diff --git a/Tethering/common/TetheringLib/Android.bp b/Tethering/common/TetheringLib/Android.bp
index f652772..6e64570 100644
--- a/Tethering/common/TetheringLib/Android.bp
+++ b/Tethering/common/TetheringLib/Android.bp
@@ -22,7 +22,19 @@
     defaults: ["framework-module-defaults"],
     impl_library_visibility: [
         "//packages/modules/Connectivity/Tethering:__subpackages__",
+
+        // Using for test only
+        "//cts/tests/netlegacy22.api",
+        "//external/sl4a:__subpackages__",
+        "//frameworks/base/packages/Connectivity/tests:__subpackages__",
+        "//frameworks/libs/net/common/testutils",
+        "//frameworks/libs/net/common/tests:__subpackages__",
+        "//frameworks/opt/telephony/tests/telephonytests",
+        "//packages/modules/CaptivePortalLogin/tests",
+        "//packages/modules/Connectivity/Tethering/tests:__subpackages__",
         "//packages/modules/Connectivity/tests:__subpackages__",
+        "//packages/modules/NetworkStack/tests:__subpackages__",
+        "//packages/modules/Wifi/service/tests/wifitests",
     ],
 
     srcs: [":framework-tethering-srcs"],
diff --git a/Tethering/common/TetheringLib/src/android/net/ITetheringConnector.aidl b/Tethering/common/TetheringLib/src/android/net/ITetheringConnector.aidl
index cf094aa..77e78bd 100644
--- a/Tethering/common/TetheringLib/src/android/net/ITetheringConnector.aidl
+++ b/Tethering/common/TetheringLib/src/android/net/ITetheringConnector.aidl
@@ -49,4 +49,6 @@
 
     void stopAllTethering(String callerPkg, String callingAttributionTag,
             IIntResultListener receiver);
+
+    void setPreferTestNetworks(boolean prefer, IIntResultListener listener);
 }
diff --git a/Tethering/common/TetheringLib/src/android/net/TetheringManager.java b/Tethering/common/TetheringLib/src/android/net/TetheringManager.java
index 73ab908..65b2650 100644
--- a/Tethering/common/TetheringLib/src/android/net/TetheringManager.java
+++ b/Tethering/common/TetheringLib/src/android/net/TetheringManager.java
@@ -1538,4 +1538,25 @@
                     }
                 }));
     }
+
+    /**
+     * Whether to treat networks that have TRANSPORT_TEST as Tethering upstreams. The effects of
+     * this method apply to any test networks that are already present on the system.
+     *
+     * @throws SecurityException If the caller doesn't have the NETWORK_SETTINGS permission.
+     * @hide
+     */
+    @RequiresPermission(android.Manifest.permission.NETWORK_SETTINGS)
+    public void setPreferTestNetworks(final boolean prefer) {
+        Log.i(TAG, "setPreferTestNetworks caller: " + mContext.getOpPackageName());
+
+        final RequestDispatcher dispatcher = new RequestDispatcher();
+        final int ret = dispatcher.waitForResult((connector, listener) -> {
+            try {
+                connector.setPreferTestNetworks(prefer, listener);
+            } catch (RemoteException e) {
+                throw new IllegalStateException(e);
+            }
+        });
+    }
 }
diff --git a/Tethering/src/com/android/networkstack/tethering/Tethering.java b/Tethering/src/com/android/networkstack/tethering/Tethering.java
index 08170f9..1559f3b 100644
--- a/Tethering/src/com/android/networkstack/tethering/Tethering.java
+++ b/Tethering/src/com/android/networkstack/tethering/Tethering.java
@@ -2601,4 +2601,13 @@
     private static String[] copy(String[] strarray) {
         return Arrays.copyOf(strarray, strarray.length);
     }
+
+    void setPreferTestNetworks(final boolean prefer, IIntResultListener listener) {
+        mHandler.post(() -> {
+            mUpstreamNetworkMonitor.setPreferTestNetworks(prefer);
+            try {
+                listener.onResult(TETHER_ERROR_NO_ERROR);
+            } catch (RemoteException e) { }
+        });
+    }
 }
diff --git a/Tethering/src/com/android/networkstack/tethering/TetheringInterfaceUtils.java b/Tethering/src/com/android/networkstack/tethering/TetheringInterfaceUtils.java
index ff38f71..d0a1ac3 100644
--- a/Tethering/src/com/android/networkstack/tethering/TetheringInterfaceUtils.java
+++ b/Tethering/src/com/android/networkstack/tethering/TetheringInterfaceUtils.java
@@ -16,6 +16,10 @@
 
 package com.android.networkstack.tethering;
 
