTO_TCP, null /* data */);
+        checkTcpv4Packet(false /* hasEther */, false /* hasData */, packet);
+        assertArrayEquals(TEST_PACKET_IPV4HDR_TCPHDR, packet.array());
+    }
+
+    @Test
+    public void testBuildPacketIPv4TcpData() throws Exception {
+        final ByteBuffer packet = buildIpv4Packet(null /* srcMac */, null /* dstMac */,
+                IPPROTO_TCP, DATA);
+        checkTcpv4Packet(false /* hasEther */, true /* hasData */, packet);
+        assertArrayEquals(TEST_PACKET_IPV4HDR_TCPHDR_DATA, packet.array());
+    }
+
+    @Test
+    public void testBuildPacketEtherIPv4Udp() throws Exception {
+        final ByteBuffer packet = buildIpv4Packet(SRC_MAC, DST_MAC, IPPROTO_UDP, null /* data */);
+        checkUdpv4Packet(true /* hasEther */, false /* hasData */, packet);
+        assertArrayEquals(TEST_PACKET_ETHERHDR_IPV4HDR_UDPHDR, packet.array());
+    }
+
+    @Test
+    public void testBuildPacketEtherIPv4UdpData() throws Exception {
+        final ByteBuffer packet = buildIpv4Packet(SRC_MAC, DST_MAC, IPPROTO_UDP, DATA);
+        checkUdpv4Packet(true /* hasEther */, true /* hasData */, packet);
+        assertArrayEquals(TEST_PACKET_ETHERHDR_IPV4HDR_UDPHDR_DATA, packet.array());
+    }
+
+    @Test
+    public void testBuildPacketIPv4Udp() throws Exception {
+        final ByteBuffer packet = buildIpv4Packet(null /* srcMac */, null /* dstMac */,
+                IPPROTO_UDP, null /*data*/);
+        checkUdpv4Packet(false /* hasEther */, false /* hasData */, packet);
+        assertArrayEquals(TEST_PACKET_IPV4HDR_UDPHDR, packet.array());
+    }
+
+    @Test
+    public void testBuildPacketIPv4UdpData() throws Exception {
+        final ByteBuffer packet = buildIpv4Packet(null /* srcMac */, null /* dstMac */,
+                IPPROTO_UDP, DATA);
+        checkUdpv4Packet(false /* hasEther */, true /* hasData */, packet);
+        assertArrayEquals(TEST_PACKET_IPV4HDR_UDPHDR_DATA, packet.array());
+    }
+
+    @Test
+    public void testFinalizePacketWithoutIpv4Header() throws Exception {
+        final ByteBuffer buffer = PacketBuilder.allocate(false /* hasEther */, IPPROTO_IP,
+                IPPROTO_TCP, 0 /* payloadLen */);
+        final PacketBuilder packetBuilder = new PacketBuilder(buffer);
+        packetBuilder.writeTcpHeader(SRC_PORT, DST_PORT, SEQ_NO, ACK_NO,
+                TCPHDR_ACK, WINDOW, URGENT_POINTER);
+        assertThrows("java.io.IOException: Packet is missing IPv4 header", IOException.class,
+                () -> packetBuilder.finalizePacket());
+    }
+
+    @Test
+    public void testFinalizePacketWithoutL4Header() throws Exception {
+        final ByteBuffer buffer = PacketBuilder.allocate(false /* hasEther */, IPPROTO_IP,
+                IPPROTO_TCP, 0 /* payloadLen */);
+        final PacketBuilder packetBuilder = new PacketBuilder(buffer);
+        packetBuilder.writeIpv4Header(TYPE_OF_SERVICE, ID, FLAGS_AND_FRAGMENT_OFFSET,
+                TIME_TO_LIVE, (byte) IPPROTO_TCP, IPV4_SRC_ADDR, IPV4_DST_ADDR);
+        assertThrows("java.io.IOException: Packet is missing neither TCP nor UDP header",
+                IOException.class, () -> packetBuilder.finalizePacket());
+    }
+
+    @Test
+    public void testWriteL2HeaderToInsufficientBuffer() throws Exception {
+        final PacketBuilder packetBuilder = new PacketBuilder(ByteBuffer.allocate(1));
+        assertThrows(IOException.class,
+                () -> packetBuilder.writeL2Header(SRC_MAC, DST_MAC, (short) ETHER_TYPE_IPV4));
+    }
+
+    @Test
+    public void testWriteIpv4HeaderToInsufficientBuffer() throws Exception {
+        final PacketBuilder packetBuilder = new PacketBuilder(ByteBuffer.allocate(1));
+        assertThrows(IOException.class,
+                () -> packetBuilder.writeIpv4Header(TYPE_OF_SERVICE, ID, FLAGS_AND_FRAGMENT_OFFSET,
+                        TIME_TO_LIVE, (byte) IPPROTO_TCP, IPV4_SRC_ADDR, IPV4_DST_ADDR));
+    }
+
+    @Test
+    public void testWriteTcpHeaderToInsufficientBuffer() throws Exception {
+        final PacketBuilder packetBuilder = new PacketBuilder(ByteBuffer.allocate(1));
+        assertThrows(IOException.class,
+                () -> packetBuilder.writeTcpHeader(SRC_PORT, DST_PORT, SEQ_NO, ACK_NO,
+                        TCPHDR_ACK, WINDOW, URGENT_POINTER));
+    }
+
+    @Test
+    public void testWriteUdpHeaderToInsufficientBuffer() throws Exception {
+        final PacketBuilder packetBuilder = new PacketBuilder(ByteBuffer.allocate(1));
+        assertThrows(IOException.class, () -> packetBuilder.writeUdpHeader(SRC_PORT, DST_PORT));
+    }
+
+    private static Inet4Address addr(String addr) {
+        return (Inet4Address) InetAddresses.parseNumericAddress(addr);
+    }
+}