diff --git a/tests/cts/net/src/android/net/cts/IpSecBaseTest.java b/tests/cts/net/src/android/net/cts/IpSecBaseTest.java
index 35d0f48..cbb9c19 100644
--- a/tests/cts/net/src/android/net/cts/IpSecBaseTest.java
+++ b/tests/cts/net/src/android/net/cts/IpSecBaseTest.java
@@ -52,6 +52,17 @@
     protected static final int[] DIRECTIONS =
             new int[] {IpSecManager.DIRECTION_IN, IpSecManager.DIRECTION_OUT};
 
+    protected static final int TCP_HDRLEN_WITH_OPTIONS = 32;
+    protected static final int UDP_HDRLEN = 8;
+    protected static final int IP4_HDRLEN = 20;
+    protected static final int IP6_HDRLEN = 40;
+
+    // Encryption parameters
+    protected static final int AES_GCM_IV_LEN = 8;
+    protected static final int AES_CBC_IV_LEN = 16;
+    protected static final int AES_GCM_BLK_SIZE = 4;
+    protected static final int AES_CBC_BLK_SIZE = 16;
+
     protected static final byte[] TEST_DATA = "Best test data ever!".getBytes();
     protected static final int DATA_BUFFER_LEN = 4096;
     protected static final int SOCK_TIMEOUT = 500;
diff --git a/tests/cts/net/src/android/net/cts/IpSecManagerTest.java b/tests/cts/net/src/android/net/cts/IpSecManagerTest.java
index 3387064..e788d96 100644
--- a/tests/cts/net/src/android/net/cts/IpSecManagerTest.java
+++ b/tests/cts/net/src/android/net/cts/IpSecManagerTest.java
@@ -53,17 +53,6 @@
 
     private static final byte[] AEAD_KEY = getKey(288);
 
-    private static final int TCP_HDRLEN_WITH_OPTIONS = 32;
-    private static final int UDP_HDRLEN = 8;
-    private static final int IP4_HDRLEN = 20;
-    private static final int IP6_HDRLEN = 40;
-
-    // Encryption parameters
-    private static final int AES_GCM_IV_LEN = 8;
-    private static final int AES_CBC_IV_LEN = 16;
-    private static final int AES_GCM_BLK_SIZE = 4;
-    private static final int AES_CBC_BLK_SIZE = 16;
-
     protected void setUp() throws Exception {
         super.setUp();
     }
diff --git a/tests/cts/net/src/android/net/cts/TunUtils.java b/tests/cts/net/src/android/net/cts/TunUtils.java
new file mode 100644
index 0000000..ca233ce
--- /dev/null
+++ b/tests/cts/net/src/android/net/cts/TunUtils.java
@@ -0,0 +1,252 @@
+/*
+ * Copyright (C) 2018 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 android.net.cts;
+
+import static android.system.OsConstants.IPPROTO_UDP;
+import static org.junit.Assert.assertEquals;
+import static org.junit.Assert.assertFalse;
+import static org.junit.Assert.fail;
+
+import android.os.ParcelFileDescriptor;
+
+import java.io.FileInputStream;
+import java.io.FileOutputStream;
+import java.io.IOException;
+import java.util.ArrayList;
+import java.util.Arrays;
+import java.util.Collections;
+import java.util.List;
+import java.util.function.Predicate;
+
+public class TunUtils {
+    private static final String TAG = TunUtils.class.getSimpleName();
+
+    private static final int DATA_BUFFER_LEN = 4096;
+    private static final int TIMEOUT = 100;
+
+    private static final int IP4_PROTO_OFFSET = 9;
+    private static final int IP6_PROTO_OFFSET = 6;
+
+    private static final int IP4_ADDR_OFFSET = 12;
+    private static final int IP4_ADDR_LEN = 4;
+    private static final int IP6_ADDR_OFFSET = 8;
+    private static final int IP6_ADDR_LEN = 16;
+
+    // Not defined in OsConstants
+    private static final int IPPROTO_ESP = 50;
+
+    private final ParcelFileDescriptor mTunFd;
+    private final List<byte[]> mPackets = new ArrayList<>();
+    private final Thread mReaderThread;
+
+    public TunUtils(ParcelFileDescriptor tunFd) {
+        mTunFd = tunFd;
+
+        // Start background reader thread
+        mReaderThread =
+                new Thread(
+                        () -> {
+                            try {
+                                // Loop will exit and thread will quit when tunFd is closed.
