diff --git a/core/java/android/os/Build.java b/core/java/android/os/Build.java
index d7d3e58..f88b0ae 100755
--- a/core/java/android/os/Build.java
+++ b/core/java/android/os/Build.java
@@ -138,7 +138,7 @@
      */
     @UnsupportedAppUsage
     @TestApi
-    public static final boolean IS_EMULATOR = getString("ro.kernel.qemu").equals("1");
+    public static final boolean IS_EMULATOR = getString("ro.boot.qemu").equals("1");
 
     /**
      * A hardware serial number, if available. Alphanumeric only, case-insensitive.
diff --git a/libs/hwui/Properties.cpp b/libs/hwui/Properties.cpp
index 446e81e..c6ab119 100644
--- a/libs/hwui/Properties.cpp
+++ b/libs/hwui/Properties.cpp
@@ -124,7 +124,7 @@
     SkAndroidFrameworkTraceUtil::setEnableTracing(
             base::GetBoolProperty(PROPERTY_SKIA_ATRACE_ENABLED, false));
 
-    runningInEmulator = base::GetBoolProperty(PROPERTY_QEMU_KERNEL, false);
+    runningInEmulator = base::GetBoolProperty(PROPERTY_IS_EMULATOR, false);
 
     defaultRenderAhead = std::max(-1, std::min(2, base::GetIntProperty(PROPERTY_RENDERAHEAD,
             render_ahead().value_or(0))));
diff --git a/libs/hwui/Properties.h b/libs/hwui/Properties.h
index d3ecb54d..6ea208e 100644
--- a/libs/hwui/Properties.h
+++ b/libs/hwui/Properties.h
@@ -160,7 +160,7 @@
 /**
  * Property for whether this is running in the emulator.
  */
-#define PROPERTY_QEMU_KERNEL "ro.kernel.qemu"
+#define PROPERTY_IS_EMULATOR "ro.boot.qemu"
 
 #define PROPERTY_RENDERAHEAD "debug.hwui.render_ahead"
 
diff --git a/packages/Connectivity/framework/src/android/net/ConnectivityManager.java b/packages/Connectivity/framework/src/android/net/ConnectivityManager.java
index 8b66928..81590a8 100644
--- a/packages/Connectivity/framework/src/android/net/ConnectivityManager.java
+++ b/packages/Connectivity/framework/src/android/net/ConnectivityManager.java
@@ -18,8 +18,8 @@
 import static android.annotation.SystemApi.Client.MODULE_LIBRARIES;
 import static android.net.NetworkRequest.Type.BACKGROUND_REQUEST;
 import static android.net.NetworkRequest.Type.LISTEN;
+import static android.net.NetworkRequest.Type.LISTEN_FOR_BEST;
 import static android.net.NetworkRequest.Type.REQUEST;
-import static android.net.NetworkRequest.Type.TRACK_BEST;
 import static android.net.NetworkRequest.Type.TRACK_DEFAULT;
 import static android.net.NetworkRequest.Type.TRACK_SYSTEM_DEFAULT;
 import static android.net.QosCallback.QosCallbackRegistrationException;
@@ -4249,7 +4249,7 @@
             @NonNull NetworkCallback networkCallback, @NonNull Handler handler) {
         final NetworkCapabilities nc = request.networkCapabilities;
         final CallbackHandler cbHandler = new CallbackHandler(handler);
-        sendRequestForNetwork(nc, networkCallback, 0, TRACK_BEST, TYPE_NONE, cbHandler);
+        sendRequestForNetwork(nc, networkCallback, 0, LISTEN_FOR_BEST, TYPE_NONE, cbHandler);
     }
 
     /**
diff --git a/packages/Connectivity/framework/src/android/net/NetworkRequest.java b/packages/Connectivity/framework/src/android/net/NetworkRequest.java
index 3fd95ee..dbe3ecc 100644
--- a/packages/Connectivity/framework/src/android/net/NetworkRequest.java
+++ b/packages/Connectivity/framework/src/android/net/NetworkRequest.java
@@ -140,7 +140,7 @@
         REQUEST,
         BACKGROUND_REQUEST,
         TRACK_SYSTEM_DEFAULT,
-        TRACK_BEST,
+        LISTEN_FOR_BEST,
     };
 
