diff --git a/core/java/android/net/ConnectivityManager.java b/core/java/android/net/ConnectivityManager.java
index 5f1043b..f0673ff 100644
--- a/core/java/android/net/ConnectivityManager.java
+++ b/core/java/android/net/ConnectivityManager.java
@@ -2744,7 +2744,9 @@
         if (networkCallback == null) {
             throw new IllegalArgumentException("null NetworkCallback");
         }
-        if (need == null) throw new IllegalArgumentException("null NetworkCapabilities");
+        if (need == null && action != REQUEST) {
+            throw new IllegalArgumentException("null NetworkCapabilities");
+        }
         try {
             incCallbackHandlerRefCount();
             synchronized(sNetworkCallback) {
@@ -2767,7 +2769,7 @@
     }
 
     /**
-     * Helper function to requests a network with a particular legacy type.
+     * Helper function to request a network with a particular legacy type.
      *
      * This is temporarily public @hide so it can be called by system code that uses the
      * NetworkRequest API to request networks but relies on CONNECTIVITY_ACTION broadcasts for
@@ -3011,6 +3013,28 @@
     }
 
     /**
+     * Registers to receive notifications about whichever network currently satisfies the
+     * system default {@link NetworkRequest}.  The callbacks will continue to be called until
+     * either the application exits or {@link #unregisterNetworkCallback} is called
+     * <p>This method requires the caller to hold the permission
+     * {@link android.Manifest.permission#ACCESS_NETWORK_STATE}.
+     *
+     * @param networkCallback The {@link NetworkCallback} that the system will call as the
+     *                        system default network changes.
+     * @hide
+     */
+    public void registerDefaultNetworkCallback(NetworkCallback networkCallback) {
+        // This works because if the NetworkCapabilities are null,
+        // ConnectivityService takes them from the default request.
+        //
+        // Since the capabilities are exactly the same as the default request's
+        // capabilities, this request is guaranteed, at all times, to be
+        // satisfied by the same network, if any, that satisfies the default
+        // request, i.e., the system default network.
+        sendRequestForNetwork(null, networkCallback, 0, REQUEST, TYPE_NONE);
+    }
+
+    /**
      * Requests bandwidth update for a given {@link Network} and returns whether the update request
      * is accepted by ConnectivityService. Once accepted, ConnectivityService will poll underlying
      * network connection for updated bandwidth information. The caller will be notified via
diff --git a/services/core/java/com/android/server/ConnectivityService.java b/services/core/java/com/android/server/ConnectivityService.java
index b7fca1a..5b01062 100644
--- a/services/core/java/com/android/server/ConnectivityService.java
+++ b/services/core/java/com/android/server/ConnectivityService.java
@@ -3845,8 +3845,16 @@
     @Override
     public NetworkRequest requestNetwork(NetworkCapabilities networkCapabilities,
             Messenger messenger, int timeoutMs, IBinder binder, int legacyType) {
-        networkCapabilities = new NetworkCapabilities(networkCapabilities);
-        enforceNetworkRequestPermissions(networkCapabilities);
+        // If the requested networkCapabilities is null, take them instead from
+        // the default network request. This allows callers to keep track of
+        // the system default network.
+        if (networkCapabilities == null) {
+            networkCapabilities = new NetworkCapabilities(mDefaultRequest.networkCapabilities);
+            enforceAccessPermission();
+        } else {
+            networkCapabilities = new NetworkCapabilities(networkCapabilities);
+            enforceNetworkRequestPermissions(networkCapabilities);
+        }
         enforceMeteredApnPolicy(networkCapabilities);
         ensureRequestableCapabilities(networkCapabilities);
 
diff --git a/services/tests/servicestests/src/com/android/server/ConnectivityServiceTest.java b/services/tests/servicestests/src/com/android/server/ConnectivityServiceTest.java
index 34f2e2e..8e11511 100644
--- a/services/tests/servicestests/src/com/android/server/ConnectivityServiceTest.java
+++ b/services/tests/servicestests/src/com/android/server/ConnectivityServiceTest.java
@@ -1008,29 +1008,41 @@
     private class TestNetworkCallback extends NetworkCallback {
         private final ConditionVariable mConditionVariable = new ConditionVariable();
         private CallbackState mLastCallback = CallbackState.NONE;
+        private Network mLastNetwork;
 
         public void onAvailable(Network network) {
             assertEquals(CallbackState.NONE, mLastCallback);
             mLastCallback = CallbackState.AVAILABLE;
+            mLastNetwork = network;
             mConditionVariable.open();
         }
 