+                                // Receiving either EOF or an exception will exit this reader loop.
+                                // FileInputStream in uninterruptable, so there's no good way to
+                                // ensure that this thread shuts down except upon FD closure.
+                                while (true) {
+                                    byte[] intercepted = receiveFromTun();
+                                    if (intercepted == null) {
+                                        // Exit once we've hit EOF
+                                        return;
+                                    } else if (intercepted.length > 0) {
+                                        // Only save packet if we've received any bytes.
+                                        synchronized (mPackets) {
+                                            mPackets.add(intercepted);
+                                            mPackets.notifyAll();
+                                        }
+                                    }
+                                }
+                            } catch (IOException ignored) {
+                                // Simply exit this reader thread
+                                return;
+                            }
+                        });
+        mReaderThread.start();
+    }
+
+    private byte[] receiveFromTun() throws IOException {
+        FileInputStream in = new FileInputStream(mTunFd.getFileDescriptor());
+        byte[] inBytes = new byte[DATA_BUFFER_LEN];
+        int bytesRead = in.read(inBytes);
+
+        if (bytesRead < 0) {
+            return null; // return null for EOF
+        } else if (bytesRead >= DATA_BUFFER_LEN) {
+            throw new IllegalStateException("Too big packet. Fragmentation unsupported");
+        }
+        return Arrays.copyOf(inBytes, bytesRead);
+    }
+
+    private byte[] getFirstMatchingPacket(Predicate<byte[]> verifier, int startIndex) {
+        synchronized (mPackets) {
+            for (int i = startIndex; i < mPackets.size(); i++) {
+                byte[] pkt = mPackets.get(i);
+                if (verifier.test(pkt)) {
+                    return pkt;
+                }
+            }
+        }
+        return null;
+    }
+
+    /**
+     * Checks if the specified bytes were ever sent in plaintext.
+     *
+     * <p>Only checks for known plaintext bytes to prevent triggering on ICMP/RA packets or the like
+     *
+     * @param plaintext the plaintext bytes to check for
+     * @param startIndex the index in the list to check for
+     */
+    public boolean hasPlaintextPacket(byte[] plaintext, int startIndex) {
+        Predicate<byte[]> verifier =
+                (pkt) -> {
+                    return Collections.indexOfSubList(Arrays.asList(pkt), Arrays.asList(plaintext))
+                            != -1;
+                };
+        return getFirstMatchingPacket(verifier, startIndex) != null;
+    }
+
+    public byte[] getEspPacket(int spi, boolean encap, int startIndex) {
+        return getFirstMatchingPacket(
+                (pkt) -> {
+                    return isEsp(pkt, spi, encap);
+                },
+                startIndex);
+    }
+
+    public byte[] awaitEspPacketNoPlaintext(
+            int spi, byte[] plaintext, boolean useEncap, int expectedPacketSize) throws Exception {
+        long endTime = System.currentTimeMillis() + TIMEOUT;
+        int startIndex = 0;
+
+        synchronized (mPackets) {
+            while (System.currentTimeMillis() < endTime) {
+                byte[] espPkt = getEspPacket(spi, useEncap, startIndex);
+                if (espPkt != null) {
+                    // Validate packet size
+                    assertEquals(expectedPacketSize, espPkt.length);
+
+                    // Always check plaintext from start
+                    assertFalse(hasPlaintextPacket(plaintext, 0));
+                    return espPkt; // We've found the packet we're looking for.