+import static android.net.NetworkCapabilities.TRANSPORT_CELLULAR;
+import static android.net.NetworkCapabilities.TRANSPORT_TEST;
+
+import android.annotation.NonNull;
 import android.annotation.Nullable;
 import android.net.LinkProperties;
 import android.net.NetworkCapabilities;
@@ -78,13 +82,17 @@
                 // Minimal amount of IPv6 provisioning:
                 && ns.linkProperties.hasGlobalIpv6Address()
                 // Temporary approximation of "dedicated prefix":
-                && ns.networkCapabilities.hasTransport(NetworkCapabilities.TRANSPORT_CELLULAR);
+                && allowIpv6Tethering(ns.networkCapabilities);
 
         return canTether
                 ? getInterfaceForDestination(ns.linkProperties, IN6ADDR_ANY)
                 : null;
     }
 
+    private static boolean allowIpv6Tethering(@NonNull final NetworkCapabilities nc) {
+        return nc.hasTransport(TRANSPORT_CELLULAR) || nc.hasTransport(TRANSPORT_TEST);
+    }
+
     private static String getInterfaceForDestination(LinkProperties lp, InetAddress dst) {
         final RouteInfo ri = (lp != null)
                 ? NetUtils.selectBestRoute(lp.getAllRoutes(), dst)
diff --git a/Tethering/src/com/android/networkstack/tethering/TetheringService.java b/Tethering/src/com/android/networkstack/tethering/TetheringService.java
index 5ab3401..d3d2962 100644
--- a/Tethering/src/com/android/networkstack/tethering/TetheringService.java
+++ b/Tethering/src/com/android/networkstack/tethering/TetheringService.java
@@ -17,6 +17,7 @@
 package com.android.networkstack.tethering;
 
 import static android.Manifest.permission.ACCESS_NETWORK_STATE;
+import static android.Manifest.permission.NETWORK_SETTINGS;
 import static android.Manifest.permission.NETWORK_STACK;
 import static android.Manifest.permission.TETHER_PRIVILEGED;
 import static android.content.pm.PackageManager.PERMISSION_GRANTED;
@@ -199,6 +200,17 @@
             } catch (RemoteException e) { }
         }
 
+        public void setPreferTestNetworks(boolean prefer, IIntResultListener listener) {
+            if (!checkCallingOrSelfPermission(NETWORK_SETTINGS)) {
+                try {
+                    listener.onResult(TETHER_ERROR_NO_CHANGE_TETHERING_PERMISSION);
+                } catch (RemoteException e) { }
+                return;
+            }
+
+            mTethering.setPreferTestNetworks(prefer, listener);
+        }
+
         @Override
         protected void dump(@NonNull FileDescriptor fd, @NonNull PrintWriter writer,
                     @Nullable String[] args) {
diff --git a/Tethering/src/com/android/networkstack/tethering/UpstreamNetworkMonitor.java b/Tethering/src/com/android/networkstack/tethering/UpstreamNetworkMonitor.java
index 5584db2..5a86b84 100644
--- a/Tethering/src/com/android/networkstack/tethering/UpstreamNetworkMonitor.java
+++ b/Tethering/src/com/android/networkstack/tethering/UpstreamNetworkMonitor.java
@@ -135,6 +135,7 @@
     private Network mDefaultInternetNetwork;
     // The current upstream network used for tethering.
     private Network mTetheringUpstreamNetwork;
+    private boolean mPreferTestNetworks;
 
     public UpstreamNetworkMonitor(Context ctx, StateMachine tgt, SharedLog log, int what) {
         mContext = ctx;
@@ -325,6 +326,11 @@
         final UpstreamNetworkState dfltState = (mDefaultInternetNetwork != null)
                 ? mNetworkMap.get(mDefaultInternetNetwork)
                 : null;
+        if (mPreferTestNetworks) {
+            final UpstreamNetworkState testState = findFirstTestNetwork(mNetworkMap.values());
+            if (testState != null) return testState;
+        }
+
         if (isNetworkUsableAndNotCellular(dfltState)) return dfltState;
 
         if (!isCellularUpstreamPermitted()) return null;
@@ -656,6 +662,20 @@
         return null;
     }
 
+    static boolean isTestNetwork(UpstreamNetworkState ns) {
+        return ((ns != null) && (ns.networkCapabilities != null)
+                && ns.networkCapabilities.hasTransport(NetworkCapabilities.TRANSPORT_TEST));
+    }
+
+    private UpstreamNetworkState findFirstTestNetwork(
+            Iterable<UpstreamNetworkState> netStates) {
+        for (UpstreamNetworkState ns : netStates) {
+            if (isTestNetwork(ns)) return ns;
+        }
+
+        return null;
+    }
+
     /**
      * Given a legacy type (TYPE_WIFI, ...) returns the corresponding NetworkCapabilities instance.
      * This function is used for deprecated legacy type and be disabled by default.
@@ -681,4 +701,9 @@
         }
         return builder.build();
     }
+
+    /** Set test network as preferred upstream. */
+    public void setPreferTestNetworks(boolean prefer) {
+        mPreferTestNetworks = prefer;
+    }
 }
diff --git a/Tethering/tests/integration/Android.bp b/Tethering/tests/integration/Android.bp
index 2593b1b..c7f5527 100644
--- a/Tethering/tests/integration/Android.bp
+++ b/Tethering/tests/integration/Android.bp
@@ -19,6 +19,7 @@
 
 java_defaults {
     name: "TetheringIntegrationTestsDefaults",
+    defaults: ["framework-connectivity-test-defaults"],
     srcs: [
         "src/**/*.java",
         "src/**/*.kt",
diff --git a/Tethering/tests/integration/AndroidManifest.xml b/Tethering/tests/integration/AndroidManifest.xml
index fddfaad..c89c556 100644
--- a/Tethering/tests/integration/AndroidManifest.xml
+++ b/Tethering/tests/integration/AndroidManifest.xml
@@ -17,6 +17,11 @@
           package="com.android.networkstack.tethering.tests.integration">
 