     /**
@@ -514,6 +514,15 @@
     }
 
     /**
+     * Returns true iff. this NetworkRequest is of type LISTEN_FOR_BEST.
+     *
+     * @hide
+     */
+    public boolean isListenForBest() {
+        return type == Type.LISTEN_FOR_BEST;
+    }
+
+    /**
      * Returns true iff. the contained NetworkRequest is one that:
      *
      *     - should be associated with at most one satisfying network
diff --git a/packages/SettingsProvider/src/com/android/providers/settings/DatabaseHelper.java b/packages/SettingsProvider/src/com/android/providers/settings/DatabaseHelper.java
index 6568bff..268603f 100644
--- a/packages/SettingsProvider/src/com/android/providers/settings/DatabaseHelper.java
+++ b/packages/SettingsProvider/src/com/android/providers/settings/DatabaseHelper.java
@@ -2445,8 +2445,8 @@
                     R.bool.def_auto_time_zone); // Sync timezone to NITZ
 
             loadSetting(stmt, Settings.Global.STAY_ON_WHILE_PLUGGED_IN,
-                    ("1".equals(SystemProperties.get("ro.kernel.qemu")) ||
-                        res.getBoolean(R.bool.def_stay_on_while_plugged_in))
+                    ("1".equals(SystemProperties.get("ro.boot.qemu"))
+                        || res.getBoolean(R.bool.def_stay_on_while_plugged_in))
                      ? 1 : 0);
 
             loadIntegerSetting(stmt, Settings.Global.WIFI_SLEEP_POLICY,
diff --git a/services/core/java/com/android/server/ConnectivityService.java b/services/core/java/com/android/server/ConnectivityService.java
index 92d9b08..f666490 100644
--- a/services/core/java/com/android/server/ConnectivityService.java
+++ b/services/core/java/com/android/server/ConnectivityService.java
@@ -55,6 +55,7 @@
 import static android.net.NetworkCapabilities.TRANSPORT_VPN;
 import static android.net.NetworkPolicyManager.RULE_NONE;
 import static android.net.NetworkPolicyManager.uidRulesToString;
+import static android.net.NetworkRequest.Type.LISTEN_FOR_BEST;
 import static android.net.shared.NetworkMonitorUtils.isPrivateDnsValidationRequired;
 import static android.os.Process.INVALID_UID;
 import static android.os.Process.VPN_UID;
@@ -3663,6 +3664,7 @@
             mNetworkRequestInfoLogs.log("REGISTER " + nri);
             for (final NetworkRequest req : nri.mRequests) {
                 mNetworkRequests.put(req, nri);
+                // TODO: Consider update signal strength for other types.
                 if (req.isListen()) {
                     for (final NetworkAgentInfo network : mNetworkAgentInfos) {
                         if (req.networkCapabilities.hasSignalStrength()
@@ -3755,18 +3757,19 @@
         // listen requests won't keep up a network satisfying it. If this is not a multilayer
         // request, return immediately. For multilayer requests, check to see if any of the
         // multilayer requests may have a potential satisfier.
-        if (!nri.isMultilayerRequest() && nri.mRequests.get(0).isListen()) {
+        if (!nri.isMultilayerRequest() && (nri.mRequests.get(0).isListen()
+                || nri.mRequests.get(0).isListenForBest())) {
             return false;
         }
         for (final NetworkRequest req : nri.mRequests) {
             // This multilayer listen request is satisfied therefore no further requests need to be
             // evaluated deeming this network not a potential satisfier.
-            if (req.isListen() && nri.getActiveRequest() == req) {
+            if ((req.isListen() || req.isListenForBest()) && nri.getActiveRequest() == req) {
                 return false;
             }
             // As non-multilayer listen requests have already returned, the below would only happen
             // for a multilayer request therefore continue to the next request if available.
-            if (req.isListen()) {
+            if (req.isListen() || req.isListenForBest()) {
                 continue;
             }
             // If this Network is already the highest scoring Network for a request, or if
@@ -5549,8 +5552,10 @@
                 //  request if the app changes network state. http://b/29964605
                 enforceMeteredApnPolicy(networkCapabilities);
                 break;
-            case TRACK_BEST:
-                throw new UnsupportedOperationException("Not implemented yet");
+            case LISTEN_FOR_BEST:
+                enforceAccessPermission();
+                networkCapabilities = new NetworkCapabilities(networkCapabilities);
+                break;
             default:
                 throw new IllegalArgumentException("Unsupported request type " + reqType);
         }
@@ -5558,11 +5563,17 @@
         ensureSufficientPermissionsForRequest(networkCapabilities,
                 Binder.getCallingPid(), callingUid, callingPackageName);
 