+                }
+
+                startIndex = mPackets.size();
+
+                // Try to prevent waiting too long. If waitTimeout <= 0, we've already hit timeout
+                long waitTimeout = endTime - System.currentTimeMillis();
+                if (waitTimeout > 0) {
+                    mPackets.wait(waitTimeout);
+                }
+            }
+
+            fail("No such ESP packet found with SPI " + spi);
+        }
+        return null;
+    }
+
+    private static boolean isSpiEqual(byte[] pkt, int espOffset, int spi) {
+        // Check SPI byte by byte.
+        return pkt[espOffset] == (byte) ((spi >>> 24) & 0xff)
+                && pkt[espOffset + 1] == (byte) ((spi >>> 16) & 0xff)
+                && pkt[espOffset + 2] == (byte) ((spi >>> 8) & 0xff)
+                && pkt[espOffset + 3] == (byte) (spi & 0xff);
+    }
+
+    private static boolean isEsp(byte[] pkt, int spi, boolean encap) {
+        if (isIpv6(pkt)) {
+            // IPv6 UDP encap not supported by kernels; assume non-encap.
+            return pkt[IP6_PROTO_OFFSET] == IPPROTO_ESP
+                    && isSpiEqual(pkt, IpSecBaseTest.IP6_HDRLEN, spi);
+        } else {
+            // Use default IPv4 header length (assuming no options)
+            if (encap) {
+                return pkt[IP4_PROTO_OFFSET] == IPPROTO_UDP
+                        && isSpiEqual(
+                                pkt, IpSecBaseTest.IP4_HDRLEN + IpSecBaseTest.UDP_HDRLEN, spi);
+            } else {
+                return pkt[IP4_PROTO_OFFSET] == IPPROTO_ESP
+                        && isSpiEqual(pkt, IpSecBaseTest.IP4_HDRLEN, spi);
+            }
+        }
+    }
+
+    private static boolean isIpv6(byte[] pkt) {
+        // First nibble shows IP version. 0x60 for IPv6
+        return (pkt[0] & (byte) 0xF0) == (byte) 0x60;
+    }
+
+    private static byte[] getReflectedPacket(byte[] pkt) {
+        byte[] reflected = Arrays.copyOf(pkt, pkt.length);
+
+        if (isIpv6(pkt)) {
+            // Set reflected packet's dst to that of the original's src
+            System.arraycopy(
+                    pkt, // src
+                    IP6_ADDR_OFFSET + IP6_ADDR_LEN, // src offset
+                    reflected, // dst
+                    IP6_ADDR_OFFSET, // dst offset
+                    IP6_ADDR_LEN); // len
+            // Set reflected packet's src IP to that of the original's dst IP
+            System.arraycopy(
+                    pkt, // src
+                    IP6_ADDR_OFFSET, // src offset
+                    reflected, // dst
+                    IP6_ADDR_OFFSET + IP6_ADDR_LEN, // dst offset
+                    IP6_ADDR_LEN); // len
+        } else {
+            // Set reflected packet's dst to that of the original's src
+            System.arraycopy(
+                    pkt, // src
+                    IP4_ADDR_OFFSET + IP4_ADDR_LEN, // src offset
+                    reflected, // dst
+                    IP4_ADDR_OFFSET, // dst offset
+                    IP4_ADDR_LEN); // len
+            // Set reflected packet's src IP to that of the original's dst IP
+            System.arraycopy(
+                    pkt, // src
+                    IP4_ADDR_OFFSET, // src offset
+                    reflected, // dst
+                    IP4_ADDR_OFFSET + IP4_ADDR_LEN, // dst offset
+                    IP4_ADDR_LEN); // len
+        }
+        return reflected;
+    }
+
+    /** Takes all captured packets, flips the src/dst, and re-injects them. */
+    public void reflectPackets() throws IOException {
+        synchronized (mPackets) {
+            for (byte[] pkt : mPackets) {
+                injectPacket(getReflectedPacket(pkt));
+            }
+        }
+    }
+
+    public void injectPacket(byte[] pkt) throws IOException {
+        FileOutputStream out = new FileOutputStream(mTunFd.getFileDescriptor());
+        out.write(pkt);
+        out.flush();
+    }
+}