     <uses-permission android:name="android.permission.INTERNET"/>
+    <!-- The test need CHANGE_NETWORK_STATE permission to use requestNetwork API to setup test
+         network. Since R shell application don't have such permission, grant permission to the test
+         here. TODO: Remove CHANGE_NETWORK_STATE permission here and use adopt shell perssion to
+         obtain CHANGE_NETWORK_STATE for testing once R device is no longer supported. -->
+    <uses-permission android:name="android.permission.CHANGE_NETWORK_STATE"/>
 
     <application android:debuggable="true">
         <uses-library android:name="android.test.runner" />
diff --git a/Tethering/tests/integration/src/android/net/EthernetTetheringTest.java b/Tethering/tests/integration/src/android/net/EthernetTetheringTest.java
index 26297a2..15f07f2 100644
--- a/Tethering/tests/integration/src/android/net/EthernetTetheringTest.java
+++ b/Tethering/tests/integration/src/android/net/EthernetTetheringTest.java
@@ -21,6 +21,7 @@
 import static android.Manifest.permission.MANAGE_TEST_NETWORKS;
 import static android.Manifest.permission.NETWORK_SETTINGS;
 import static android.Manifest.permission.TETHER_PRIVILEGED;
+import static android.net.InetAddresses.parseNumericAddress;
 import static android.net.TetheringManager.CONNECTIVITY_SCOPE_GLOBAL;
 import static android.net.TetheringManager.CONNECTIVITY_SCOPE_LOCAL;
 import static android.net.TetheringManager.TETHERING_ETHERNET;
@@ -29,6 +30,7 @@
 import static com.android.net.module.util.ConnectivityUtils.isIPv6ULA;
 import static com.android.net.module.util.NetworkStackConstants.ETHER_TYPE_IPV6;
 import static com.android.net.module.util.NetworkStackConstants.ICMPV6_ROUTER_ADVERTISEMENT;
+import static com.android.testutils.TestNetworkTrackerKt.initTestNetwork;
 
 import static org.junit.Assert.assertEquals;
 import static org.junit.Assert.assertNotNull;
@@ -45,14 +47,10 @@
 import android.net.TetheringManager.StartTetheringCallback;
 import android.net.TetheringManager.TetheringEventCallback;
 import android.net.TetheringManager.TetheringRequest;
-import android.net.dhcp.DhcpAckPacket;
-import android.net.dhcp.DhcpOfferPacket;
-import android.net.dhcp.DhcpPacket;
 import android.os.Handler;
 import android.os.HandlerThread;
 import android.os.SystemClock;
 import android.os.SystemProperties;
-import android.system.Os;
 import android.util.Log;
 
 import androidx.test.InstrumentationRegistry;
@@ -65,6 +63,7 @@
 import com.android.net.module.util.structs.Ipv6Header;
 import com.android.testutils.HandlerUtils;
 import com.android.testutils.TapPacketReader;
+import com.android.testutils.TestNetworkTracker;
 
 import org.junit.After;
 import org.junit.Before;
@@ -72,12 +71,12 @@
 import org.junit.runner.RunWith;
 
 import java.io.FileDescriptor;
-import java.net.Inet4Address;
 import java.net.InetAddress;
 import java.net.InterfaceAddress;
 import java.net.NetworkInterface;
 import java.net.SocketException;
 import java.nio.ByteBuffer;
+import java.util.Arrays;
 import java.util.Collection;
 import java.util.List;
 import java.util.Random;
@@ -93,24 +92,19 @@
 
     private static final String TAG = EthernetTetheringTest.class.getSimpleName();
     private static final int TIMEOUT_MS = 5000;
-    private static final int PACKET_READ_TIMEOUT_MS = 100;
-    private static final int DHCP_DISCOVER_ATTEMPTS = 10;
-    private static final byte[] DHCP_REQUESTED_PARAMS = new byte[] {
-            DhcpPacket.DHCP_SUBNET_MASK,
-            DhcpPacket.DHCP_ROUTER,
-            DhcpPacket.DHCP_DNS_SERVER,
-            DhcpPacket.DHCP_LEASE_TIME,
-    };
-    private static final String DHCP_HOSTNAME = "testhostname";
+    private static final LinkAddress TEST_IP4_ADDR = new LinkAddress("10.0.0.1/8");
+    private static final LinkAddress TEST_IP6_ADDR = new LinkAddress("2001:db8:1::101/64");
+    private static final InetAddress TEST_IP4_DNS = parseNumericAddress("8.8.8.8");
+    private static final InetAddress TEST_IP6_DNS = parseNumericAddress("2001:db8:1::888");
 
     private final Context mContext = InstrumentationRegistry.getContext();
     private final EthernetManager mEm = mContext.getSystemService(EthernetManager.class);
     private final TetheringManager mTm = mContext.getSystemService(TetheringManager.class);
 
