diff --git a/Tethering/tests/unit/Android.bp b/Tethering/tests/unit/Android.bp
index 089bbd3..da62107 100644
--- a/Tethering/tests/unit/Android.bp
+++ b/Tethering/tests/unit/Android.bp
@@ -43,5 +43,8 @@
     name: "tethering-tests-src",
     srcs: [
         "src/com/android/server/connectivity/tethering/TetheringConfigurationTest.java",
+        "src/android/net/dhcp/DhcpServingParamsParcelExtTest.java",
+        "src/android/net/ip/IpServerTest.java",
+        "src/android/net/util/InterfaceSetTest.java",
     ],
 }
diff --git a/Tethering/tests/unit/src/android/net/dhcp/DhcpServingParamsParcelExtTest.java b/Tethering/tests/unit/src/android/net/dhcp/DhcpServingParamsParcelExtTest.java
new file mode 100644
index 0000000..e01ac7f
--- /dev/null
+++ b/Tethering/tests/unit/src/android/net/dhcp/DhcpServingParamsParcelExtTest.java
@@ -0,0 +1,114 @@
+/*
+ * 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.dhcp;
+
+import static android.net.InetAddresses.parseNumericAddress;
+
+import static com.google.android.collect.Sets.newHashSet;
+
+import static org.junit.Assert.assertEquals;
+import static org.junit.Assert.assertFalse;
+import static org.junit.Assert.assertTrue;
+
+import android.net.LinkAddress;
+
+import androidx.test.filters.SmallTest;
+import androidx.test.runner.AndroidJUnit4;
+
+import org.junit.Before;
+import org.junit.Test;
+import org.junit.runner.RunWith;
+
+import java.net.Inet4Address;
+import java.util.Set;
+import java.util.stream.Collectors;
+import java.util.stream.IntStream;
+
+@RunWith(AndroidJUnit4.class)
+@SmallTest
+public class DhcpServingParamsParcelExtTest {
+    private static final Inet4Address TEST_ADDRESS = inet4Addr("192.168.0.123");
+    private static final int TEST_ADDRESS_PARCELED = 0xc0a8007b;
+    private static final int TEST_PREFIX_LENGTH = 17;
+    private static final int TEST_LEASE_TIME_SECS = 120;
+    private static final int TEST_MTU = 1000;
+    private static final Set<Inet4Address> TEST_ADDRESS_SET =
+            newHashSet(inet4Addr("192.168.1.123"), inet4Addr("192.168.1.124"));
+    private static final Set<Integer> TEST_ADDRESS_SET_PARCELED =
+            newHashSet(0xc0a8017b, 0xc0a8017c);
+
+    private DhcpServingParamsParcelExt mParcel;
+
+    @Before
+    public void setUp() {
+        mParcel = new DhcpServingParamsParcelExt();
+    }
+
+    @Test
+    public void testSetServerAddr() {
+        mParcel.setServerAddr(new LinkAddress(TEST_ADDRESS, TEST_PREFIX_LENGTH));
+
+        assertEquals(TEST_ADDRESS_PARCELED, mParcel.serverAddr);
+        assertEquals(TEST_PREFIX_LENGTH, mParcel.serverAddrPrefixLength);
+    }
+
+    @Test
+    public void testSetDefaultRouters() {
+        mParcel.setDefaultRouters(TEST_ADDRESS_SET);
+        assertEquals(TEST_ADDRESS_SET_PARCELED, asSet(mParcel.defaultRouters));
+    }
+
+    @Test
+    public void testSetDnsServers() {
+        mParcel.setDnsServers(TEST_ADDRESS_SET);
+        assertEquals(TEST_ADDRESS_SET_PARCELED, asSet(mParcel.dnsServers));
+    }
+
+    @Test
+    public void testSetExcludedAddrs() {
+        mParcel.setExcludedAddrs(TEST_ADDRESS_SET);
+        assertEquals(TEST_ADDRESS_SET_PARCELED, asSet(mParcel.excludedAddrs));
+    }
+
+    @Test
+    public void testSetDhcpLeaseTimeSecs() {
+        mParcel.setDhcpLeaseTimeSecs(TEST_LEASE_TIME_SECS);
+        assertEquals(TEST_LEASE_TIME_SECS, mParcel.dhcpLeaseTimeSecs);
+    }
+
+    @Test
+    public void testSetLinkMtu() {
+        mParcel.setLinkMtu(TEST_MTU);
+        assertEquals(TEST_MTU, mParcel.linkMtu);
+    }
+
+    @Test
+    public void testSetMetered() {
+        mParcel.setMetered(true);
+        assertTrue(mParcel.metered);
+        mParcel.setMetered(false);
+        assertFalse(mParcel.metered);
+    }
+
+    private static Inet4Address inet4Addr(String addr) {
+        return (Inet4Address) parseNumericAddress(addr);
+    }
+
+    private static Set<Integer> asSet(int[] ints) {
+        return IntStream.of(ints).boxed().collect(Collectors.toSet());
+    }
+}
diff --git a/Tethering/tests/unit/src/android/net/ip/IpServerTest.java b/Tethering/tests/unit/src/android/net/ip/IpServerTest.java
