diff --git a/core/api/current.txt b/core/api/current.txt
index 10957ff..15aa188 100644
--- a/core/api/current.txt
+++ b/core/api/current.txt
@@ -9010,8 +9010,6 @@
     method @Deprecated public boolean hasNotificationAccess(android.content.ComponentName);
     method public void requestNotificationAccess(android.content.ComponentName);
     method @RequiresPermission(android.Manifest.permission.REQUEST_OBSERVE_COMPANION_DEVICE_PRESENCE) public void startObservingDevicePresence(@NonNull String) throws android.companion.DeviceNotAssociatedException;
-    method @Deprecated public void startSystemDataTransfer(int) throws android.companion.DeviceNotAssociatedException;
-    method public void startSystemDataTransfer(int, @NonNull java.util.concurrent.Executor, @NonNull android.os.OutcomeReceiver<java.lang.Void,android.companion.CompanionException>) throws android.companion.DeviceNotAssociatedException;
     method @RequiresPermission(android.Manifest.permission.REQUEST_OBSERVE_COMPANION_DEVICE_PRESENCE) public void stopObservingDevicePresence(@NonNull String) throws android.companion.DeviceNotAssociatedException;
     field public static final String EXTRA_ASSOCIATION = "android.companion.extra.ASSOCIATION";
     field @Deprecated public static final String EXTRA_DEVICE = "android.companion.extra.DEVICE";
@@ -9027,19 +9025,14 @@
 
   public abstract class CompanionDeviceService extends android.app.Service {
     ctor public CompanionDeviceService();
-    method @RequiresPermission(android.Manifest.permission.DELIVER_COMPANION_MESSAGES) public final void dispatchMessageToSystem(int, int, @NonNull byte[]) throws android.companion.DeviceNotAssociatedException;
     method @Nullable public final android.os.IBinder onBind(@NonNull android.content.Intent);
     method @Deprecated @MainThread public void onDeviceAppeared(@NonNull String);
     method @MainThread public void onDeviceAppeared(@NonNull android.companion.AssociationInfo);
     method @Deprecated @MainThread public void onDeviceDisappeared(@NonNull String);
     method @MainThread public void onDeviceDisappeared(@NonNull android.companion.AssociationInfo);
-    method public void onMessageDispatchedFromSystem(int, int, @NonNull byte[]);
     field public static final String SERVICE_INTERFACE = "android.companion.CompanionDeviceService";
   }
 
-  public class CompanionException extends java.lang.RuntimeException {
-  }
-
   public interface DeviceFilter<D extends android.os.Parcelable> extends android.os.Parcelable {
   }
 
diff --git a/core/java/android/companion/CompanionDeviceManager.java b/core/java/android/companion/CompanionDeviceManager.java
index 1b51faf3..633b734 100644
--- a/core/java/android/companion/CompanionDeviceManager.java
+++ b/core/java/android/companion/CompanionDeviceManager.java
@@ -800,18 +800,15 @@
      *
      * @hide
      */
+    @Deprecated
     @RequiresPermission(android.Manifest.permission.DELIVER_COMPANION_MESSAGES)
     public void dispatchMessage(int messageId, int associationId, @NonNull byte[] message)
             throws DeviceNotAssociatedException {
-        try {
-            mService.dispatchMessage(messageId, associationId, message);
-        } catch (RemoteException e) {
-            ExceptionUtils.propagateIfInstanceOf(e.getCause(), DeviceNotAssociatedException.class);
-            throw e.rethrowFromSystemServer();
-        }
+        Log.w(LOG_TAG, "dispatchMessage replaced by attachSystemDataTransport");
     }
 
     /** {@hide} */
+    @RequiresPermission(android.Manifest.permission.DELIVER_COMPANION_MESSAGES)
     public final void attachSystemDataTransport(int associationId, @NonNull InputStream in,
             @NonNull OutputStream out) throws DeviceNotAssociatedException {
         synchronized (mTransports) {
@@ -830,6 +827,7 @@
     }
 
     /** {@hide} */
+    @RequiresPermission(android.Manifest.permission.DELIVER_COMPANION_MESSAGES)
     public final void detachSystemDataTransport(int associationId)
             throws DeviceNotAssociatedException {
         synchronized (mTransports) {
@@ -927,7 +925,7 @@
      *
      * <p>The permission transfer doesn't happen immediately after the call or user consented.
      * The app needs to trigger the system data transfer manually by calling
-     * {@link #startSystemDataTransfer(int)}, when it confirms the communication channel between
+     * {@code #startSystemDataTransfer(int)}, when it confirms the communication channel between
      * the two devices is established.</p>
      *
      * @param associationId The unique {@link AssociationInfo#getId ID} assigned to the association
@@ -965,8 +963,8 @@
      * @param associationId The unique {@link AssociationInfo#getId ID} assigned to the Association
      *                      of the companion device recorded by CompanionDeviceManager
      * @throws DeviceNotAssociatedException Exception if the companion device is not associated
-     *
      * @deprecated Use {@link #startSystemDataTransfer(int, Executor, OutcomeReceiver)} instead.
+     * @hide
      */
     @Deprecated
     @UserHandleAware
@@ -993,6 +991,7 @@
      * @param executor The executor which will be used to invoke the result callback.
      * @param result The callback to notify the app of the result of the system data transfer.
      * @throws DeviceNotAssociatedException Exception if the companion device is not associated
+     * @hide
      */
     @UserHandleAware
     public void startSystemDataTransfer(
@@ -1125,12 +1124,14 @@
 
         public void start() throws IOException {
             final ParcelFileDescriptor[] pair = ParcelFileDescriptor.createSocketPair();
-            mLocalIn = new ParcelFileDescriptor.AutoCloseInputStream(pair[0]);
-            mLocalOut = new ParcelFileDescriptor.AutoCloseOutputStream(pair[0]);
+            final ParcelFileDescriptor localFd = pair[0];
+            final ParcelFileDescriptor remoteFd = pair[1];
+            mLocalIn = new ParcelFileDescriptor.AutoCloseInputStream(localFd);
+            mLocalOut = new ParcelFileDescriptor.AutoCloseOutputStream(localFd);
 
             try {
                 mService.attachSystemDataTransport(mContext.getPackageName(),
-                        mContext.getUserId(), mAssociationId, pair[1]);
+                        mContext.getUserId(), mAssociationId, remoteFd);
             } catch (RemoteException e) {
                 throw new IOException("Failed to configure transport", e);
             }
diff --git a/core/java/android/companion/CompanionDeviceService.java b/core/java/android/companion/CompanionDeviceService.java
index 83d2713..a79983a 100644
--- a/core/java/android/companion/CompanionDeviceService.java
+++ b/core/java/android/companion/CompanionDeviceService.java
@@ -156,9 +156,12 @@
      * @param messageId system assigned id of the message to be sent
      * @param associationId association id of the associated device
      * @param message message to be sent
+     * @hide
      */
+    @Deprecated
     public void onMessageDispatchedFromSystem(int messageId, int associationId,
             @NonNull byte[] message) {
+        Log.w(LOG_TAG, "Replaced by attachSystemDataTransport");
         // do nothing. Companion apps can override this function for system to send messages.
     }
 
@@ -185,22 +188,14 @@
      * @param messageId id of the message
      * @param associationId id of the associated device
      * @param message message received from the associated device
+     * @hide
      */
+    @Deprecated
     @RequiresPermission(android.Manifest.permission.DELIVER_COMPANION_MESSAGES)
     public final void dispatchMessageToSystem(int messageId, int associationId,
             @NonNull byte[] message)
             throws DeviceNotAssociatedException {
-        if (getBaseContext() == null) {
-            Log.e(LOG_TAG, "Dispatch failed. Start your service before calling this method.");
-            return;
-        }
-        CompanionDeviceManager companionDeviceManager =
-                getSystemService(CompanionDeviceManager.class);
-        if (companionDeviceManager != null) {
-            companionDeviceManager.dispatchMessage(messageId, associationId, message);
-        } else {
-            Log.e(LOG_TAG, "CompanionDeviceManager is null. Can't dispatch messages.");
-        }
+        Log.w(LOG_TAG, "Replaced by attachSystemDataTransport");
     }
 
     /**
@@ -223,10 +218,13 @@
      *            device
      * @hide
      */
+    @RequiresPermission(android.Manifest.permission.DELIVER_COMPANION_MESSAGES)
     public final void attachSystemDataTransport(int associationId, @NonNull InputStream in,
             @NonNull OutputStream out) throws DeviceNotAssociatedException {
         getSystemService(CompanionDeviceManager.class)
-                .attachSystemDataTransport(associationId, in, out);
+                .attachSystemDataTransport(associationId,
+                        Objects.requireNonNull(in),
+                        Objects.requireNonNull(out));
     }
 