         public void onLosing(Network network, int maxMsToLive) {
             assertEquals(CallbackState.NONE, mLastCallback);
             mLastCallback = CallbackState.LOSING;
+            mLastNetwork = network;
             mConditionVariable.open();
         }
 
         public void onLost(Network network) {
             assertEquals(CallbackState.NONE, mLastCallback);
             mLastCallback = CallbackState.LOST;
+            mLastNetwork = network;
             mConditionVariable.open();
         }
 
         void expectCallback(CallbackState state) {
+            expectCallback(state, null);
+        }
+
+        void expectCallback(CallbackState state, MockNetworkAgent mockAgent) {
             waitFor(mConditionVariable);
             assertEquals(state, mLastCallback);
+            if (mockAgent != null) {
+                assertEquals(mockAgent.getNetwork(), mLastNetwork);
+            }
             mLastCallback = CallbackState.NONE;
+            mLastNetwork = null;
             mConditionVariable.close();
         }
 
@@ -1389,6 +1401,55 @@
                 execptionCalled);
     }
 
+    @LargeTest
+    public void testRegisterDefaultNetworkCallback() throws Exception {
+        final TestNetworkCallback defaultNetworkCallback = new TestNetworkCallback();
+        mCm.registerDefaultNetworkCallback(defaultNetworkCallback);
+        defaultNetworkCallback.assertNoCallback();
+
+        // Create a TRANSPORT_CELLULAR request to keep the mobile interface up
+        // whenever Wi-Fi is up. Without this, the mobile network agent is
+        // reaped before any other activity can take place.
+        final TestNetworkCallback cellNetworkCallback = new TestNetworkCallback();
+        final NetworkRequest cellRequest = new NetworkRequest.Builder()
+                .addTransportType(TRANSPORT_CELLULAR).build();
+        mCm.requestNetwork(cellRequest, cellNetworkCallback);
+        cellNetworkCallback.assertNoCallback();
+
+        // Bring up cell and expect CALLBACK_AVAILABLE.
+        mCellNetworkAgent = new MockNetworkAgent(TRANSPORT_CELLULAR);
+        mCellNetworkAgent.connect(true);
+        cellNetworkCallback.expectCallback(CallbackState.AVAILABLE, mCellNetworkAgent);
+        defaultNetworkCallback.expectCallback(CallbackState.AVAILABLE, mCellNetworkAgent);
+
+        // Bring up wifi and expect CALLBACK_AVAILABLE.
+        mWiFiNetworkAgent = new MockNetworkAgent(TRANSPORT_WIFI);
+        mWiFiNetworkAgent.connect(true);
+        cellNetworkCallback.assertNoCallback();
+        defaultNetworkCallback.expectCallback(CallbackState.AVAILABLE, mWiFiNetworkAgent);
+
+        // Bring down cell. Expect no default network callback, since it wasn't the default.
+        mCellNetworkAgent.disconnect();
+        cellNetworkCallback.expectCallback(CallbackState.LOST, mCellNetworkAgent);
+        defaultNetworkCallback.assertNoCallback();
+
+        // Bring up cell. Expect no default network callback, since it won't be the default.
+        mCellNetworkAgent = new MockNetworkAgent(TRANSPORT_CELLULAR);
+        mCellNetworkAgent.connect(true);
+        cellNetworkCallback.expectCallback(CallbackState.AVAILABLE, mCellNetworkAgent);
+        defaultNetworkCallback.assertNoCallback();
+
+        // Bring down wifi. Expect the default network callback to notified of LOST wifi
+        // followed by AVAILABLE cell.
+        mWiFiNetworkAgent.disconnect();
+        cellNetworkCallback.assertNoCallback();
+        defaultNetworkCallback.expectCallback(CallbackState.LOST, mWiFiNetworkAgent);
+        defaultNetworkCallback.expectCallback(CallbackState.AVAILABLE, mCellNetworkAgent);
+        mCellNetworkAgent.disconnect();
+        cellNetworkCallback.expectCallback(CallbackState.LOST, mCellNetworkAgent);
+        defaultNetworkCallback.expectCallback(CallbackState.LOST, mCellNetworkAgent);
+    }
+
     private static class TestKeepaliveCallback extends PacketKeepaliveCallback {
 
         public static enum CallbackType { ON_STARTED, ON_STOPPED, ON_ERROR };