new file mode 100644
index 0000000..4358cd6
--- /dev/null
+++ b/Tethering/tests/unit/src/android/net/ip/IpServerTest.java
@@ -0,0 +1,526 @@
+/*
+ * Copyright (C) 2016 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.ip;
+
+import static android.net.ConnectivityManager.TETHERING_BLUETOOTH;
+import static android.net.ConnectivityManager.TETHERING_USB;
+import static android.net.ConnectivityManager.TETHERING_WIFI;
+import static android.net.ConnectivityManager.TETHERING_WIFI_P2P;
+import static android.net.ConnectivityManager.TETHER_ERROR_ENABLE_NAT_ERROR;
+import static android.net.ConnectivityManager.TETHER_ERROR_NO_ERROR;
+import static android.net.ConnectivityManager.TETHER_ERROR_TETHER_IFACE_ERROR;
+import static android.net.dhcp.IDhcpServer.STATUS_SUCCESS;
+import static android.net.ip.IpServer.STATE_AVAILABLE;
+import static android.net.ip.IpServer.STATE_LOCAL_ONLY;
+import static android.net.ip.IpServer.STATE_TETHERED;
+import static android.net.ip.IpServer.STATE_UNAVAILABLE;
+import static android.net.shared.Inet4AddressUtils.intToInet4AddressHTH;
+
+import static org.junit.Assert.assertEquals;
+import static org.junit.Assert.assertFalse;
+import static org.junit.Assert.assertNotNull;
+import static org.junit.Assert.assertTrue;
+import static org.junit.Assert.fail;
+import static org.mockito.ArgumentMatchers.argThat;
+import static org.mockito.Matchers.any;
+import static org.mockito.Matchers.anyString;
+import static org.mockito.Matchers.eq;
+import static org.mockito.Mockito.doAnswer;
+import static org.mockito.Mockito.doThrow;
+import static org.mockito.Mockito.inOrder;
+import static org.mockito.Mockito.never;
+import static org.mockito.Mockito.reset;
+import static org.mockito.Mockito.timeout;
+import static org.mockito.Mockito.times;
+import static org.mockito.Mockito.verify;
+import static org.mockito.Mockito.verifyNoMoreInteractions;
+import static org.mockito.Mockito.when;
+
+import android.net.INetd;
+import android.net.INetworkStatsService;
+import android.net.InterfaceConfiguration;
+import android.net.IpPrefix;
+import android.net.LinkAddress;
+import android.net.LinkProperties;
+import android.net.MacAddress;
+import android.net.RouteInfo;
+import android.net.dhcp.DhcpServingParamsParcel;
+import android.net.dhcp.IDhcpServer;
+import android.net.dhcp.IDhcpServerCallbacks;
+import android.net.util.InterfaceParams;
+import android.net.util.InterfaceSet;
+import android.net.util.SharedLog;
+import android.os.INetworkManagementService;
+import android.os.RemoteException;
+import android.os.test.TestLooper;
+import android.text.TextUtils;
+
+import androidx.test.filters.SmallTest;
+import androidx.test.runner.AndroidJUnit4;
+
+import org.junit.Before;
+import org.junit.Test;
+import org.junit.runner.RunWith;
+import org.mockito.ArgumentCaptor;
+import org.mockito.Captor;
+import org.mockito.InOrder;
+import org.mockito.Mock;
+import org.mockito.MockitoAnnotations;
+
+import java.net.Inet4Address;
+
+@RunWith(AndroidJUnit4.class)
+@SmallTest
+public class IpServerTest {
+    private static final String IFACE_NAME = "testnet1";
+    private static final String UPSTREAM_IFACE = "upstream0";
+    private static final String UPSTREAM_IFACE2 = "upstream1";
+    private static final int DHCP_LEASE_TIME_SECS = 3600;
+
+    private static final InterfaceParams TEST_IFACE_PARAMS = new InterfaceParams(
+            IFACE_NAME, 42 /* index */, MacAddress.ALL_ZEROS_ADDRESS, 1500 /* defaultMtu */);
+
+    private static final int MAKE_DHCPSERVER_TIMEOUT_MS = 1000;
+
+    @Mock private INetworkManagementService mNMService;
+    @Mock private INetd mNetd;
+    @Mock private INetworkStatsService mStatsService;
+    @Mock private IpServer.Callback mCallback;
+    @Mock private InterfaceConfiguration mInterfaceConfiguration;
+    @Mock private SharedLog mSharedLog;
+    @Mock private IDhcpServer mDhcpServer;
+    @Mock private RouterAdvertisementDaemon mRaDaemon;
+    @Mock private IpServer.Dependencies mDependencies;
+
+    @Captor private ArgumentCaptor<DhcpServingParamsParcel> mDhcpParamsCaptor;