     /**
@@ -236,6 +234,7 @@
      * @param associationId id of the associated device
      * @hide
      */
+    @RequiresPermission(android.Manifest.permission.DELIVER_COMPANION_MESSAGES)
     public final void detachSystemDataTransport(int associationId)
             throws DeviceNotAssociatedException {
         getSystemService(CompanionDeviceManager.class)
@@ -299,13 +298,5 @@
         public void onDeviceDisappeared(AssociationInfo associationInfo) {
             mMainHandler.postAtFrontOfQueue(() -> mService.onDeviceDisappeared(associationInfo));
         }
-
-        @Override
-        public void onMessageDispatchedFromSystem(int messageId, int associationId,
-                @NonNull byte[] message) {
-            mMainHandler.postAtFrontOfQueue(
-                    () -> mService.onMessageDispatchedFromSystem(messageId, associationId,
-                            message));
-        }
     }
 }
diff --git a/core/java/android/companion/CompanionException.java b/core/java/android/companion/CompanionException.java
index fb78e8d..9a92401 100644
--- a/core/java/android/companion/CompanionException.java
+++ b/core/java/android/companion/CompanionException.java
@@ -19,7 +19,10 @@
 import android.annotation.NonNull;
 
 /**
- * {@code CompanionException} can be thrown during the companion system data transfer process.
+ * {@code CompanionException} can be thrown during the companion system data
+ * transfer process.
+ *
+ * @hide
  */
 public class CompanionException extends RuntimeException {
     /** @hide */
diff --git a/core/java/android/companion/ICompanionDeviceManager.aidl b/core/java/android/companion/ICompanionDeviceManager.aidl
index 42f9083..6e6e187 100644
--- a/core/java/android/companion/ICompanionDeviceManager.aidl
+++ b/core/java/android/companion/ICompanionDeviceManager.aidl
@@ -63,8 +63,6 @@
     void createAssociation(in String packageName, in String macAddress, int userId,
         in byte[] certificate);
 
-    void dispatchMessage(int messageId, int associationId, in byte[] message);
-
     void addOnAssociationsChangedListener(IOnAssociationsChangedListener listener, int userId);
 
     void removeOnAssociationsChangedListener(IOnAssociationsChangedListener listener, int userId);
diff --git a/core/java/android/companion/ICompanionDeviceService.aidl b/core/java/android/companion/ICompanionDeviceService.aidl
index 3c90b86..fa68508 100644
--- a/core/java/android/companion/ICompanionDeviceService.aidl
+++ b/core/java/android/companion/ICompanionDeviceService.aidl
@@ -22,5 +22,4 @@
 oneway interface ICompanionDeviceService {
     void onDeviceAppeared(in AssociationInfo associationInfo);
     void onDeviceDisappeared(in AssociationInfo associationInfo);
-    void onMessageDispatchedFromSystem(in int messageId, in int associationId, in byte[] message);
 }
diff --git a/core/tests/companiontests/src/android/companion/SystemDataTransportTest.java b/core/tests/companiontests/src/android/companion/SystemDataTransportTest.java
index be04b6c..6bd498c 100644
--- a/core/tests/companiontests/src/android/companion/SystemDataTransportTest.java
+++ b/core/tests/companiontests/src/android/companion/SystemDataTransportTest.java
@@ -16,12 +16,15 @@
 
 package android.companion;
 
+import android.content.Context;
 import android.os.SystemClock;
 import android.test.InstrumentationTestCase;
 import android.util.Log;
 
 import com.android.internal.util.HexDump;
 
+import libcore.util.EmptyArray;
+
 import java.io.ByteArrayInputStream;
 import java.io.ByteArrayOutputStream;
 import java.io.FilterInputStream;
@@ -31,69 +34,81 @@
 import java.io.OutputStream;
 import java.nio.ByteBuffer;
 import java.nio.charset.StandardCharsets;
+import java.util.List;
 import java.util.Random;
 
 public class SystemDataTransportTest extends InstrumentationTestCase {
     private static final String TAG = "SystemDataTransportTest";
 
-    private static final int COMMAND_INVALID = 0xF00DCAFE;
-    private static final int COMMAND_PING_V0 = 0x50490000;
-    private static final int COMMAND_PONG_V0 = 0x504F0000;
+    private static final int MESSAGE_INVALID = 0xF00DCAFE;
 
+    private static final int MESSAGE_REQUEST_INVALID = 0x63636363; // ????
+    private static final int MESSAGE_REQUEST_PING = 0x63807378; // ?PIN
+
+    private static final int MESSAGE_RESPONSE_INVALID = 0x33333333; // !!!!
+    private static final int MESSAGE_RESPONSE_SUCCESS = 0x33838567; // !SUC
+    private static final int MESSAGE_RESPONSE_FAILURE = 0x33706573; // !FAI
+
+    private Context mContext;
     private CompanionDeviceManager mCdm;
+    private int mAssociationId;
 
     @Override
     protected void setUp() throws Exception {
         super.setUp();
 
-        mCdm = getInstrumentation().getTargetContext()
-                .getSystemService(CompanionDeviceManager.class);
+        mContext = getInstrumentation().getTargetContext();
+        mCdm = mContext.getSystemService(CompanionDeviceManager.class);
+        mAssociationId = createAssociation();
+    }
+
+    @Override
+    protected void tearDown() throws Exception {
+        super.tearDown();
+
+        mCdm.disassociate(mAssociationId);
     }
 
     public void testPingHandRolled() {
         // NOTE: These packets are explicitly hand-rolled to verify wire format;
         // the remainder of the tests are fine using generated packets
 
-        // PING v0 with payload "HELLO WORLD!"
+        // MESSAGE_REQUEST_PING with payload "HELLO WORLD!"
         final byte[] input = new byte[] {
-                0x50, 0x49, 0x00, 0x00,
-                0x00, 0x00, 0x00, 0x0C,
+                0x63, (byte) 0x80, 0x73, 0x78, // message: MESSAGE_REQUEST_PING
+                0x00, 0x00, 0x00, 0x2A, // sequence: 42
+                0x00, 0x00, 0x00, 0x0C, // length: 12
                 0x48, 0x45, 0x4C, 0x4C, 0x4F, 0x20, 0x57, 0x4F, 0x52, 0x4C, 0x44, 0x21,
         };
-        // PONG v0 with payload "HELLO WORLD!"
+        // MESSAGE_RESPONSE_SUCCESS with payload "HELLO WORLD!"
         final byte[] expected = new byte[] {
-                0x50, 0x4F, 0x00, 0x00,
-                0x00, 0x00, 0x00, 0x0C,
+                0x33, (byte) 0x83, (byte) 0x85, 0x67, // message: MESSAGE_RESPONSE_SUCCESS
+                0x00, 0x00, 0x00, 0x2A, // sequence: 42
+                0x00, 0x00, 0x00, 0x0C, // length: 12
                 0x48, 0x45, 0x4C, 0x4C, 0x4F, 0x20, 0x57, 0x4F, 0x52, 0x4C, 0x44, 0x21,
         };
-
-        final ByteArrayInputStream in = new ByteArrayInputStream(input);
-        final ByteArrayOutputStream out = new ByteArrayOutputStream();
-        mCdm.attachSystemDataTransport(42, in, out);
-
-        final byte[] actual = waitForByteArray(out, expected.length);
-        assertEquals(HexDump.toHexString(expected), HexDump.toHexString(actual));
+        assertTransportBehavior(input, expected);
     }
 
     public void testPingTrickle() {
-        final byte[] input = generatePacket(COMMAND_PING_V0, TAG);
-        final byte[] expected = generatePacket(COMMAND_PONG_V0, TAG);
+        final byte[] input = generatePacket(MESSAGE_REQUEST_PING, /* sequence */ 1, TAG);
+        final byte[] expected = generatePacket(MESSAGE_RESPONSE_SUCCESS, /* sequence */ 1, TAG);
 
         final ByteArrayInputStream in = new ByteArrayInputStream(input);
         final ByteArrayOutputStream out = new ByteArrayOutputStream();
-        mCdm.attachSystemDataTransport(42, new TrickleInputStream(in), out);
+        mCdm.attachSystemDataTransport(mAssociationId, new TrickleInputStream(in), out);
 
         final byte[] actual = waitForByteArray(out, expected.length);
         assertEquals(HexDump.toHexString(expected), HexDump.toHexString(actual));
     }
 
     public void testPingDelay() {
-        final byte[] input = generatePacket(COMMAND_PING_V0, TAG);
-        final byte[] expected = generatePacket(COMMAND_PONG_V0, TAG);
+        final byte[] input = generatePacket(MESSAGE_REQUEST_PING, /* sequence */ 1, TAG);
+        final byte[] expected = generatePacket(MESSAGE_RESPONSE_SUCCESS, /* sequence */ 1, TAG);
 
         final ByteArrayInputStream in = new ByteArrayInputStream(input);
         final ByteArrayOutputStream out = new ByteArrayOutputStream();
-        mCdm.attachSystemDataTransport(42, new DelayingInputStream(in, 1000),
+        mCdm.attachSystemDataTransport(mAssociationId, new DelayingInputStream(in, 1000),
                 new DelayingOutputStream(out, 1000));
 
         final byte[] actual = waitForByteArray(out, expected.length);
@@ -104,49 +119,54 @@
         final byte[] blob = new byte[500_000];
         new Random().nextBytes(blob);
 