-    private TestNetworkInterface mTestIface;
+    private TestNetworkInterface mDownstreamIface;
     private HandlerThread mHandlerThread;
     private Handler mHandler;
-    private TapPacketReader mTapPacketReader;
+    private TapPacketReader mDownstreamReader;
 
     private TetheredInterfaceRequester mTetheredInterfaceRequester;
     private MyTetheringEventCallback mTetheringEventCallback;
@@ -119,6 +113,8 @@
             InstrumentationRegistry.getInstrumentation().getUiAutomation();
     private boolean mRunTests;
 
+    private TestNetworkTracker mUpstreamTracker;
+
     @Before
     public void setUp() throws Exception {
         // Needed to create a TestNetworkInterface, to call requestTetheredInterface, and to receive
@@ -138,16 +134,23 @@
     }
 
     private void cleanUp() throws Exception {
+        mTm.setPreferTestNetworks(false);
+
+        if (mUpstreamTracker != null) {
+            mUpstreamTracker.teardown();
+            mUpstreamTracker = null;
+        }
+
         mTm.stopTethering(TETHERING_ETHERNET);
         if (mTetheringEventCallback != null) {
             mTetheringEventCallback.awaitInterfaceUntethered();
             mTetheringEventCallback.unregister();
             mTetheringEventCallback = null;
         }
-        if (mTapPacketReader != null) {
-            TapPacketReader reader = mTapPacketReader;
+        if (mDownstreamReader != null) {
+            TapPacketReader reader = mDownstreamReader;
             mHandler.post(() -> reader.stop());
-            mTapPacketReader = null;
+            mDownstreamReader = null;
         }
         mHandlerThread.quitSafely();
         mTetheredInterfaceRequester.release();
@@ -169,21 +172,21 @@
         // This test requires manipulating packets. Skip if there is a physical Ethernet connected.
         assumeFalse(mEm.isAvailable());
 
-        mTestIface = createTestInterface();
+        mDownstreamIface = createTestInterface();
         // This must be done now because as soon as setIncludeTestInterfaces(true) is called, the
         // interface will be placed in client mode, which will delete the link-local address.
         // At that point NetworkInterface.getByName() will cease to work on the interface, because
         // starting in R NetworkInterface can no longer see interfaces without IP addresses.
-        int mtu = getMTU(mTestIface);
+        int mtu = getMTU(mDownstreamIface);
 
         Log.d(TAG, "Including test interfaces");
         mEm.setIncludeTestInterfaces(true);
 
         final String iface = mTetheredInterfaceRequester.getInterface();
         assertEquals("TetheredInterfaceCallback for unexpected interface",
-                mTestIface.getInterfaceName(), iface);
+                mDownstreamIface.getInterfaceName(), iface);
 
-        checkVirtualEthernet(mTestIface, mtu);
+        checkVirtualEthernet(mDownstreamIface, mtu);
     }
 
     @Test
@@ -195,13 +198,13 @@
 
         mEm.setIncludeTestInterfaces(true);
 
-        mTestIface = createTestInterface();
+        mDownstreamIface = createTestInterface();
 
         final String iface = futureIface.get(TIMEOUT_MS, TimeUnit.MILLISECONDS);
         assertEquals("TetheredInterfaceCallback for unexpected interface",
-                mTestIface.getInterfaceName(), iface);
+                mDownstreamIface.getInterfaceName(), iface);
 
-        checkVirtualEthernet(mTestIface, getMTU(mTestIface));
+        checkVirtualEthernet(mDownstreamIface, getMTU(mDownstreamIface));
     }
 
     @Test
@@ -210,11 +213,11 @@
 
         mEm.setIncludeTestInterfaces(true);
 
-        mTestIface = createTestInterface();
+        mDownstreamIface = createTestInterface();
 
         final String iface = mTetheredInterfaceRequester.getInterface();
         assertEquals("TetheredInterfaceCallback for unexpected interface",
-                mTestIface.getInterfaceName(), iface);
+                mDownstreamIface.getInterfaceName(), iface);
 
         assertInvalidStaticIpv4Request(iface, null, null);
         assertInvalidStaticIpv4Request(iface, "2001:db8::1/64", "2001:db8:2::/64");
@@ -235,13 +238,14 @@
         byte[] client1 = MacAddress.fromString("1:2:3:4:5:6").toByteArray();
         byte[] client2 = MacAddress.fromString("a:b:c:d:e:f").toByteArray();
 