+
+    private final TestLooper mLooper = new TestLooper();
+    private final ArgumentCaptor<LinkProperties> mLinkPropertiesCaptor =
+            ArgumentCaptor.forClass(LinkProperties.class);
+    private IpServer mIpServer;
+
+    private void initStateMachine(int interfaceType) throws Exception {
+        initStateMachine(interfaceType, false /* usingLegacyDhcp */);
+    }
+
+    private void initStateMachine(int interfaceType, boolean usingLegacyDhcp) throws Exception {
+        doAnswer(inv -> {
+            final IDhcpServerCallbacks cb = inv.getArgument(2);
+            new Thread(() -> {
+                try {
+                    cb.onDhcpServerCreated(STATUS_SUCCESS, mDhcpServer);
+                } catch (RemoteException e) {
+                    fail(e.getMessage());
+                }
+            }).run();
+            return null;
+        }).when(mDependencies).makeDhcpServer(any(), mDhcpParamsCaptor.capture(), any());
+        when(mDependencies.getRouterAdvertisementDaemon(any())).thenReturn(mRaDaemon);
+        when(mDependencies.getInterfaceParams(IFACE_NAME)).thenReturn(TEST_IFACE_PARAMS);
+        when(mDependencies.getNetdService()).thenReturn(mNetd);
+
+        mIpServer = new IpServer(
+                IFACE_NAME, mLooper.getLooper(), interfaceType, mSharedLog,
+                mNMService, mStatsService, mCallback, usingLegacyDhcp, mDependencies);
+        mIpServer.start();
+        // Starting the state machine always puts us in a consistent state and notifies
+        // the rest of the world that we've changed from an unknown to available state.
+        mLooper.dispatchAll();
+        reset(mNMService, mStatsService, mCallback);
+        when(mNMService.getInterfaceConfig(IFACE_NAME)).thenReturn(mInterfaceConfiguration);
+
+        when(mRaDaemon.start()).thenReturn(true);
+    }
+
+    private void initTetheredStateMachine(int interfaceType, String upstreamIface)
+            throws Exception {
+        initTetheredStateMachine(interfaceType, upstreamIface, false);
+    }
+
+    private void initTetheredStateMachine(int interfaceType, String upstreamIface,
+            boolean usingLegacyDhcp) throws Exception {
+        initStateMachine(interfaceType, usingLegacyDhcp);
+        dispatchCommand(IpServer.CMD_TETHER_REQUESTED, STATE_TETHERED);
+        if (upstreamIface != null) {
+            dispatchTetherConnectionChanged(upstreamIface);
+        }
+        reset(mNMService, mStatsService, mCallback);
+        when(mNMService.getInterfaceConfig(IFACE_NAME)).thenReturn(mInterfaceConfiguration);
+    }
+
+    @Before public void setUp() throws Exception {
+        MockitoAnnotations.initMocks(this);
+        when(mSharedLog.forSubComponent(anyString())).thenReturn(mSharedLog);
+    }
+
+    @Test
+    public void startsOutAvailable() {
+        mIpServer = new IpServer(IFACE_NAME, mLooper.getLooper(),
+                TETHERING_BLUETOOTH, mSharedLog, mNMService, mStatsService, mCallback,
+                false /* usingLegacyDhcp */, mDependencies);
+        mIpServer.start();
+        mLooper.dispatchAll();
+        verify(mCallback).updateInterfaceState(
+                mIpServer, STATE_AVAILABLE, TETHER_ERROR_NO_ERROR);
+        verify(mCallback).updateLinkProperties(eq(mIpServer), any(LinkProperties.class));
+        verifyNoMoreInteractions(mCallback, mNMService, mStatsService);
+    }
+
+    @Test
+    public void shouldDoNothingUntilRequested() throws Exception {
+        initStateMachine(TETHERING_BLUETOOTH);
+        final int [] noOp_commands = {
+            IpServer.CMD_TETHER_UNREQUESTED,
+            IpServer.CMD_IP_FORWARDING_ENABLE_ERROR,
+            IpServer.CMD_IP_FORWARDING_DISABLE_ERROR,
+            IpServer.CMD_START_TETHERING_ERROR,
+            IpServer.CMD_STOP_TETHERING_ERROR,
+            IpServer.CMD_SET_DNS_FORWARDERS_ERROR,
+            IpServer.CMD_TETHER_CONNECTION_CHANGED
+        };
+        for (int command : noOp_commands) {
+            // None of these commands should trigger us to request action from
+            // the rest of the system.