-        final byte[] input = generatePacket(COMMAND_PING_V0, blob);
-        final byte[] expected = generatePacket(COMMAND_PONG_V0, blob);
-
-        final ByteArrayInputStream in = new ByteArrayInputStream(input);
-        final ByteArrayOutputStream out = new ByteArrayOutputStream();
-        mCdm.attachSystemDataTransport(42, in, out);
-
-        final byte[] actual = waitForByteArray(out, expected.length);
-        assertEquals(HexDump.toHexString(expected), HexDump.toHexString(actual));
+        final byte[] input = generatePacket(MESSAGE_REQUEST_PING, /* sequence */ 1, blob);
+        final byte[] expected = generatePacket(MESSAGE_RESPONSE_SUCCESS, /* sequence */ 1, blob);
+        assertTransportBehavior(input, expected);
     }
 
     public void testMutiplePingPing() {
-        final byte[] input = concat(generatePacket(COMMAND_PING_V0, "red"),
-                generatePacket(COMMAND_PING_V0, "green"));
-        final byte[] expected = concat(generatePacket(COMMAND_PONG_V0, "red"),
-                generatePacket(COMMAND_PONG_V0, "green"));
-
-        final ByteArrayInputStream in = new ByteArrayInputStream(input);
-        final ByteArrayOutputStream out = new ByteArrayOutputStream();
-        mCdm.attachSystemDataTransport(42, in, out);
-
-        final byte[] actual = waitForByteArray(out, expected.length);
-        assertEquals(HexDump.toHexString(expected), HexDump.toHexString(actual));
+        final byte[] input = concat(
+                generatePacket(MESSAGE_REQUEST_PING, /* sequence */ 1, "red"),
+                generatePacket(MESSAGE_REQUEST_PING, /* sequence */ 2, "green"));
+        final byte[] expected = concat(
+                generatePacket(MESSAGE_RESPONSE_SUCCESS, /* sequence */ 1, "red"),
+                generatePacket(MESSAGE_RESPONSE_SUCCESS, /* sequence */ 2, "green"));
+        assertTransportBehavior(input, expected);
     }
 
     public void testMultipleInvalidPing() {
-        final byte[] input = concat(generatePacket(COMMAND_INVALID, "red"),
-                generatePacket(COMMAND_PING_V0, "green"));
-        final byte[] expected = generatePacket(COMMAND_PONG_V0, "green");
+        final byte[] input = concat(
+                generatePacket(MESSAGE_INVALID, /* sequence */ 1, "red"),
+                generatePacket(MESSAGE_REQUEST_PING, /* sequence */ 2, "green"));
+        final byte[] expected =
+                generatePacket(MESSAGE_RESPONSE_SUCCESS, /* sequence */ 2, "green");
+        assertTransportBehavior(input, expected);
+    }
 
-        final ByteArrayInputStream in = new ByteArrayInputStream(input);
-        final ByteArrayOutputStream out = new ByteArrayOutputStream();
-        mCdm.attachSystemDataTransport(42, in, out);
+    public void testMultipleInvalidRequestPing() {
+        final byte[] input = concat(
+                generatePacket(MESSAGE_REQUEST_INVALID, /* sequence */ 1, "red"),
+                generatePacket(MESSAGE_REQUEST_PING, /* sequence */ 2, "green"));
+        final byte[] expected = concat(
+                generatePacket(MESSAGE_RESPONSE_FAILURE, /* sequence */ 1),
+                generatePacket(MESSAGE_RESPONSE_SUCCESS, /* sequence */ 2, "green"));
+        assertTransportBehavior(input, expected);
+    }
 
-        final byte[] actual = waitForByteArray(out, expected.length);
-        assertEquals(HexDump.toHexString(expected), HexDump.toHexString(actual));
+    public void testMultipleInvalidResponsePing() {
+        final byte[] input = concat(
+                generatePacket(MESSAGE_RESPONSE_INVALID, /* sequence */ 1, "red"),
+                generatePacket(MESSAGE_REQUEST_PING, /* sequence */ 2, "green"));
+        final byte[] expected =
+                generatePacket(MESSAGE_RESPONSE_SUCCESS, /* sequence */ 2, "green");
+        assertTransportBehavior(input, expected);
     }
 
     public void testDoubleAttach() {
         // Connect an empty connection that is stalled out
         final InputStream in = new EmptyInputStream();
         final OutputStream out = new ByteArrayOutputStream();
-        mCdm.attachSystemDataTransport(42, in, out);
+        mCdm.attachSystemDataTransport(mAssociationId, in, out);
         SystemClock.sleep(1000);
 
         // Attach a second transport that has some packets; it should disconnect
@@ -154,17 +174,57 @@
         testPingHandRolled();
     }
 
-    public static byte[] concat(byte[] a, byte[] b) {
-        return ByteBuffer.allocate(a.length + b.length).put(a).put(b).array();
+    public static byte[] concat(byte[]... blobs) {
+        int length = 0;
+        for (byte[] blob : blobs) {
+            length += blob.length;
+        }
+        final ByteBuffer buf = ByteBuffer.allocate(length);
+        for (byte[] blob : blobs) {
+            buf.put(blob);
+        }
+        return buf.array();
     }
 
-    public static byte[] generatePacket(int command, String data) {
-        return generatePacket(command, data.getBytes(StandardCharsets.UTF_8));
+    public static byte[] generatePacket(int message, int sequence) {
+        return generatePacket(message, sequence, EmptyArray.BYTE);
     }
 
-    public static byte[] generatePacket(int command, byte[] data) {
-        return ByteBuffer.allocate(data.length + 8)
-                .putInt(command).putInt(data.length).put(data).array();
+    public static byte[] generatePacket(int message, int sequence, String data) {
+        return generatePacket(message, sequence, data.getBytes(StandardCharsets.UTF_8));
+    }
+
+    public static byte[] generatePacket(int message, int sequence, byte[] data) {
+        return ByteBuffer.allocate(data.length + 12)
+                .putInt(message)
+                .putInt(sequence)
+                .putInt(data.length)
+                .put(data)
+                .array();
+    }
+
+    private int createAssociation() {
+        getInstrumentation().getUiAutomation().executeShellCommand("cmd companiondevice associate "
+                + mContext.getUserId() + " " + mContext.getPackageName() + " AA:BB:CC:DD:EE:FF");
+        List<AssociationInfo> infos;
+        for (int i = 0; i < 100; i++) {
+            infos = mCdm.getMyAssociations();
+            if (!infos.isEmpty()) {
+                return infos.get(0).getId();
+            } else {
+                SystemClock.sleep(100);
+            }
+        }
+        throw new AssertionError();
+    }
+
+    private void assertTransportBehavior(byte[] input, byte[] expected) {
+        final ByteArrayInputStream in = new ByteArrayInputStream(input);
+        final ByteArrayOutputStream out = new ByteArrayOutputStream();
+        mCdm.attachSystemDataTransport(mAssociationId, in, out);
+
+        final byte[] actual = waitForByteArray(out, expected.length);
+        assertEquals(HexDump.toHexString(expected), HexDump.toHexString(actual));
     }
 
     private static byte[] waitForByteArray(ByteArrayOutputStream out, int size) {
diff --git a/services/companion/Android.bp b/services/companion/Android.bp
index d3ef6dc..cdeb2dc 100644
--- a/services/companion/Android.bp
+++ b/services/companion/Android.bp
@@ -11,7 +11,6 @@
     name: "services.companion-sources",
     srcs: [
         "java/**/*.java",
-        "java/**/*.proto",
     ],
     path: "java",
     visibility: ["//frameworks/base/services"],
@@ -20,9 +19,6 @@
 java_library_static {
     name: "services.companion",
     defaults: ["platform_service_defaults"],
-    proto: {
-        type: "stream",
-    },
     srcs: [":services.companion-sources"],
     libs: [
         "app-compat-annotations",
diff --git a/services/companion/java/com/android/server/companion/CompanionApplicationController.java b/services/companion/java/com/android/server/companion/CompanionApplicationController.java
index 2ab1aa8..2acb65e 100644
--- a/services/companion/java/com/android/server/companion/CompanionApplicationController.java
+++ b/services/companion/java/com/android/server/companion/CompanionApplicationController.java
@@ -244,29 +244,6 @@
         primaryServiceConnector.postOnDeviceDisappeared(association);
     }
 