-        FileDescriptor fd = mTestIface.getFileDescriptor().getFileDescriptor();
-        mTapPacketReader = makePacketReader(fd, getMTU(mTestIface));
-        DhcpResults dhcpResults = runDhcp(fd, client1);
+        FileDescriptor fd = mDownstreamIface.getFileDescriptor().getFileDescriptor();
+        mDownstreamReader = makePacketReader(fd, getMTU(mDownstreamIface));
+        TetheringTester tester = new TetheringTester(mDownstreamReader);
+        DhcpResults dhcpResults = tester.runDhcp(client1);
         assertEquals(new LinkAddress(clientAddr), dhcpResults.ipAddress);
 
         try {
-            runDhcp(fd, client2);
+            tester.runDhcp(client2);
             fail("Only one client should get an IP address");
         } catch (TimeoutException expected) { }
 
@@ -301,11 +305,11 @@
 
         mEm.setIncludeTestInterfaces(true);
 
-        mTestIface = createTestInterface();
+        mDownstreamIface = createTestInterface();
 
         final String iface = mTetheredInterfaceRequester.getInterface();
         assertEquals("TetheredInterfaceCallback for unexpected interface",
-                mTestIface.getInterfaceName(), iface);
+                mDownstreamIface.getInterfaceName(), iface);
 
         final TetheringRequest request = new TetheringRequest.Builder(TETHERING_ETHERNET)
                 .setConnectivityScope(CONNECTIVITY_SCOPE_LOCAL).build();
@@ -316,10 +320,9 @@
         // does not have an IP address, and unprivileged apps cannot see interfaces without IP
         // addresses. This shouldn't be flaky because the TAP interface will buffer all packets even
         // before the reader is started.
-        FileDescriptor fd = mTestIface.getFileDescriptor().getFileDescriptor();
-        mTapPacketReader = makePacketReader(fd, getMTU(mTestIface));
+        mDownstreamReader = makePacketReader(mDownstreamIface);
 
-        expectRouterAdvertisement(mTapPacketReader, iface, 2000 /* timeoutMs */);
+        expectRouterAdvertisement(mDownstreamReader, iface, 2000 /* timeoutMs */);
         expectLocalOnlyAddresses(iface);
     }
 
@@ -354,12 +357,14 @@
         private final CountDownLatch mLocalOnlyStartedLatch = new CountDownLatch(1);
         private final CountDownLatch mLocalOnlyStoppedLatch = new CountDownLatch(1);
         private final CountDownLatch mClientConnectedLatch = new CountDownLatch(1);
+        private final CountDownLatch mUpstreamConnectedLatch = new CountDownLatch(1);
         private final TetheringInterface mIface;
 
         private volatile boolean mInterfaceWasTethered = false;
         private volatile boolean mInterfaceWasLocalOnly = false;
         private volatile boolean mUnregistered = false;
         private volatile Collection<TetheredClient> mClients = null;
+        private volatile Network mUpstream = null;
 
         MyTetheringEventCallback(TetheringManager tm, String iface) {
             mTm = tm;
@@ -465,6 +470,22 @@
                     mClientConnectedLatch.await(TIMEOUT_MS, TimeUnit.MILLISECONDS));
             return mClients;
         }
+
+        @Override
+        public void onUpstreamChanged(Network network) {
+            // Ignore stale callbacks registered by previous test cases.
+            if (mUnregistered) return;
+
+            Log.d(TAG, "Got upstream changed: " + network);
+            mUpstream = network;
+            if (mUpstream != null) mUpstreamConnectedLatch.countDown();
+        }
+
+        public Network awaitFirstUpstreamConnected() throws Exception {
+            assertTrue("Did not receive upstream connected callback after " + TIMEOUT_MS + "ms",
+                    mUpstreamConnectedLatch.await(TIMEOUT_MS, TimeUnit.MILLISECONDS));
+            return mUpstream;
+        }
     }
 
     private MyTetheringEventCallback enableEthernetTethering(String iface,
@@ -508,6 +529,11 @@
         return nif.getMTU();
     }
 
+    private TapPacketReader makePacketReader(final TestNetworkInterface iface) throws Exception {
+        FileDescriptor fd = iface.getFileDescriptor().getFileDescriptor();
+        return makePacketReader(fd, getMTU(iface));
+    }
+
     private TapPacketReader makePacketReader(FileDescriptor fd, int mtu) {
         final TapPacketReader reader = new TapPacketReader(mHandler, fd, mtu);
         mHandler.post(() -> reader.start());
@@ -517,40 +543,18 @@
 
     private void checkVirtualEthernet(TestNetworkInterface iface, int mtu) throws Exception {
         FileDescriptor fd = iface.getFileDescriptor().getFileDescriptor();
-        mTapPacketReader = makePacketReader(fd, mtu);
+        mDownstreamReader = makePacketReader(fd, mtu);
         mTetheringEventCallback = enableEthernetTethering(iface.getInterfaceName());
-        checkTetheredClientCallbacks(fd);
+        checkTetheredClientCallbacks(mDownstreamReader);
     }
 