+            dispatchCommand(command);
+            verifyNoMoreInteractions(mNMService, mStatsService, mCallback);
+        }
+    }
+
+    @Test
+    public void handlesImmediateInterfaceDown() throws Exception {
+        initStateMachine(TETHERING_BLUETOOTH);
+
+        dispatchCommand(IpServer.CMD_INTERFACE_DOWN);
+        verify(mCallback).updateInterfaceState(
+                mIpServer, STATE_UNAVAILABLE, TETHER_ERROR_NO_ERROR);
+        verify(mCallback).updateLinkProperties(eq(mIpServer), any(LinkProperties.class));
+        verifyNoMoreInteractions(mNMService, mStatsService, mCallback);
+    }
+
+    @Test
+    public void canBeTethered() throws Exception {
+        initStateMachine(TETHERING_BLUETOOTH);
+
+        dispatchCommand(IpServer.CMD_TETHER_REQUESTED, STATE_TETHERED);
+        InOrder inOrder = inOrder(mCallback, mNMService);
+        inOrder.verify(mNMService).tetherInterface(IFACE_NAME);
+        inOrder.verify(mCallback).updateInterfaceState(
+                mIpServer, STATE_TETHERED, TETHER_ERROR_NO_ERROR);
+        inOrder.verify(mCallback).updateLinkProperties(
+                eq(mIpServer), any(LinkProperties.class));
+        verifyNoMoreInteractions(mNMService, mStatsService, mCallback);
+    }
+
+    @Test
+    public void canUnrequestTethering() throws Exception {
+        initTetheredStateMachine(TETHERING_BLUETOOTH, null);
+
+        dispatchCommand(IpServer.CMD_TETHER_UNREQUESTED);
+        InOrder inOrder = inOrder(mNMService, mNetd, mStatsService, mCallback);
+        inOrder.verify(mNMService).untetherInterface(IFACE_NAME);
+        inOrder.verify(mNetd).interfaceSetCfg(argThat(cfg -> IFACE_NAME.equals(cfg.ifName)));
+        inOrder.verify(mCallback).updateInterfaceState(
+                mIpServer, STATE_AVAILABLE, TETHER_ERROR_NO_ERROR);
+        inOrder.verify(mCallback).updateLinkProperties(
+                eq(mIpServer), any(LinkProperties.class));
+        verifyNoMoreInteractions(mNMService, mStatsService, mCallback);
+    }
+
+    @Test
+    public void canBeTetheredAsUsb() throws Exception {
+        initStateMachine(TETHERING_USB);
+
+        dispatchCommand(IpServer.CMD_TETHER_REQUESTED, STATE_TETHERED);
+        InOrder inOrder = inOrder(mCallback, mNMService);
+        inOrder.verify(mNMService).getInterfaceConfig(IFACE_NAME);
+        inOrder.verify(mNMService).setInterfaceConfig(IFACE_NAME, mInterfaceConfiguration);
+        inOrder.verify(mNMService).tetherInterface(IFACE_NAME);
+        inOrder.verify(mCallback).updateInterfaceState(
+                mIpServer, STATE_TETHERED, TETHER_ERROR_NO_ERROR);
+        inOrder.verify(mCallback).updateLinkProperties(
+                eq(mIpServer), mLinkPropertiesCaptor.capture());
+        assertIPv4AddressAndDirectlyConnectedRoute(mLinkPropertiesCaptor.getValue());
+        verifyNoMoreInteractions(mNMService, mStatsService, mCallback);
+    }
+
+    @Test
+    public void canBeTetheredAsWifiP2p() throws Exception {
+        initStateMachine(TETHERING_WIFI_P2P);
+
+        dispatchCommand(IpServer.CMD_TETHER_REQUESTED, STATE_LOCAL_ONLY);
+        InOrder inOrder = inOrder(mCallback, mNMService);
+        inOrder.verify(mNMService).getInterfaceConfig(IFACE_NAME);
+        inOrder.verify(mNMService).setInterfaceConfig(IFACE_NAME, mInterfaceConfiguration);
+        inOrder.verify(mNMService).tetherInterface(IFACE_NAME);