-    /** Pass an encrypted secure message to the companion application for transporting. */
-    public void dispatchMessage(@UserIdInt int userId, @NonNull String packageName,
-            int associationId, int messageId, @NonNull byte[] message) {
-        if (DEBUG) {
-            Log.i(TAG, "dispatchMessage() u" + userId + "/" + packageName
-                    + " associationId=" + associationId);
-        }
-
-        final CompanionDeviceServiceConnector primaryServiceConnector =
-                getPrimaryServiceConnector(userId, packageName);
-        if (primaryServiceConnector == null) {
-            if (DEBUG) {
-                Log.e(TAG, "dispatchMessage(): "
-                        + "u" + userId + "/" + packageName + " is NOT bound.");
-                Log.d(TAG, "Stacktrace", new Throwable());
-            }
-            return;
-        }
-
-        primaryServiceConnector.postOnMessageDispatchedFromSystem(associationId, messageId,
-                message);
-    }
-      
     void dump(@NonNull PrintWriter out) {
         out.append("Companion Device Application Controller: \n");
 
diff --git a/services/companion/java/com/android/server/companion/CompanionDeviceManagerService.java b/services/companion/java/com/android/server/companion/CompanionDeviceManagerService.java
index c354561..fa043f8 100644
--- a/services/companion/java/com/android/server/companion/CompanionDeviceManagerService.java
+++ b/services/companion/java/com/android/server/companion/CompanionDeviceManagerService.java
@@ -106,11 +106,10 @@
 import com.android.server.FgThread;
 import com.android.server.LocalServices;
 import com.android.server.SystemService;
-import com.android.server.companion.datatransfer.CompanionMessageProcessor;
 import com.android.server.companion.datatransfer.SystemDataTransferProcessor;
 import com.android.server.companion.datatransfer.SystemDataTransferRequestStore;
 import com.android.server.companion.presence.CompanionDevicePresenceMonitor;
-import com.android.server.companion.securechannel.CompanionSecureCommunicationsManager;
+import com.android.server.companion.transport.CompanionTransportManager;
 import com.android.server.pm.UserManagerInternal;
 import com.android.server.wm.ActivityTaskManagerInternal;
 
@@ -151,10 +150,9 @@
     private final SystemDataTransferRequestStore mSystemDataTransferRequestStore;
     private AssociationRequestsProcessor mAssociationRequestsProcessor;
     private SystemDataTransferProcessor mSystemDataTransferProcessor;
-    private CompanionMessageProcessor mCompanionMessageProcessor;
     private CompanionDevicePresenceMonitor mDevicePresenceMonitor;
     private CompanionApplicationController mCompanionAppController;
-    private CompanionSecureCommunicationsManager mSecureCommsManager;
+    private CompanionTransportManager mTransportManager;
 
     private final ActivityTaskManagerInternal mAtmInternal;
     private final ActivityManagerInternal mAmInternal;
@@ -238,11 +236,9 @@
                 /* cdmService */this, mAssociationStore);
         mCompanionAppController = new CompanionApplicationController(
                 context, mApplicationControllerCallback);
-        mSecureCommsManager = new CompanionSecureCommunicationsManager(
-                mAssociationStore, mCompanionAppController);
-        mCompanionMessageProcessor = new CompanionMessageProcessor(mSecureCommsManager);
+        mTransportManager = new CompanionTransportManager(context);
         mSystemDataTransferProcessor = new SystemDataTransferProcessor(this, mAssociationStore,
-                mSystemDataTransferRequestStore, mCompanionMessageProcessor);
+                mSystemDataTransferRequestStore, mTransportManager);
 
         // Publish "binder" service.
         final CompanionDeviceManagerImpl impl = new CompanionDeviceManagerImpl();
@@ -319,18 +315,32 @@
                 MINUTES.toMillis(10));
     }
 
-    @Nullable
+    @NonNull
     AssociationInfo getAssociationWithCallerChecks(
             @UserIdInt int userId, @NonNull String packageName, @NonNull String macAddress) {
-        final AssociationInfo association = mAssociationStore.getAssociationsForPackageWithAddress(
+        AssociationInfo association = mAssociationStore.getAssociationsForPackageWithAddress(
                 userId, packageName, macAddress);
-        return sanitizeWithCallerChecks(getContext(), association);
+        association = sanitizeWithCallerChecks(getContext(), association);
+        if (association != null) {
+            return association;
+        } else {
+            throw new IllegalArgumentException("Association does not exist "
+                    + "or the caller does not have permissions to manage it "
+                    + "(ie. it belongs to a different package or a different user).");
+        }
     }
 
-    @Nullable
+    @NonNull
     AssociationInfo getAssociationWithCallerChecks(int associationId) {
-        final AssociationInfo association = mAssociationStore.getAssociationById(associationId);
-        return sanitizeWithCallerChecks(getContext(), association);
+        AssociationInfo association = mAssociationStore.getAssociationById(associationId);
+        association = sanitizeWithCallerChecks(getContext(), association);
+        if (association != null) {
+            return association;
+        } else {
+            throw new IllegalArgumentException("Association does not exist "
+                    + "or the caller does not have permissions to manage it "
+                    + "(ie. it belongs to a different package or a different user).");
+        }
     }
 
     private void onDeviceAppearedInternal(int associationId) {
@@ -609,12 +619,6 @@
 
             final AssociationInfo association =
                     getAssociationWithCallerChecks(userId, packageName, deviceMacAddress);
-            if (association == null) {
-                throw new IllegalArgumentException("Association does not exist "
-                        + "or the caller does not have permissions to manage it "
-                        + "(ie. it belongs to a different package or a different user).");
-            }
-
             disassociateInternal(association.getId());
         }
 
@@ -622,15 +626,9 @@
         public void disassociate(int associationId) {
             if (DEBUG) Log.i(TAG, "disassociate() associationId=" + associationId);
 
-            final AssociationInfo association = getAssociationWithCallerChecks(associationId);
-            if (association == null) {
-                throw new IllegalArgumentException("Association with ID " + associationId + " "
-                        + "does not exist "
-                        + "or belongs to a different package "
-                        + "or belongs to a different user");
-            }
-
-            disassociateInternal(associationId);
+            final AssociationInfo association =
+                    getAssociationWithCallerChecks(associationId);
+            disassociateInternal(association.getId());
         }
 
         @Override
@@ -698,27 +696,6 @@
         }
 
         @Override
-        public void dispatchMessage(int messageId, int associationId, @NonNull byte[] message) {
-            if (DEBUG) {
-                Log.i(TAG, "dispatchMessage() associationId=" + associationId + "\n"
-                        + " message(Base64)=" + Base64.encodeToString(message, 0));
-            }
-
-            getContext().enforceCallingOrSelfPermission(DELIVER_COMPANION_MESSAGES,
-                    "dispatchMessage");
-
-            AssociationInfo association = getAssociationWithCallerChecks(associationId);
-            if (association == null) {
-                throw new IllegalArgumentException("Association with ID " + associationId + " "
-                        + "does not exist "
-                        + "or belongs to a different package "
-                        + "or belongs to a different user");
-            }
-
-            mSecureCommsManager.receiveSecureMessage(messageId, associationId, message);
-        }
-
-        @Override
         public PendingIntent buildPermissionTransferUserConsentIntent(String packageName,
                 int userId, int associationId) {
             return mSystemDataTransferProcessor.buildPermissionTransferUserConsentIntent(
@@ -735,14 +712,14 @@
         @Override
         public void attachSystemDataTransport(String packageName, int userId, int associationId,
                 ParcelFileDescriptor fd) {
-            mSystemDataTransferProcessor.attachSystemDataTransport(packageName, userId,
-                    associationId, fd);
+            getAssociationWithCallerChecks(associationId);
+            mTransportManager.attachSystemDataTransport(packageName, userId, associationId, fd);
         }
 
         @Override
         public void detachSystemDataTransport(String packageName, int userId, int associationId) {
-            mSystemDataTransferProcessor.detachSystemDataTransport(packageName, userId,
-                    associationId);
+            getAssociationWithCallerChecks(associationId);
+            mTransportManager.detachSystemDataTransport(packageName, userId, associationId);
         }
 
         @Override
@@ -750,13 +727,6 @@
             if (DEBUG) Log.i(TAG, "notifyDevice_Appeared() id=" + associationId);
 
             AssociationInfo association = getAssociationWithCallerChecks(associationId);
-            if (association == null) {
-                throw new IllegalArgumentException("Association with ID " + associationId + " "
-                        + "does not exist "
-                        + "or belongs to a different package "
-                        + "or belongs to a different user");
-            }
-
             if (!association.isSelfManaged()) {
                 throw new IllegalArgumentException("Association with ID " + associationId
                         + " is not self-managed. notifyDeviceAppeared(int) can only be called for"
@@ -777,13 +747,6 @@
             if (DEBUG) Log.i(TAG, "notifyDevice_Disappeared() id=" + associationId);
 
             final AssociationInfo association = getAssociationWithCallerChecks(associationId);
-            if (association == null) {
-                throw new IllegalArgumentException("Association with ID " + associationId + " "
-                        + "does not exist "
-                        + "or belongs to a different package "
-                        + "or belongs to a different user");
-            }
-
             if (!association.isSelfManaged()) {
                 throw new IllegalArgumentException("Association with ID " + associationId
                         + " is not self-managed. notifyDeviceAppeared(int) can only be called for"
@@ -892,7 +855,6 @@
             final CompanionDeviceShellCommand cmd = new CompanionDeviceShellCommand(
                     CompanionDeviceManagerService.this,
                     mAssociationStore,
-                    mSecureCommsManager,
                     mDevicePresenceMonitor);
             cmd.exec(this, in, out, err, args, callback, resultReceiver);
         }
diff --git a/services/companion/java/com/android/server/companion/CompanionDeviceServiceConnector.java b/services/companion/java/com/android/server/companion/CompanionDeviceServiceConnector.java
index 7360f08..a288f7b 100644
--- a/services/companion/java/com/android/server/companion/CompanionDeviceServiceConnector.java
+++ b/services/companion/java/com/android/server/companion/CompanionDeviceServiceConnector.java
@@ -99,12 +99,6 @@
         post(companionService -> companionService.onDeviceDisappeared(associationInfo));
     }
 