-        // Set the UID range for this request to the single UID of the requester, or to an empty
-        // set of UIDs if the caller has the appropriate permission and UIDs have not been set.
+        // Enforce FOREGROUND if the caller does not have permission to use background network.
+        if (reqType == LISTEN_FOR_BEST) {
+            restrictBackgroundRequestForCaller(networkCapabilities);
+        }
+
+        // Set the UID range for this request to the single UID of the requester, unless the
+        // requester has the permission to specify other UIDs.
         // This will overwrite any allowed UIDs in the requested capabilities. Though there
         // are no visible methods to set the UIDs, an app could use reflection to try and get
         // networks for other apps so it's essential that the UIDs are overwritten.
+        // Also set the requester UID and package name in the request.
         restrictRequestUidsForCallerAndSetRequestorInfo(networkCapabilities,
                 callingUid, callingPackageName);
 
diff --git a/services/core/java/com/android/server/clipboard/ClipboardService.java b/services/core/java/com/android/server/clipboard/ClipboardService.java
index d2b642c..b355730 100644
--- a/services/core/java/com/android/server/clipboard/ClipboardService.java
+++ b/services/core/java/com/android/server/clipboard/ClipboardService.java
@@ -158,7 +158,7 @@
 
     private static final String TAG = "ClipboardService";
     private static final boolean IS_EMULATOR =
-        SystemProperties.getBoolean("ro.kernel.qemu", false);
+            SystemProperties.getBoolean("ro.boot.qemu", false);
 
     private final ActivityManagerInternal mAmInternal;
     private final IUriGrantsManager mUgm;
diff --git a/services/core/java/com/android/server/connectivity/NetworkAgentInfo.java b/services/core/java/com/android/server/connectivity/NetworkAgentInfo.java
index 9dcc55b..e44dcf5 100644
--- a/services/core/java/com/android/server/connectivity/NetworkAgentInfo.java
+++ b/services/core/java/com/android/server/connectivity/NetworkAgentInfo.java
@@ -719,6 +719,7 @@
                 break;
 
             case LISTEN:
+            case LISTEN_FOR_BEST:
             case TRACK_DEFAULT:
             case TRACK_SYSTEM_DEFAULT:
                 break;
diff --git a/services/java/com/android/server/SystemServer.java b/services/java/com/android/server/SystemServer.java
index 7bde760..f710f7c 100644
--- a/services/java/com/android/server/SystemServer.java
+++ b/services/java/com/android/server/SystemServer.java
@@ -1127,7 +1127,7 @@
                 false);
         boolean enableLeftyService = SystemProperties.getBoolean("config.enable_lefty", false);
 
-        boolean isEmulator = SystemProperties.get("ro.kernel.qemu").equals("1");
+        boolean isEmulator = SystemProperties.get("ro.boot.qemu").equals("1");
 
         boolean isWatch = context.getPackageManager().hasSystemFeature(
                 PackageManager.FEATURE_WATCH);
diff --git a/tests/net/java/com/android/server/ConnectivityServiceTest.java b/tests/net/java/com/android/server/ConnectivityServiceTest.java
index cc58edf..b894b02 100644
--- a/tests/net/java/com/android/server/ConnectivityServiceTest.java
+++ b/tests/net/java/com/android/server/ConnectivityServiceTest.java
@@ -1079,6 +1079,10 @@
         public void triggerUnfulfillable(NetworkRequest r) {
             super.releaseRequestAsUnfulfillableByAnyFactory(r);
         }
+
+        public void assertNoRequestChanged() {
+            assertNull(mRequestHistory.poll(0, r -> true));
+        }
     }
 
     private Set<UidRange> uidRangesForUids(int... uids) {
@@ -11121,11 +11125,99 @@
         mCm.unregisterNetworkCallback(cellCb);
     }
 