+        inOrder.verify(mCallback).updateInterfaceState(
+                mIpServer, STATE_LOCAL_ONLY, TETHER_ERROR_NO_ERROR);
+        inOrder.verify(mCallback).updateLinkProperties(
+                eq(mIpServer), mLinkPropertiesCaptor.capture());
+        assertIPv4AddressAndDirectlyConnectedRoute(mLinkPropertiesCaptor.getValue());
+        verifyNoMoreInteractions(mNMService, mStatsService, mCallback);
+    }
+
+    @Test
+    public void handlesFirstUpstreamChange() throws Exception {
+        initTetheredStateMachine(TETHERING_BLUETOOTH, null);
+
+        // Telling the state machine about its upstream interface triggers
+        // a little more configuration.
+        dispatchTetherConnectionChanged(UPSTREAM_IFACE);
+        InOrder inOrder = inOrder(mNMService);
+        inOrder.verify(mNMService).enableNat(IFACE_NAME, UPSTREAM_IFACE);
+        inOrder.verify(mNMService).startInterfaceForwarding(IFACE_NAME, UPSTREAM_IFACE);
+        verifyNoMoreInteractions(mNMService, mStatsService, mCallback);
+    }
+
+    @Test
+    public void handlesChangingUpstream() throws Exception {
+        initTetheredStateMachine(TETHERING_BLUETOOTH, UPSTREAM_IFACE);
+
+        dispatchTetherConnectionChanged(UPSTREAM_IFACE2);
+        InOrder inOrder = inOrder(mNMService, mStatsService);
+        inOrder.verify(mStatsService).forceUpdate();
+        inOrder.verify(mNMService).stopInterfaceForwarding(IFACE_NAME, UPSTREAM_IFACE);
+        inOrder.verify(mNMService).disableNat(IFACE_NAME, UPSTREAM_IFACE);
+        inOrder.verify(mNMService).enableNat(IFACE_NAME, UPSTREAM_IFACE2);
+        inOrder.verify(mNMService).startInterfaceForwarding(IFACE_NAME, UPSTREAM_IFACE2);
+        verifyNoMoreInteractions(mNMService, mStatsService, mCallback);
+    }
+
+    @Test
+    public void handlesChangingUpstreamNatFailure() throws Exception {
+        initTetheredStateMachine(TETHERING_WIFI, UPSTREAM_IFACE);
+
+        doThrow(RemoteException.class).when(mNMService).enableNat(IFACE_NAME, UPSTREAM_IFACE2);
+
+        dispatchTetherConnectionChanged(UPSTREAM_IFACE2);
+        InOrder inOrder = inOrder(mNMService, mStatsService);
+        inOrder.verify(mStatsService).forceUpdate();
+        inOrder.verify(mNMService).stopInterfaceForwarding(IFACE_NAME, UPSTREAM_IFACE);
+        inOrder.verify(mNMService).disableNat(IFACE_NAME, UPSTREAM_IFACE);
+        inOrder.verify(mNMService).enableNat(IFACE_NAME, UPSTREAM_IFACE2);
+        inOrder.verify(mStatsService).forceUpdate();
+        inOrder.verify(mNMService).stopInterfaceForwarding(IFACE_NAME, UPSTREAM_IFACE2);
+        inOrder.verify(mNMService).disableNat(IFACE_NAME, UPSTREAM_IFACE2);
+    }
+
+    @Test
+    public void handlesChangingUpstreamInterfaceForwardingFailure() throws Exception {
+        initTetheredStateMachine(TETHERING_WIFI, UPSTREAM_IFACE);
+
+        doThrow(RemoteException.class).when(mNMService).startInterfaceForwarding(
+                IFACE_NAME, UPSTREAM_IFACE2);
+
+        dispatchTetherConnectionChanged(UPSTREAM_IFACE2);
+        InOrder inOrder = inOrder(mNMService, mStatsService);
+        inOrder.verify(mStatsService).forceUpdate();
+        inOrder.verify(mNMService).stopInterfaceForwarding(IFACE_NAME, UPSTREAM_IFACE);
+        inOrder.verify(mNMService).disableNat(IFACE_NAME, UPSTREAM_IFACE);
+        inOrder.verify(mNMService).enableNat(IFACE_NAME, UPSTREAM_IFACE2);