-    void postOnMessageDispatchedFromSystem(int associationId, int messageId,
-            @NonNull byte[] message) {
-        post(companionService ->
-                companionService.onMessageDispatchedFromSystem(messageId, associationId, message));
-    }
-
     /**
      * Post "unbind" job, which will run *after* all previously posted jobs complete.
      *
diff --git a/services/companion/java/com/android/server/companion/CompanionDeviceShellCommand.java b/services/companion/java/com/android/server/companion/CompanionDeviceShellCommand.java
index 0b7bc03..322ad46 100644
--- a/services/companion/java/com/android/server/companion/CompanionDeviceShellCommand.java
+++ b/services/companion/java/com/android/server/companion/CompanionDeviceShellCommand.java
@@ -16,14 +16,9 @@
 
 package com.android.server.companion;
 
-import static java.nio.charset.StandardCharsets.UTF_8;
-
 import android.companion.AssociationInfo;
 import android.os.Binder;
 import android.os.ShellCommand;
-import android.util.Base64;
-
-import com.android.server.companion.securechannel.CompanionSecureCommunicationsManager;
 
 import com.android.server.companion.presence.CompanionDevicePresenceMonitor;
 
@@ -35,16 +30,13 @@
 
     private final CompanionDeviceManagerService mService;
     private final AssociationStore mAssociationStore;
-    private final CompanionSecureCommunicationsManager mSecureCommsManager;
     private final CompanionDevicePresenceMonitor mDevicePresenceMonitor;
 
     CompanionDeviceShellCommand(CompanionDeviceManagerService service,
             AssociationStore associationStore,
-            CompanionSecureCommunicationsManager secureCommsManager,
             CompanionDevicePresenceMonitor devicePresenceMonitor) {
         mService = service;
         mAssociationStore = associationStore;
-        mSecureCommsManager = secureCommsManager;
         mDevicePresenceMonitor = devicePresenceMonitor;
     }
 
@@ -92,32 +84,6 @@
                     mService.loadAssociationsFromDisk();
                     break;
 