-    private DhcpResults runDhcp(FileDescriptor fd, byte[] clientMacAddr) throws Exception {
-        // We have to retransmit DHCP requests because IpServer declares itself to be ready before
-        // its DhcpServer is actually started. TODO: fix this race and remove this loop.
-        DhcpPacket offerPacket = null;
-        for (int i = 0; i < DHCP_DISCOVER_ATTEMPTS; i++) {
-            Log.d(TAG, "Sending DHCP discover");
-            sendDhcpDiscover(fd, clientMacAddr);
-            offerPacket = getNextDhcpPacket();
-            if (offerPacket instanceof DhcpOfferPacket) break;
-        }
-        if (!(offerPacket instanceof DhcpOfferPacket)) {
-            throw new TimeoutException("No DHCPOFFER received on interface within timeout");
-        }
-
-        sendDhcpRequest(fd, offerPacket, clientMacAddr);
-        DhcpPacket ackPacket = getNextDhcpPacket();
-        if (!(ackPacket instanceof DhcpAckPacket)) {
-            throw new TimeoutException("No DHCPACK received on interface within timeout");
-        }
-
-        return ackPacket.toDhcpResults();
-    }
-
-    private void checkTetheredClientCallbacks(FileDescriptor fd) throws Exception {
+    private void checkTetheredClientCallbacks(TapPacketReader packetReader) throws Exception {
         // Create a fake client.
         byte[] clientMacAddr = new byte[6];
         new Random().nextBytes(clientMacAddr);
 
-        DhcpResults dhcpResults = runDhcp(fd, clientMacAddr);
+        TetheringTester tester = new TetheringTester(packetReader);
+        DhcpResults dhcpResults = tester.runDhcp(clientMacAddr);
 
         final Collection<TetheredClient> clients = mTetheringEventCallback.awaitClientConnected();
         assertEquals(1, clients.size());
@@ -563,7 +567,7 @@
         // Check the hostname.
         assertEquals(1, client.getAddresses().size());
         TetheredClient.AddressInfo info = client.getAddresses().get(0);
-        assertEquals(DHCP_HOSTNAME, info.getHostname());
+        assertEquals(TetheringTester.DHCP_HOSTNAME, info.getHostname());
 
         // Check the address is the one that was handed out in the DHCP ACK.
         assertLinkAddressMatches(dhcpResults.ipAddress, info.getAddress());
@@ -576,18 +580,6 @@
         assertTrue(msg, Math.abs(dhcpResults.leaseDuration - actualLeaseDuration) < 10);
     }
 
-    private DhcpPacket getNextDhcpPacket() throws ParseException {
-        byte[] packet;
-        while ((packet = mTapPacketReader.popPacket(PACKET_READ_TIMEOUT_MS)) != null) {
-            try {
-                return DhcpPacket.decodeFullPacket(packet, packet.length, DhcpPacket.ENCAP_L2);
-            } catch (DhcpPacket.ParseException e) {
-                // Not a DHCP packet. Continue.
-            }
-        }
-        return null;
-    }
-
     private static final class TetheredInterfaceRequester implements TetheredInterfaceCallback {
         private final Handler mHandler;
         private final EthernetManager mEm;
@@ -631,31 +623,6 @@
         }
     }
 
-    private void sendDhcpDiscover(FileDescriptor fd, byte[] macAddress) throws Exception {
-        ByteBuffer packet = DhcpPacket.buildDiscoverPacket(DhcpPacket.ENCAP_L2,
-                new Random().nextInt() /* transactionId */, (short) 0 /* secs */,
-                macAddress,  false /* unicast */, DHCP_REQUESTED_PARAMS,
-                false /* rapid commit */,  DHCP_HOSTNAME);
-        sendPacket(fd, packet);
-    }
-
-    private void sendDhcpRequest(FileDescriptor fd, DhcpPacket offerPacket, byte[] macAddress)
-            throws Exception {
-        DhcpResults results = offerPacket.toDhcpResults();
-        Inet4Address clientIp = (Inet4Address) results.ipAddress.getAddress();
-        Inet4Address serverIdentifier = results.serverAddress;
-        ByteBuffer packet = DhcpPacket.buildRequestPacket(DhcpPacket.ENCAP_L2,
-                0 /* transactionId */, (short) 0 /* secs */, DhcpPacket.INADDR_ANY /* clientIp */,
-                false /* broadcast */, macAddress, clientIp /* requestedIpAddress */,
-                serverIdentifier, DHCP_REQUESTED_PARAMS, DHCP_HOSTNAME);
-        sendPacket(fd, packet);
-    }
-
-    private void sendPacket(FileDescriptor fd, ByteBuffer packet) throws Exception {
-        assertNotNull("Only tests on virtual interfaces can send packets", fd);
-        Os.write(fd, packet);
-    }
-
     public void assertLinkAddressMatches(LinkAddress l1, LinkAddress l2) {
         // Check all fields except the deprecation and expiry times.
         String msg = String.format("LinkAddresses do not match. expected: %s actual: %s", l1, l2);
@@ -701,10 +668,45 @@
     }
 