+        inOrder.verify(mNMService).startInterfaceForwarding(IFACE_NAME, UPSTREAM_IFACE2);
+        inOrder.verify(mStatsService).forceUpdate();
+        inOrder.verify(mNMService).stopInterfaceForwarding(IFACE_NAME, UPSTREAM_IFACE2);
+        inOrder.verify(mNMService).disableNat(IFACE_NAME, UPSTREAM_IFACE2);
+    }
+
+    @Test
+    public void canUnrequestTetheringWithUpstream() throws Exception {
+        initTetheredStateMachine(TETHERING_BLUETOOTH, UPSTREAM_IFACE);
+
+        dispatchCommand(IpServer.CMD_TETHER_UNREQUESTED);
+        InOrder inOrder = inOrder(mNMService, mNetd, mStatsService, mCallback);
+        inOrder.verify(mStatsService).forceUpdate();
+        inOrder.verify(mNMService).stopInterfaceForwarding(IFACE_NAME, UPSTREAM_IFACE);
+        inOrder.verify(mNMService).disableNat(IFACE_NAME, UPSTREAM_IFACE);
+        inOrder.verify(mNMService).untetherInterface(IFACE_NAME);
+        inOrder.verify(mNetd).interfaceSetCfg(argThat(cfg -> IFACE_NAME.equals(cfg.ifName)));
+        inOrder.verify(mCallback).updateInterfaceState(
+                mIpServer, STATE_AVAILABLE, TETHER_ERROR_NO_ERROR);
+        inOrder.verify(mCallback).updateLinkProperties(
+                eq(mIpServer), any(LinkProperties.class));
+        verifyNoMoreInteractions(mNMService, mStatsService, mCallback);
+    }
+
+    @Test
+    public void interfaceDownLeadsToUnavailable() throws Exception {
+        for (boolean shouldThrow : new boolean[]{true, false}) {
+            initTetheredStateMachine(TETHERING_USB, null);
+
+            if (shouldThrow) {
+                doThrow(RemoteException.class).when(mNMService).untetherInterface(IFACE_NAME);
+            }
+            dispatchCommand(IpServer.CMD_INTERFACE_DOWN);
+            InOrder usbTeardownOrder = inOrder(mNMService, mInterfaceConfiguration, mCallback);
+            usbTeardownOrder.verify(mInterfaceConfiguration).setInterfaceDown();
+            usbTeardownOrder.verify(mNMService).setInterfaceConfig(
+                    IFACE_NAME, mInterfaceConfiguration);
+            usbTeardownOrder.verify(mCallback).updateInterfaceState(
+                    mIpServer, STATE_UNAVAILABLE, TETHER_ERROR_NO_ERROR);
+            usbTeardownOrder.verify(mCallback).updateLinkProperties(
+                    eq(mIpServer), mLinkPropertiesCaptor.capture());
+            assertNoAddressesNorRoutes(mLinkPropertiesCaptor.getValue());
+        }
+    }
+
+    @Test
+    public void usbShouldBeTornDownOnTetherError() throws Exception {
+        initStateMachine(TETHERING_USB);
+
+        doThrow(RemoteException.class).when(mNMService).tetherInterface(IFACE_NAME);
+        dispatchCommand(IpServer.CMD_TETHER_REQUESTED, STATE_TETHERED);
+        InOrder usbTeardownOrder = inOrder(mNMService, mInterfaceConfiguration, mCallback);
+        usbTeardownOrder.verify(mInterfaceConfiguration).setInterfaceDown();
+        usbTeardownOrder.verify(mNMService).setInterfaceConfig(
+                IFACE_NAME, mInterfaceConfiguration);
+        usbTeardownOrder.verify(mCallback).updateInterfaceState(
+                mIpServer, STATE_AVAILABLE, TETHER_ERROR_TETHER_IFACE_ERROR);
+        usbTeardownOrder.verify(mCallback).updateLinkProperties(
+                eq(mIpServer), mLinkPropertiesCaptor.capture());
+        assertNoAddressesNorRoutes(mLinkPropertiesCaptor.getValue());
+    }
+
+    @Test
+    public void shouldTearDownUsbOnUpstreamError() throws Exception {