-                case "send-secure-message":
-                    associationId = getNextIntArgRequired();
-                    final byte[] message;
-
-                    // The message should be either a UTF-8 String or Base64-encoded data.
-                    final boolean isBase64 = "--base64".equals(getNextOption());
-                    if (isBase64) {
-                        final String base64encodedMessage = getNextArgRequired();
-                        message = Base64.decode(base64encodedMessage, 0);
-                    } else {
-                        // We treat the rest of the command as the message, which should contain at
-                        // least one word (hence getNextArg_Required() below), but there may be
-                        // more.
-                        final StringBuilder sb = new StringBuilder(getNextArgRequired());
-                        // Pick up the rest.
-                        for (String word : peekRemainingArgs()) {
-                            sb.append(" ").append(word);
-                        }
-                        // And now convert to byte[]...
-                        message = sb.toString().getBytes(UTF_8);
-                    }
-
-                    mSecureCommsManager.sendSecureMessage(associationId, /* messageId */ 0,
-                            message);
-                    break;
-
                 case "simulate-device-appeared":
                     associationId = getNextIntArgRequired();
                     mDevicePresenceMonitor.simulateDeviceAppeared(associationId);
@@ -167,8 +133,6 @@
         pw.println("      Create a new Association.");
         pw.println("  disassociate USER_ID PACKAGE MAC_ADDRESS");
         pw.println("      Remove an existing Association.");
-        pw.println("  send-secure-message ASSOCIATION_ID [--base64] MESSAGE");
-        pw.println("      Send a secure message to an associated companion device.");
         pw.println("  clear-association-memory-cache");
         pw.println("      Clear the in-memory association cache and reload all association ");
         pw.println("      information from persistent storage. USE FOR DEBUGGING PURPOSES ONLY.");
diff --git a/services/companion/java/com/android/server/companion/datatransfer/CompanionMessageInfo.java b/services/companion/java/com/android/server/companion/datatransfer/CompanionMessageInfo.java
deleted file mode 100644
index 91ad93c..0000000
--- a/services/companion/java/com/android/server/companion/datatransfer/CompanionMessageInfo.java
+++ /dev/null
@@ -1,50 +0,0 @@
-/*
- * Copyright (C) 2022 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 com.android.server.companion.datatransfer;
-
-class CompanionMessageInfo {
-
-    private final long mId;
-    private final int mPage;
-    private final int mTotal;
-    private final int mType;
-    private final byte[] mData;
-
-    CompanionMessageInfo(long id, int page, int total, int type, byte[] data) {
-        mId = id;
-        mPage = page;
-        mTotal = total;
-        mType = type;
-        mData = data;
-    }
-
-    public long getId() {
-        return mId;
-    }
-
-    public int getPage() {
-        return mPage;
-    }
-
-    public int getType() {
-        return mType;
-    }
-
-    public byte[] getData() {
-        return mData;
-    }
-}
diff --git a/services/companion/java/com/android/server/companion/datatransfer/CompanionMessageProcessor.java b/services/companion/java/com/android/server/companion/datatransfer/CompanionMessageProcessor.java
deleted file mode 100644
index 98a00aa6..0000000
--- a/services/companion/java/com/android/server/companion/datatransfer/CompanionMessageProcessor.java
+++ /dev/null
@@ -1,229 +0,0 @@
-/*
- * Copyright (C) 2022 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 com.android.server.companion.datatransfer;
-
-import android.annotation.NonNull;
-import android.annotation.Nullable;
-import android.util.Slog;
-import android.util.proto.ProtoInputStream;
-import android.util.proto.ProtoOutputStream;
-
-import com.android.server.companion.proto.CompanionMessage;
-import com.android.server.companion.securechannel.CompanionSecureCommunicationsManager;
-
-import java.io.ByteArrayOutputStream;
-import java.io.IOException;
-import java.util.ArrayList;
-import java.util.Arrays;
-import java.util.Comparator;
-import java.util.HashMap;
-import java.util.List;
-import java.util.Map;
-
-/**
- * This class builds and reads CompanionMessage. And also paginate and combine messages.
- */
-public class CompanionMessageProcessor {
-
-    private static final String LOG_TAG = CompanionMessageProcessor.class.getSimpleName();
-
-    /** Listener for incoming complete messages. */
-    interface Listener {
-        /** When a complete message is received from the companion app. */
-        void onCompleteMessageReceived(@NonNull CompanionMessageInfo message);
-    }
-
-    // Rough size for each CompanionMessage, each message can exceed 50K for a little, but not
-    // too much. Hard limit is 100K, WCS data processing limit. Closer to 100K, less stable at
-    // the WCS data processing layer. Refer to
-    // https://developers.google.com/android/reference/com/google/android/gms/wearable/MessageClient
-    // #public-abstract-taskinteger-sendmessage-string-nodeid,-string-path,-byte[]-data
-    private static final int MESSAGE_SIZE_IN_BYTES = 50000;
-
-    private final CompanionSecureCommunicationsManager mSecureCommsManager;
-
-    @Nullable
-    private Listener mListener;
-
-    // Association id -> (parent id -> received messages)
-    private final Map<Integer, Map<Integer, List<CompanionMessageInfo>>> mAssociationsMessagesMap =
-            new HashMap<>();
-    // Association id -> next parent id
-    private final Map<Integer, Integer> mNextParentId = new HashMap<>();
-
-    public CompanionMessageProcessor(CompanionSecureCommunicationsManager secureCommsManager) {
-        mSecureCommsManager = secureCommsManager;
-        mSecureCommsManager.setListener(this::onDecryptedMessageReceived);
-    }
-
-    public void setListener(@NonNull Listener listener) {
-        mListener = listener;
-    }
-
-    /**
-     * Paginate the data into multiple messages with size limit. And dispatch the messages to the
-     * companion app.
-     */
-    public void paginateAndDispatchMessagesToApp(byte[] data, int messageType,
-            String packageName, int userId, int associationId) {
-        Slog.i(LOG_TAG, "Paginating " + data.length + " bytes.");
-
-        final int totalMessageCount = (data.length / MESSAGE_SIZE_IN_BYTES)
-                + ((data.length % MESSAGE_SIZE_IN_BYTES == 0) ? 0 : 1);
-        int parentMessageId = findNextParentId(associationId, totalMessageCount);
-
-        for (int i = 0; i < totalMessageCount; i++) {
-            ProtoOutputStream proto = new ProtoOutputStream();
-            int messageId = parentMessageId + i + 1;
-            proto.write(CompanionMessage.ID, messageId);
-
-            long paginationInfoToken = proto.start(CompanionMessage.PAGINATION_INFO);
-            proto.write(CompanionMessage.PaginationInfo.PARENT_ID, parentMessageId);
-            proto.write(CompanionMessage.PaginationInfo.PAGE, i + 1);
-            proto.write(CompanionMessage.PaginationInfo.TOTAL, totalMessageCount);
-            proto.end(paginationInfoToken);
-
-            proto.write(CompanionMessage.TYPE, messageType);
-            byte[] currentData = Arrays.copyOfRange(data, i * MESSAGE_SIZE_IN_BYTES,
-                    Math.min((i + 1) * MESSAGE_SIZE_IN_BYTES, data.length));
-            proto.write(CompanionMessage.DATA, currentData);
-
-            byte[] message = proto.getBytes();
-
-            Slog.i(LOG_TAG, "Sending [" + message.length + "] bytes to " + packageName);
-
-            mSecureCommsManager.sendSecureMessage(associationId, messageId, message);
-        }
-    }
-
-    /**
-     * Process the message and store it. If all the messages with the same parent id have been
-     * received, return the message with combined message data. Otherwise, return null if there's
-     * still data parts missing.
-     */
-    public CompanionMessageInfo onDecryptedMessageReceived(int messageId, int associationId,
-            byte[] message) {
-        Slog.i(LOG_TAG, "Partial message received, size [" + message.length
-                + "], reading from protobuf.");
-
-        ProtoInputStream proto = new ProtoInputStream(message);
-        try {
-            int id = 0;
-            int parentId = 0;
-            int page = 0;
-            int total = 0;
-            int type = CompanionMessage.UNKNOWN;
-            byte[] data = null;
-
-            // Read proto data
-            while (proto.nextField() != ProtoInputStream.NO_MORE_FIELDS) {
-                switch (proto.getFieldNumber()) {
-                    case (int) CompanionMessage.ID:
-                        id = proto.readInt(CompanionMessage.ID);
-                        break;
-                    case (int) CompanionMessage.PAGINATION_INFO:
-                        long paginationToken = proto.start(CompanionMessage.PAGINATION_INFO);
-                        while (proto.nextField() != ProtoInputStream.NO_MORE_FIELDS) {
-                            switch (proto.getFieldNumber()) {
-                                case (int) CompanionMessage.PaginationInfo.PARENT_ID:
-                                    parentId = proto.readInt(
-                                            CompanionMessage.PaginationInfo.PARENT_ID);
-                                    break;
-                                case (int) CompanionMessage.PaginationInfo.PAGE:
-                                    page = proto.readInt(CompanionMessage.PaginationInfo.PAGE);
-                                    break;
-                                case (int) CompanionMessage.PaginationInfo.TOTAL:
-                                    total = proto.readInt(CompanionMessage.PaginationInfo.TOTAL);
-                                    break;
-                                default:
-                                    Slog.e(LOG_TAG, "Unexpected field id "
-                                            + proto.getFieldNumber() + " for PaginationInfo.");
-                                    break;
-                            }
-                        }
-                        proto.end(paginationToken);
-                        break;
-                    case (int) CompanionMessage.TYPE:
-                        type = proto.readInt(CompanionMessage.TYPE);
-                        break;
-                    case (int) CompanionMessage.DATA:
-                        data = proto.readBytes(CompanionMessage.DATA);
-                        break;
-                    default:
-                        Slog.e(LOG_TAG, "Unexpected field id " + proto.getFieldNumber()
-                                + " for CompanionMessage.");
-                        break;
-                }
-            }
-
-            if (id == messageId) {
-                CompanionMessageInfo messageInfo = new CompanionMessageInfo(id, page, total, type,
-                        data);
-                // Add the message into mAssociationsMessagesMap
-                Map<Integer, List<CompanionMessageInfo>> associationMessages =
-                        mAssociationsMessagesMap.getOrDefault(associationId, new HashMap<>());
-                List<CompanionMessageInfo> childMessages = associationMessages.getOrDefault(
-                        parentId, new ArrayList<>());
-                childMessages.add(messageInfo);
-                associationMessages.put(parentId, childMessages);
-                mAssociationsMessagesMap.put(associationId, associationMessages);
-                // Check if all the messages with the same parentId are received.
-                if (childMessages.size() == total) {
-                    Slog.i(LOG_TAG, "All [" + total + "] messages are received for parentId ["
-                            + parentId + "]. Processing.");
-
-                    childMessages.sort(Comparator.comparing(CompanionMessageInfo::getPage));
-                    ByteArrayOutputStream stream = new ByteArrayOutputStream();
-                    for (int i = 0; i < childMessages.size(); i++) {
-                        stream.write(childMessages.get(i).getData());
-                    }
-                    mAssociationsMessagesMap.remove(parentId);
-                    mListener.onCompleteMessageReceived(
-                            new CompanionMessageInfo(parentId, 0, total, type,
-                                    stream.toByteArray()));
-                } else {
-                    Slog.i(LOG_TAG, "[" + childMessages.size() + "/" + total
-                            + "] messages are received for parentId [" + parentId + "]");
-                }
-            } else {
-                Slog.e(LOG_TAG, "Message id mismatch.");
-                return null;
-            }
-        } catch (IOException e) {
-            Slog.e(LOG_TAG, "Can't read proto from the message.");
-            return null;
-        }
-        return null;
-    }
-
-    /**
-     * Find the next parent id from [1, Integer.MAX_VALUE].
-     * The parent and child ids are incremental.
-     */
-    private int findNextParentId(int associationId, int totalMessageCount) {
-        int nextParentId = mNextParentId.getOrDefault(associationId, 1);
-
-        // If the last child message id exceeds the Integer range, start from 1 again.
-        if (nextParentId > Integer.MAX_VALUE - totalMessageCount - 1) {
-            nextParentId = 1;
-        }
-
-        mNextParentId.put(associationId, nextParentId + totalMessageCount + 1);
-
-        return nextParentId;
-    }
-}
diff --git a/services/companion/java/com/android/server/companion/datatransfer/SystemDataTransferProcessor.java b/services/companion/java/com/android/server/companion/datatransfer/SystemDataTransferProcessor.java
index 7eede55..f0a0492 100644
--- a/services/companion/java/com/android/server/companion/datatransfer/SystemDataTransferProcessor.java
+++ b/services/companion/java/com/android/server/companion/datatransfer/SystemDataTransferProcessor.java
@@ -38,34 +38,27 @@
 import android.os.Binder;
 import android.os.Bundle;
 import android.os.Handler;
-import android.os.ParcelFileDescriptor;
+import android.os.RemoteException;
 import android.os.ResultReceiver;
 import android.os.UserHandle;
 import android.permission.PermissionControllerManager;
 import android.util.Slog;
-import android.util.SparseArray;
 
-import com.android.internal.annotations.GuardedBy;
 import com.android.server.companion.AssociationStore;
 import com.android.server.companion.CompanionDeviceManagerService;
 import com.android.server.companion.PermissionsUtils;
-import com.android.server.companion.proto.CompanionMessage;
+import com.android.server.companion.transport.CompanionTransportManager;
 
-import libcore.io.IoUtils;
-import libcore.io.Streams;
-
-import java.io.IOException;
-import java.io.InputStream;
-import java.io.OutputStream;
-import java.nio.ByteBuffer;
 import java.util.List;
 import java.util.concurrent.ExecutorService;
 import java.util.concurrent.Executors;
+import java.util.concurrent.Future;
+import java.util.concurrent.TimeUnit;
 
 /**
- * This processor builds user consent intent for a given SystemDataTransferRequest and processes the
- * request when the system is ready (a secure channel is established between the handhold and the
- * companion device).
+ * This processor builds user consent intent for a
+ * {@link SystemDataTransferRequest} and processes the request once a channel is
+ * established between the local and remote companion device.
  */
 public class SystemDataTransferProcessor {
 