     private void maybeDeleteTestInterface() throws Exception {
-        if (mTestIface != null) {
-            mTestIface.getFileDescriptor().close();
-            Log.d(TAG, "Deleted test interface " + mTestIface.getInterfaceName());
-            mTestIface = null;
+        if (mDownstreamIface != null) {
+            mDownstreamIface.getFileDescriptor().close();
+            Log.d(TAG, "Deleted test interface " + mDownstreamIface.getInterfaceName());
+            mDownstreamIface = null;
         }
     }
+
+    private TestNetworkTracker createTestUpstream(final List<LinkAddress> addresses)
+            throws Exception {
+        mTm.setPreferTestNetworks(true);
+
+        return initTestNetwork(mContext, addresses, TIMEOUT_MS);
+    }
+
+    @Test
+    public void testTestNetworkUpstream() throws Exception {
+        assumeFalse(mEm.isAvailable());
+
+        // MyTetheringEventCallback currently only support await first available upstream. Tethering
+        // may select internet network as upstream if test network is not available and not be
+        // preferred yet. Create test upstream network before enable tethering.
+        mUpstreamTracker = createTestUpstream(toList(TEST_IP4_ADDR, TEST_IP6_ADDR));
+
+        mDownstreamIface = createTestInterface();
+        mEm.setIncludeTestInterfaces(true);
+
+        final String iface = mTetheredInterfaceRequester.getInterface();
+        assertEquals("TetheredInterfaceCallback for unexpected interface",
+                mDownstreamIface.getInterfaceName(), iface);
+
+        mTetheringEventCallback = enableEthernetTethering(mDownstreamIface.getInterfaceName());
+        assertEquals("onUpstreamChanged for unexpected network", mUpstreamTracker.getNetwork(),
+                mTetheringEventCallback.awaitFirstUpstreamConnected());
+
+        mDownstreamReader = makePacketReader(mDownstreamIface);
+        // TODO: do basic forwarding test here.
+    }
+
+    private <T> List<T> toList(T... array) {
+        return Arrays.asList(array);
+    }
 }
diff --git a/Tethering/tests/integration/src/android/net/TetheringTester.java b/Tethering/tests/integration/src/android/net/TetheringTester.java
new file mode 100644
index 0000000..38d74ad
--- /dev/null
+++ b/Tethering/tests/integration/src/android/net/TetheringTester.java
@@ -0,0 +1,155 @@
+/*
+ * 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 android.net;
+
+import static org.junit.Assert.fail;
+
+import android.net.dhcp.DhcpAckPacket;
+import android.net.dhcp.DhcpOfferPacket;
+import android.net.dhcp.DhcpPacket;
+import android.util.ArrayMap;
+import android.util.Log;
+
+import com.android.testutils.TapPacketReader;
+
+import java.net.Inet4Address;
+import java.nio.ByteBuffer;
+import java.util.Random;
+import java.util.concurrent.TimeoutException;
+import java.util.function.Function;
+
+/**
+ * A class simulate tethered client. When caller create TetheringTester, it would connect to
+ * tethering module that do the dhcp and slaac to obtain ipv4 and ipv6 address. Then caller can
+ * send/receive packets by this class.
+ */
+public final class TetheringTester {
+    private static final String TAG = TetheringTester.class.getSimpleName();
+    private static final int PACKET_READ_TIMEOUT_MS = 100;
+    private static final int DHCP_DISCOVER_ATTEMPTS = 10;
+    private static final byte[] DHCP_REQUESTED_PARAMS = new byte[] {
+            DhcpPacket.DHCP_SUBNET_MASK,
+            DhcpPacket.DHCP_ROUTER,
+            DhcpPacket.DHCP_DNS_SERVER,
+            DhcpPacket.DHCP_LEASE_TIME,
+    };
+
+    public static final String DHCP_HOSTNAME = "testhostname";
+
+    private final ArrayMap<MacAddress, TetheredDevice> mTetheredDevices;
+    private final TapPacketReader mDownstreamReader;
+
+    public TetheringTester(TapPacketReader downstream) {
+        if (downstream == null) fail("Downstream reader could not be NULL");
+
+        mDownstreamReader = downstream;
+        mTetheredDevices = new ArrayMap<>();
+    }
+
+    public TetheredDevice createTetheredDevice(MacAddress macAddr) throws Exception {
+        if (mTetheredDevices.get(macAddr) != null) {
+            fail("Tethered device already created");
+        }
+
+        TetheredDevice tethered = new TetheredDevice(macAddr);
+        mTetheredDevices.put(macAddr, tethered);
+
+        return tethered;
+    }
+
+    public class TetheredDevice {
+        private final MacAddress mMacAddr;
+
+        public final Inet4Address mIpv4Addr;
+
+        private TetheredDevice(MacAddress mac) throws Exception {
+            mMacAddr = mac;
+