+        initTetheredStateMachine(TETHERING_USB, null);
+
+        doThrow(RemoteException.class).when(mNMService).enableNat(anyString(), anyString());
+        dispatchTetherConnectionChanged(UPSTREAM_IFACE);
+        InOrder usbTeardownOrder = inOrder(mNMService, mInterfaceConfiguration, mCallback);
+        usbTeardownOrder.verify(mInterfaceConfiguration).setInterfaceDown();
+        usbTeardownOrder.verify(mNMService).setInterfaceConfig(IFACE_NAME, mInterfaceConfiguration);
+        usbTeardownOrder.verify(mCallback).updateInterfaceState(
+                mIpServer, STATE_AVAILABLE, TETHER_ERROR_ENABLE_NAT_ERROR);
+        usbTeardownOrder.verify(mCallback).updateLinkProperties(
+                eq(mIpServer), mLinkPropertiesCaptor.capture());
+        assertNoAddressesNorRoutes(mLinkPropertiesCaptor.getValue());
+    }
+
+    @Test
+    public void ignoresDuplicateUpstreamNotifications() throws Exception {
+        initTetheredStateMachine(TETHERING_WIFI, UPSTREAM_IFACE);
+
+        verifyNoMoreInteractions(mNMService, mStatsService, mCallback);
+
+        for (int i = 0; i < 5; i++) {
+            dispatchTetherConnectionChanged(UPSTREAM_IFACE);
+            verifyNoMoreInteractions(mNMService, mStatsService, mCallback);
+        }
+    }
+
+    @Test
+    public void startsDhcpServer() throws Exception {
+        initTetheredStateMachine(TETHERING_WIFI, UPSTREAM_IFACE);
+        dispatchTetherConnectionChanged(UPSTREAM_IFACE);
+
+        assertDhcpStarted(new IpPrefix("192.168.43.0/24"));
+    }
+
+    @Test
+    public void startsDhcpServerOnBluetooth() throws Exception {
+        initTetheredStateMachine(TETHERING_BLUETOOTH, UPSTREAM_IFACE);
+        dispatchTetherConnectionChanged(UPSTREAM_IFACE);
+
+        assertDhcpStarted(new IpPrefix("192.168.44.0/24"));
+    }
+
+    @Test
+    public void startsDhcpServerOnWifiP2p() throws Exception {
+        initTetheredStateMachine(TETHERING_WIFI_P2P, UPSTREAM_IFACE);
+        dispatchTetherConnectionChanged(UPSTREAM_IFACE);
+
+        assertDhcpStarted(new IpPrefix("192.168.49.0/24"));
+    }
+
+    @Test
+    public void doesNotStartDhcpServerIfDisabled() throws Exception {
+        initTetheredStateMachine(TETHERING_WIFI, UPSTREAM_IFACE, true /* usingLegacyDhcp */);
+        dispatchTetherConnectionChanged(UPSTREAM_IFACE);
+
+        verify(mDependencies, never()).makeDhcpServer(any(), any(), any());
+    }
+
+    private void assertDhcpStarted(IpPrefix expectedPrefix) throws Exception {
+        verify(mDependencies, times(1)).makeDhcpServer(eq(IFACE_NAME), any(), any());
+        verify(mDhcpServer, timeout(MAKE_DHCPSERVER_TIMEOUT_MS).times(1)).start(any());
+        final DhcpServingParamsParcel params = mDhcpParamsCaptor.getValue();
+        // Last address byte is random
+        assertTrue(expectedPrefix.contains(intToInet4AddressHTH(params.serverAddr)));
+        assertEquals(expectedPrefix.getPrefixLength(), params.serverAddrPrefixLength);
+        assertEquals(1, params.defaultRouters.length);
+        assertEquals(params.serverAddr, params.defaultRouters[0]);
+        assertEquals(1, params.dnsServers.length);
+        assertEquals(params.serverAddr, params.dnsServers[0]);
+        assertEquals(DHCP_LEASE_TIME_SECS, params.dhcpLeaseTimeSecs);
+    }
+
+    /**
+     * Send a command to the state machine under test, and run the event loop to idle.