@@ -85,21 +78,19 @@
     private final Context mContext;
     private final AssociationStore mAssociationStore;
     private final SystemDataTransferRequestStore mSystemDataTransferRequestStore;
-    private final CompanionMessageProcessor mCompanionMessageProcessor;
+    private final CompanionTransportManager mTransportManager;
     private final PermissionControllerManager mPermissionControllerManager;
     private final ExecutorService mExecutor;
-    @GuardedBy("mTransports")
-    private final SparseArray<Transport> mTransports = new SparseArray<>();
 
     public SystemDataTransferProcessor(CompanionDeviceManagerService service,
             AssociationStore associationStore,
             SystemDataTransferRequestStore systemDataTransferRequestStore,
-            CompanionMessageProcessor companionMessageProcessor) {
+            CompanionTransportManager transportManager) {
         mContext = service.getContext();
         mAssociationStore = associationStore;
         mSystemDataTransferRequestStore = systemDataTransferRequestStore;
-        mCompanionMessageProcessor = companionMessageProcessor;
-        mCompanionMessageProcessor.setListener(this::onCompleteMessageReceived);
+        mTransportManager = transportManager;
+        mTransportManager.setListener(this::onReceivePermissionRestore);
         mPermissionControllerManager = mContext.getSystemService(PermissionControllerManager.class);
         mExecutor = Executors.newSingleThreadExecutor();
     }
@@ -194,73 +185,42 @@
             return;
         }
 
-        // TODO: Establish a secure channel
-
         // Start permission sync
         final long callingIdentityToken = Binder.clearCallingIdentity();
         try {
+            // TODO: refactor to work with streams of data
             mPermissionControllerManager.getRuntimePermissionBackup(UserHandle.of(userId),
-                    mExecutor,
-                    backup -> mCompanionMessageProcessor.paginateAndDispatchMessagesToApp(backup,
-                        CompanionMessage.PERMISSION_SYNC, packageName, userId, associationId));
+                    mExecutor, backup -> {
+                        Future<?> result = mTransportManager
+                                .requestPermissionRestore(associationId, backup);
+                        try {
+                            result.get(15, TimeUnit.SECONDS);
+                            try {
+                                callback.onResult();
+                            } catch (RemoteException ignored) {
+                            }
+                        } catch (Exception e) {
+                            try {
+                                callback.onError(e.getMessage());
+                            } catch (RemoteException ignored) {
+                            }
+                        }
+                    });
         } finally {
             Binder.restoreCallingIdentity(callingIdentityToken);
         }
     }
 