+            DhcpResults dhcpResults = runDhcp(mMacAddr.toByteArray());
+            mIpv4Addr = (Inet4Address) dhcpResults.ipAddress.getAddress();
+        }
+    }
+
+    /** Simulate dhcp client to obtain ipv4 address. */
+    public DhcpResults runDhcp(byte[] clientMacAddr)
+            throws Exception {
+        // We have to retransmit DHCP requests because IpServer declares itself to be ready before
+        // its DhcpServer is actually started. TODO: fix this race and remove this loop.
+        DhcpPacket offerPacket = null;
+        for (int i = 0; i < DHCP_DISCOVER_ATTEMPTS; i++) {
+            Log.d(TAG, "Sending DHCP discover");
+            sendDhcpDiscover(clientMacAddr);
+            offerPacket = getNextDhcpPacket();
+            if (offerPacket instanceof DhcpOfferPacket) break;
+        }
+        if (!(offerPacket instanceof DhcpOfferPacket)) {
+            throw new TimeoutException("No DHCPOFFER received on interface within timeout");
+        }
+
+        sendDhcpRequest(offerPacket, clientMacAddr);
+        DhcpPacket ackPacket = getNextDhcpPacket();
+        if (!(ackPacket instanceof DhcpAckPacket)) {
+            throw new TimeoutException("No DHCPACK received on interface within timeout");
+        }
+
+        return ackPacket.toDhcpResults();
+    }
+
+    private void sendDhcpDiscover(byte[] macAddress) throws Exception {
+        ByteBuffer packet = DhcpPacket.buildDiscoverPacket(DhcpPacket.ENCAP_L2,
+                new Random().nextInt() /* transactionId */, (short) 0 /* secs */,
+                macAddress,  false /* unicast */, DHCP_REQUESTED_PARAMS,
+                false /* rapid commit */,  DHCP_HOSTNAME);
+        mDownstreamReader.sendResponse(packet);
+    }
+
+    private void sendDhcpRequest(DhcpPacket offerPacket, byte[] macAddress)
+            throws Exception {
+        DhcpResults results = offerPacket.toDhcpResults();
+        Inet4Address clientIp = (Inet4Address) results.ipAddress.getAddress();
+        Inet4Address serverIdentifier = results.serverAddress;
+        ByteBuffer packet = DhcpPacket.buildRequestPacket(DhcpPacket.ENCAP_L2,
+                0 /* transactionId */, (short) 0 /* secs */, DhcpPacket.INADDR_ANY /* clientIp */,
+                false /* broadcast */, macAddress, clientIp /* requestedIpAddress */,
+                serverIdentifier, DHCP_REQUESTED_PARAMS, DHCP_HOSTNAME);
+        mDownstreamReader.sendResponse(packet);
+    }
+
+    private DhcpPacket getNextDhcpPacket() {
+        return getNextMatchedPacket((p) -> {
+            try {
+                return DhcpPacket.decodeFullPacket(p, p.length, DhcpPacket.ENCAP_L2);
+            } catch (DhcpPacket.ParseException e) {
+                // Not a DHCP packet. Continue.
+            }
+
+            return null;
+        });
+    }
+
+    private <R> R getNextMatchedPacket(Function<byte[], R> match) {
+        byte[] packet;
+        R result;
+        while ((packet = mDownstreamReader.popPacket(PACKET_READ_TIMEOUT_MS)) != null) {
+            result = match.apply(packet);
+
+            if (result != null) return result;
+        }
+
+        return null;
+    }
+}
diff --git a/framework/src/android/net/ConnectivityManager.java b/framework/src/android/net/ConnectivityManager.java
index 0c95732..4f10abc 100644
--- a/framework/src/android/net/ConnectivityManager.java
+++ b/framework/src/android/net/ConnectivityManager.java
@@ -1163,10 +1163,11 @@
      * default data network.  In the event that the current active default data
      * network disconnects, the returned {@code Network} object will no longer
      * be usable.  This will return {@code null} when there is no default
-     * network.
+     * network, or when the default network is blocked.
      *
      * @return a {@link Network} object for the current default network or
-     *        {@code null} if no default network is currently active
+     *        {@code null} if no default network is currently active or if
+     *        the default network is blocked for the caller
      */
     @RequiresPermission(android.Manifest.permission.ACCESS_NETWORK_STATE)
     @Nullable
diff --git a/tests/common/Android.bp b/tests/common/Android.bp
index e1fab09..9b136d5 100644
--- a/tests/common/Android.bp
+++ b/tests/common/Android.bp
@@ -115,6 +115,7 @@
         // meaning @hide APIs in framework-connectivity are resolved before @SystemApi
         // stubs in framework
         "framework-connectivity.impl",
+        "framework-tethering.impl",
         "framework",
 
         // if sdk_version="" this gets automatically included, but here we need to add manually.