+     *
+     * @param command One of the IpServer.CMD_* constants.
+     * @param arg1 An additional argument to pass.
+     */
+    private void dispatchCommand(int command, int arg1) {
+        mIpServer.sendMessage(command, arg1);
+        mLooper.dispatchAll();
+    }
+
+    /**
+     * Send a command to the state machine under test, and run the event loop to idle.
+     *
+     * @param command One of the IpServer.CMD_* constants.
+     */
+    private void dispatchCommand(int command) {
+        mIpServer.sendMessage(command);
+        mLooper.dispatchAll();
+    }
+
+    /**
+     * Special override to tell the state machine that the upstream interface has changed.
+     *
+     * @see #dispatchCommand(int)
+     * @param upstreamIface String name of upstream interface (or null)
+     */
+    private void dispatchTetherConnectionChanged(String upstreamIface) {
+        mIpServer.sendMessage(IpServer.CMD_TETHER_CONNECTION_CHANGED,
+                new InterfaceSet(upstreamIface));
+        mLooper.dispatchAll();
+    }
+
+    private void assertIPv4AddressAndDirectlyConnectedRoute(LinkProperties lp) {
+        // Find the first IPv4 LinkAddress.
+        LinkAddress addr4 = null;
+        for (LinkAddress addr : lp.getLinkAddresses()) {
+            if (!(addr.getAddress() instanceof Inet4Address)) continue;
+            addr4 = addr;
+            break;
+        }
+        assertNotNull("missing IPv4 address", addr4);
+
+        // Assert the presence of the associated directly connected route.
+        final RouteInfo directlyConnected = new RouteInfo(addr4, null, lp.getInterfaceName());
+        assertTrue("missing directly connected route: '" + directlyConnected.toString() + "'",
+                   lp.getRoutes().contains(directlyConnected));
+    }
+
+    private void assertNoAddressesNorRoutes(LinkProperties lp) {
+        assertTrue(lp.getLinkAddresses().isEmpty());
+        assertTrue(lp.getRoutes().isEmpty());
+        // We also check that interface name is non-empty, because we should
+        // never see an empty interface name in any LinkProperties update.
+        assertFalse(TextUtils.isEmpty(lp.getInterfaceName()));
+    }
+}
diff --git a/Tethering/tests/unit/src/android/net/util/InterfaceSetTest.java b/Tethering/tests/unit/src/android/net/util/InterfaceSetTest.java
new file mode 100644
index 0000000..ea084b6
--- /dev/null
+++ b/Tethering/tests/unit/src/android/net/util/InterfaceSetTest.java
@@ -0,0 +1,61 @@
+/*
+ * 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.util;
+
+import static junit.framework.Assert.assertEquals;
+import static junit.framework.Assert.assertFalse;
+import static junit.framework.Assert.assertTrue;
+
+import androidx.test.filters.SmallTest;
+import androidx.test.runner.AndroidJUnit4;
+
+import org.junit.Test;
+import org.junit.runner.RunWith;
+
+@RunWith(AndroidJUnit4.class)
+@SmallTest
+public class InterfaceSetTest {
+    @Test
+    public void testNullNamesIgnored() {
+        final InterfaceSet set = new InterfaceSet(null, "if1", null, "if2", null);
+        assertEquals(2, set.ifnames.size());
+        assertTrue(set.ifnames.contains("if1"));
+        assertTrue(set.ifnames.contains("if2"));
+    }
+
+    @Test
+    public void testToString() {
+        final InterfaceSet set = new InterfaceSet("if1", "if2");
+        final String setString = set.toString();
+        assertTrue(setString.equals("[if1,if2]") || setString.equals("[if2,if1]"));
+    }
+
+    @Test
+    public void testToString_Empty() {
+        final InterfaceSet set = new InterfaceSet(null, null);
+        assertEquals("[]", set.toString());
+    }
+
+    @Test
+    public void testEquals() {
+        assertEquals(new InterfaceSet(null, "if1", "if2"), new InterfaceSet("if2", "if1"));
+        assertEquals(new InterfaceSet(null, null), new InterfaceSet());
+        assertFalse(new InterfaceSet("if1", "if3").equals(new InterfaceSet("if1", "if2")));
+        assertFalse(new InterfaceSet("if1", "if2").equals(new InterfaceSet("if1")));
+        assertFalse(new InterfaceSet().equals(null));
+    }
+}