diff --git a/tests/cts/net/src/android/net/cts/TunUtils.java b/tests/cts/net/src/android/net/cts/TunUtils.java
index 0377160..268d8d2 100644
--- a/tests/cts/net/src/android/net/cts/TunUtils.java
+++ b/tests/cts/net/src/android/net/cts/TunUtils.java
@@ -22,7 +22,6 @@
 import static android.net.cts.PacketUtils.UDP_HDRLEN;
 import static android.system.OsConstants.IPPROTO_UDP;
 
-import static org.junit.Assert.assertEquals;
 import static org.junit.Assert.fail;
 
 import android.os.ParcelFileDescriptor;
@@ -32,6 +31,7 @@
 import java.io.FileInputStream;
 import java.io.FileOutputStream;
 import java.io.IOException;
+import java.nio.ByteBuffer;
 import java.util.ArrayList;
 import java.util.Arrays;
 import java.util.List;
@@ -140,10 +140,8 @@
     public byte[] awaitEspPacketNoPlaintext(
             int spi, byte[] plaintext, boolean useEncap, int expectedPacketSize) throws Exception {
         final byte[] espPkt = awaitPacket(
-                (pkt) -> isEspFailIfSpecifiedPlaintextFound(pkt, spi, useEncap, plaintext));
-
-        // Validate packet size
-        assertEquals(expectedPacketSize, espPkt.length);
+            (pkt) -> expectedPacketSize == pkt.length
+                    && isEspFailIfSpecifiedPlaintextFound(pkt, spi, useEncap, plaintext));
 
         return espPkt; // We've found the packet we're looking for.
     }
@@ -153,11 +151,11 @@
     }
 
     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);
+        ByteBuffer buffer = ByteBuffer.wrap(pkt);
+        buffer.get(new byte[espOffset]); // Skip IP, UDP header
+        int actualSpi = buffer.getInt();
+
+        return actualSpi == spi;
     }
 
     /**
@@ -180,8 +178,13 @@
 
     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, IP6_HDRLEN, spi);
+            if (encap) {
+                return pkt[IP6_PROTO_OFFSET] == IPPROTO_UDP
+                        && isSpiEqual(pkt, IP6_HDRLEN + UDP_HDRLEN, spi);
+            } else {
+                return pkt[IP6_PROTO_OFFSET] == IPPROTO_ESP && isSpiEqual(pkt, IP6_HDRLEN, spi);
+            }
+
         } else {
             // Use default IPv4 header length (assuming no options)
             if (encap) {
diff --git a/tests/cts/net/util/java/android/net/cts/util/CtsNetUtils.java b/tests/cts/net/util/java/android/net/cts/util/CtsNetUtils.java
index df3a4aa..d817630 100644
--- a/tests/cts/net/util/java/android/net/cts/util/CtsNetUtils.java
+++ b/tests/cts/net/util/java/android/net/cts/util/CtsNetUtils.java
@@ -75,6 +75,13 @@
 
 public final class CtsNetUtils {
     private static final String TAG = CtsNetUtils.class.getSimpleName();
+
+    // Redefine this flag here so that IPsec code shipped in a mainline module can build on old
+    // platforms before FEATURE_IPSEC_TUNNEL_MIGRATION API is released.
+    // TODO: b/275378783 Remove this flag and use the platform API when it is available.
+    private static final String FEATURE_IPSEC_TUNNEL_MIGRATION =
+            "android.software.ipsec_tunnel_migration";
+
     private static final int SOCKET_TIMEOUT_MS = 2000;
     private static final int PRIVATE_DNS_PROBE_MS = 1_000;
 
@@ -115,6 +122,11 @@
                 || getFirstApiLevel() >= Build.VERSION_CODES.Q;
     }
 
+    /** Checks if FEATURE_IPSEC_TUNNEL_MIGRATION is enabled on the device */
+    public boolean hasIpsecTunnelMigrateFeature() {
+        return mContext.getPackageManager().hasSystemFeature(FEATURE_IPSEC_TUNNEL_MIGRATION);
+    }
+
     /**
      * Sets the given appop using shell commands
      *