+    // Cannot be part of MockNetworkFactory since it requires method of the test.
+    private void expectNoRequestChanged(@NonNull MockNetworkFactory factory) {
+        waitForIdle();
+        factory.assertNoRequestChanged();
+    }
+
     @Test
-    public void testRegisterBestMatchingNetworkCallback() throws Exception {
-        final NetworkRequest request = new NetworkRequest.Builder().build();
-        assertThrows(UnsupportedOperationException.class,
-                () -> mCm.registerBestMatchingNetworkCallback(request, new NetworkCallback(),
-                        mCsHandlerThread.getThreadHandler()));
+    public void testRegisterBestMatchingNetworkCallback_noIssueToFactory() throws Exception {
+        // Prepare mock mms factory.
+        final HandlerThread handlerThread = new HandlerThread("MockCellularFactory");
+        handlerThread.start();
+        NetworkCapabilities filter = new NetworkCapabilities()
+                .addTransportType(TRANSPORT_CELLULAR)
+                .addCapability(NET_CAPABILITY_MMS);
+        final MockNetworkFactory testFactory = new MockNetworkFactory(handlerThread.getLooper(),
+                mServiceContext, "testFactory", filter, mCsHandlerThread);
+        testFactory.setScoreFilter(40);
+
+        try {
+            // Register the factory and expect it will see default request, because all requests
+            // are sent to all factories.
+            testFactory.register();
+            testFactory.expectRequestAdd();
+            testFactory.assertRequestCountEquals(1);
+            // The factory won't try to start the network since the default request doesn't
+            // match the filter (no INTERNET capability).
+            assertFalse(testFactory.getMyStartRequested());
+
+            // Register callback for listening best matching network. Verify that the request won't
+            // be sent to factory.
+            final TestNetworkCallback bestMatchingCb = new TestNetworkCallback();
+            mCm.registerBestMatchingNetworkCallback(
+                    new NetworkRequest.Builder().addCapability(NET_CAPABILITY_MMS).build(),
+                    bestMatchingCb, mCsHandlerThread.getThreadHandler());
+            bestMatchingCb.assertNoCallback();
+            expectNoRequestChanged(testFactory);
+            testFactory.assertRequestCountEquals(1);
+            assertFalse(testFactory.getMyStartRequested());
+
+            // Fire a normal mms request, verify the factory will only see the request.
+            final TestNetworkCallback mmsNetworkCallback = new TestNetworkCallback();
+            final NetworkRequest mmsRequest = new NetworkRequest.Builder()
+                    .addCapability(NET_CAPABILITY_MMS).build();
+            mCm.requestNetwork(mmsRequest, mmsNetworkCallback);
+            testFactory.expectRequestAdd();
+            testFactory.assertRequestCountEquals(2);
+            assertTrue(testFactory.getMyStartRequested());
+
+            // Unregister best matching callback, verify factory see no change.
+            mCm.unregisterNetworkCallback(bestMatchingCb);
+            expectNoRequestChanged(testFactory);
+            testFactory.assertRequestCountEquals(2);
+            assertTrue(testFactory.getMyStartRequested());
+        } finally {
+            testFactory.terminate();
+        }
+    }
+
+    @Test
+    public void testRegisterBestMatchingNetworkCallback_trackBestNetwork() throws Exception {
+        final TestNetworkCallback bestMatchingCb = new TestNetworkCallback();
+        mCm.registerBestMatchingNetworkCallback(
+                new NetworkRequest.Builder().addCapability(NET_CAPABILITY_TRUSTED).build(),
+                bestMatchingCb, mCsHandlerThread.getThreadHandler());
+
+        mCellNetworkAgent = new TestNetworkAgentWrapper(TRANSPORT_CELLULAR);
+        mCellNetworkAgent.connect(true);
+        bestMatchingCb.expectAvailableThenValidatedCallbacks(mCellNetworkAgent);
+
+        mWiFiNetworkAgent = new TestNetworkAgentWrapper(TRANSPORT_WIFI);
+        mWiFiNetworkAgent.connect(true);
+        bestMatchingCb.expectAvailableDoubleValidatedCallbacks(mWiFiNetworkAgent);
+
+        // Change something on cellular to trigger capabilities changed, since the callback
+        // only cares about the best network, verify it received nothing from cellular.
+        mCellNetworkAgent.addCapability(NET_CAPABILITY_TEMPORARILY_NOT_METERED);
+        bestMatchingCb.assertNoCallback();
+
+        // Make cellular the best network again, verify the callback now tracks cellular.
+        mWiFiNetworkAgent.adjustScore(-50);
+        bestMatchingCb.expectAvailableCallbacksValidated(mCellNetworkAgent);
+
+        // Make cellular temporary non-trusted, which will not satisfying the request.
+        // Verify the callback switch from/to the other network accordingly.
+        mCellNetworkAgent.removeCapability(NET_CAPABILITY_TRUSTED);
+        bestMatchingCb.expectAvailableCallbacksValidated(mWiFiNetworkAgent);
+        mCellNetworkAgent.addCapability(NET_CAPABILITY_TRUSTED);
+        bestMatchingCb.expectAvailableDoubleValidatedCallbacks(mCellNetworkAgent);
+
+        // Verify the callback doesn't care about wifi disconnect.
+        mWiFiNetworkAgent.disconnect();
+        bestMatchingCb.assertNoCallback();
+        mCellNetworkAgent.disconnect();
+        bestMatchingCb.expectCallback(CallbackEntry.LOST, mCellNetworkAgent);
     }
 }