-    public void attachSystemDataTransport(String packageName, int userId, int associationId,
-            ParcelFileDescriptor fd) {
-        synchronized (mTransports) {
-            // TODO: restore once testing has evolved
-            // resolveAssociation(packageName, userId, associationId);
-
-            if (mTransports.contains(associationId)) {
-                detachSystemDataTransport(packageName, userId, associationId);
-            }
-
-            final Transport transport = new Transport(fd);
-            transport.start();
-            mTransports.put(associationId, transport);
-        }
-    }
-
-    public void detachSystemDataTransport(String packageName, int userId, int associationId) {
-        synchronized (mTransports) {
-            // TODO: restore once testing has evolved
-            // resolveAssociation(packageName, userId, associationId);
-
-            final Transport transport = mTransports.get(associationId);
-            if (transport != null) {
-                mTransports.delete(associationId);
-                transport.stop();
-            }
-        }
-    }
-
-    /**
-     * Process a complete decrypted message reported by the companion app.
-     */
-    public void onCompleteMessageReceived(@NonNull CompanionMessageInfo completeMessage) {
-        switch (completeMessage.getType()) {
-            case CompanionMessage.PERMISSION_SYNC:
-                processPermissionSyncMessage(completeMessage);
-                break;
-            default:
-                Slog.e(LOG_TAG, "Unknown message type [" + completeMessage.getType()
-                        + "]. Unable to process.");
-        }
-    }
-
-    private void processPermissionSyncMessage(CompanionMessageInfo messageInfo) {
+    private void onReceivePermissionRestore(byte[] message) {
         Slog.i(LOG_TAG, "Applying permissions.");
         // Start applying permissions
         UserHandle user = mContext.getUser();
         final long callingIdentityToken = Binder.clearCallingIdentity();
         try {
+            // TODO: refactor to work with streams of data
             mPermissionControllerManager.stageAndApplyRuntimePermissionsBackup(
-                    messageInfo.getData(), user);
+                    message, user);
         } finally {
-            Slog.i(LOG_TAG, "Permissions applied.");
             Binder.restoreCallingIdentity(callingIdentityToken);
         }
     }
@@ -291,74 +251,4 @@
                     Slog.e(LOG_TAG, "Unknown result code:" + resultCode);
                 }
             };
-
-    private class Transport {
-        private final InputStream mRemoteIn;
-        private final OutputStream mRemoteOut;
-
-        private volatile boolean mStopped;
-
-        public Transport(ParcelFileDescriptor fd) {
-            mRemoteIn = new ParcelFileDescriptor.AutoCloseInputStream(fd);
-            mRemoteOut = new ParcelFileDescriptor.AutoCloseOutputStream(fd);
-        }
-
-        public void start() {
-            new Thread(() -> {
-                try {
-                    while (!mStopped) {
-                        processNextCommand();
-                    }
-                } catch (IOException e) {
-                    if (!mStopped) {
-                        Slog.w(LOG_TAG, "Trouble during transport", e);
-                        stop();
-                    }
-                }
-            }).start();
-        }
-
-        public void stop() {
-            mStopped = true;
-
-            IoUtils.closeQuietly(mRemoteIn);
-            IoUtils.closeQuietly(mRemoteOut);
-        }
-
-        private void processNextCommand() throws IOException {
-            Slog.d(LOG_TAG, "Waiting for next command...");
-
-            // Read message header
-            final byte[] headerBytes = new byte[8];
-            Streams.readFully(mRemoteIn, headerBytes);
-            final ByteBuffer header = ByteBuffer.wrap(headerBytes);
-            final int command = header.getInt();
-            final int length = header.getInt();
-
-            Slog.d(LOG_TAG, "Received command 0x" + Integer.toHexString(command)
-                    + " length " + length);
-            switch (command) {
-                case 0x50490000: // PI(NG) version 0
-                    // Repeat back the given payload, within reason
-                    final int target = Math.min(length, 1_000_000);
-                    final byte[] payload = new byte[target];
-                    Streams.readFully(mRemoteIn, payload);
-                    Streams.skipByReading(mRemoteIn, length - target);
-
-                    // Respond with PO(NG) version 0
-                    header.rewind();
-                    header.putInt(0x504F0000);
-                    header.putInt(target);
-                    mRemoteOut.write(header.array());
-                    mRemoteOut.write(payload);
-                    break;
-
-                default:
-                    // Emit local warning, and skip message to
-                    // handle next one
-                    Slog.w(LOG_TAG, "Unknown command 0x" + Integer.toHexString(command));
-                    mRemoteIn.skip(length);
-            }
-        }
-    }
 }
diff --git a/services/companion/java/com/android/server/companion/proto/companion_message.proto b/services/companion/java/com/android/server/companion/proto/companion_message.proto
deleted file mode 100644
index 893a0db..0000000
--- a/services/companion/java/com/android/server/companion/proto/companion_message.proto
+++ /dev/null
@@ -1,57 +0,0 @@
-/*
- * Copyright (C) 2022 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.
- */
-
-syntax = "proto3";
-
-option java_multiple_files = true;
-
-package com.android.server.companion.proto;
-
-/* Represents a message between companion devices */
-message CompanionMessage {
-  int32 id = 1;
-
-  PaginationInfo paginationInfo = 2;
-
-  Type type = 3;
-
-  // message body data
-  bytes data = 4;
-
-  /* Message pagination info */
-  message PaginationInfo {
-    // id of the parent message, which should be the same for all the paginated messages
-    int32 parentId = 1;
-
-    // page number of the current message in [1, total]
-    int32 page = 2;
-
-    // total number of messages
-    int32 total = 3;
-  }
-
-  /* Message type */
-  enum Type {
-    // default value for proto3
-    UNKNOWN = 0;
-
-    // handshake message to establish secure channel
-    SECURE_CHANNEL_HANDSHAKE = 1;
-
-    // permission sync
-    PERMISSION_SYNC = 2;
-  }
-}
diff --git a/services/companion/java/com/android/server/companion/securechannel/CompanionSecureCommunicationsManager.java b/services/companion/java/com/android/server/companion/securechannel/CompanionSecureCommunicationsManager.java
deleted file mode 100644
index 16a74ec..0000000
--- a/services/companion/java/com/android/server/companion/securechannel/CompanionSecureCommunicationsManager.java
+++ /dev/null
@@ -1,121 +0,0 @@
-/*
- * Copyright (C) 2022 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 com.android.server.companion.securechannel;
-
-import android.annotation.NonNull;
-import android.annotation.Nullable;
-import android.annotation.SuppressLint;
-import android.companion.AssociationInfo;
-import android.util.Base64;
-import android.util.Log;
-import android.util.Slog;
-
-import com.android.server.companion.AssociationStore;
-import com.android.server.companion.CompanionApplicationController;
-
-/** Secure Comms Manager */
-@SuppressLint("LongLogTag")
-public class CompanionSecureCommunicationsManager {
-    static final String TAG = "CompanionDevice_SecureComms";
-    static final boolean DEBUG = false;
-
-    /** Listener for incoming decrypted messages. */
-    public interface Listener {
-        /** When an incoming message is decrypted. */
-        void onDecryptedMessageReceived(int messageId, int associationId, byte[] message);
-    }
-
-    private final AssociationStore mAssociationStore;
-    private final CompanionApplicationController mCompanionAppController;
-
-    @Nullable
-    private Listener mListener;
-
-    /** Constructor */
-    public CompanionSecureCommunicationsManager(AssociationStore associationStore,
-            CompanionApplicationController companionApplicationController) {
-        mAssociationStore = associationStore;
-        mCompanionAppController = companionApplicationController;
-    }
-
-    public void setListener(@NonNull Listener listener) {
-        mListener = listener;
-    }
-
-    /**
-     * Send a data to the associated companion device via secure channel (establishing one if
-     * needed).
-     * @param associationId associationId of the "recipient" companion device.
-     * @param messageId id of the message
-     * @param message data to be sent securely.
-     */
-    public void sendSecureMessage(int associationId, int messageId, @NonNull byte[] message) {
-        if (DEBUG) {
-            Log.d(TAG, "sendSecureMessage() associationId=" + associationId + "\n"
-                    + "   message (Base64)=\"" + Base64.encodeToString(message, 0) + "\"");
-        }
-
-        final AssociationInfo association = mAssociationStore.getAssociationById(associationId);
-        if (association == null) {
-            throw new IllegalArgumentException(
-                    "Association with ID " + associationId + " does not exist");
-        }
-        if (DEBUG) Log.d(TAG, "  association=" + association);
-
-        final int userId = association.getUserId();
-        final String packageName = association.getPackageName();
-
-        // Bind to the app if it hasn't been bound.
-        if (!mCompanionAppController.isCompanionApplicationBound(userId, packageName)) {
-            Slog.d(TAG, "userId [" + userId + "] packageName [" + packageName
-                    + "] is not bound. Binding it now to send a secure message.");
-            mCompanionAppController.bindCompanionApplication(userId, packageName,
-                    association.isSelfManaged());
-
-            // TODO(b/202926196): implement: encrypt and pass on the companion application for
-            //  transporting
-            mCompanionAppController.dispatchMessage(userId, packageName, associationId, messageId,
-                    message);
-
-            Slog.d(TAG, "Unbinding userId [" + userId + "] packageName [" + packageName
-                    + "]");
-            mCompanionAppController.unbindCompanionApplication(userId, packageName);
-        }
-
-        // TODO(b/202926196): implement: encrypt and pass on the companion application for
-        //  transporting
-        mCompanionAppController.dispatchMessage(userId, packageName, associationId, messageId,
-                message);
-    }
-
-    /**
-     * Decrypt and dispatch message received from an associated companion device.
-     * @param associationId associationId of the "sender" companion device.
-     * @param encryptedMessage data.
-     */
-    public void receiveSecureMessage(int messageId, int associationId,
-            @NonNull byte[] encryptedMessage) {
-        if (DEBUG) {
-            Log.d(TAG, "sendSecureMessage() associationId=" + associationId + "\n"
-                    + "   message (Base64)=\"" + Base64.encodeToString(encryptedMessage, 0) + "\"");
-        }
-
-        // TODO(b/202926196): implement: decrypt and dispatch
-
-        mListener.onDecryptedMessageReceived(messageId, associationId, encryptedMessage);
-    }
-}
diff --git a/services/companion/java/com/android/server/companion/securechannel/OWNERS b/services/companion/java/com/android/server/companion/securechannel/OWNERS
deleted file mode 100644
index ecb97f4..0000000
--- a/services/companion/java/com/android/server/companion/securechannel/OWNERS
+++ /dev/null
@@ -1,4 +0,0 @@
-set noparent
-
-sergeynv@google.com
-ewol@google.com
diff --git a/services/companion/java/com/android/server/companion/transport/CompanionTransportManager.java b/services/companion/java/com/android/server/companion/transport/CompanionTransportManager.java
new file mode 100644
index 0000000..f01bdf2
--- /dev/null
+++ b/services/companion/java/com/android/server/companion/transport/CompanionTransportManager.java
@@ -0,0 +1,318 @@
+/*
+ * Copyright (C) 2022 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 com.android.server.companion.transport;
+
+import static android.Manifest.permission.DELIVER_COMPANION_MESSAGES;
+
+import android.annotation.NonNull;
+import android.annotation.Nullable;
+import android.annotation.SuppressLint;
+import android.app.ActivityManagerInternal;
+import android.content.Context;
+import android.content.pm.ApplicationInfo;
+import android.content.pm.PackageManager;
+import android.content.pm.PackageManager.NameNotFoundException;
+import android.os.Binder;
+import android.os.Build;
+import android.os.ParcelFileDescriptor;
+import android.util.Slog;
+import android.util.SparseArray;
+
+import com.android.internal.annotations.GuardedBy;
+import com.android.server.LocalServices;
+
+import libcore.io.IoUtils;
+import libcore.io.Streams;
+import libcore.util.EmptyArray;
+
+import java.io.IOException;
+import java.io.InputStream;
+import java.io.OutputStream;
+import java.nio.ByteBuffer;
+import java.util.concurrent.CompletableFuture;
+import java.util.concurrent.Future;
+import java.util.concurrent.atomic.AtomicInteger;
+
+@SuppressLint("LongLogTag")
+public class CompanionTransportManager {
+    private static final String TAG = "CompanionTransportManager";
+    // TODO: flip to false
+    private static final boolean DEBUG = true;
+
+    private static final int HEADER_LENGTH = 12;
+    // TODO: refactor message processing to use streams to remove this limit
+    private static final int MAX_PAYLOAD_LENGTH = 1_000_000;
+
+    private static final int MESSAGE_REQUEST_PING = 0x63807378; // ?PIN
+    private static final int MESSAGE_REQUEST_PERMISSION_RESTORE = 0x63826983; // ?RES
+
+    private static final int MESSAGE_RESPONSE_SUCCESS = 0x33838567; // !SUC
+    private static final int MESSAGE_RESPONSE_FAILURE = 0x33706573; // !FAI
+
+    private static boolean isRequest(int message) {
+        return (message & 0xFF000000) == 0x63000000;
+    }
+
+    private static boolean isResponse(int message) {
+        return (message & 0xFF000000) == 0x33000000;
+    }
+
+    public interface Listener {
+        void onRequestPermissionRestore(byte[] data);
+    }
+
+    private final Context mContext;
+
+    @GuardedBy("mTransports")
+    private final SparseArray<Transport> mTransports = new SparseArray<>();
+
+    @Nullable
+    private Listener mListener;
+
+    public CompanionTransportManager(Context context) {
+        mContext = context;
+    }
+
+    public void setListener(@NonNull Listener listener) {
+        mListener = listener;
+    }
+
+    /**
+     * For the moment, we only offer transporting of system data to built-in
+     * companion apps; future work will improve the security model to support
+     * third-party companion apps.
+     */
+    private void enforceCallerCanTransportSystemData(String packageName, int userId) {
+        mContext.enforceCallingOrSelfPermission(DELIVER_COMPANION_MESSAGES, TAG);
+
+        try {
+            final ApplicationInfo info = mContext.getPackageManager().getApplicationInfoAsUser(
+                    packageName, 0, userId);
+            final int instrumentationUid = LocalServices.getService(ActivityManagerInternal.class)
+                    .getInstrumentationSourceUid(Binder.getCallingUid());
+            if (!Build.isDebuggable() && !info.isSystemApp()
+                    && instrumentationUid == android.os.Process.INVALID_UID) {
+                throw new SecurityException("Transporting of system data currently only available "
+                        + "to built-in companion apps or tests");
+            }
+        } catch (NameNotFoundException e) {
+            throw new IllegalArgumentException(e);
+        }
+    }
+
+    public void attachSystemDataTransport(String packageName, int userId, int associationId,
+            ParcelFileDescriptor fd) {
+        enforceCallerCanTransportSystemData(packageName, userId);
+        synchronized (mTransports) {
+            if (mTransports.contains(associationId)) {
+                detachSystemDataTransport(packageName, userId, associationId);
+            }
+
+            final Transport transport = new Transport(associationId, fd);
+            transport.start();
+            mTransports.put(associationId, transport);
+        }
+    }
+
+    public void detachSystemDataTransport(String packageName, int userId, int associationId) {
+        enforceCallerCanTransportSystemData(packageName, userId);
+        synchronized (mTransports) {
+            final Transport transport = mTransports.get(associationId);
+            if (transport != null) {
+                mTransports.delete(associationId);
+                transport.stop();
+            }
+        }
+    }
+
+    public Future<?> requestPermissionRestore(int associationId, byte[] data) {
+        synchronized (mTransports) {
+            final Transport transport = mTransports.get(associationId);
+            if (transport != null) {
+                return transport.requestForResponse(MESSAGE_REQUEST_PERMISSION_RESTORE, data);
+            } else {
+                return CompletableFuture.failedFuture(new IOException("Missing transport"));
+            }
+        }
+    }
+
+    private class Transport {
+        private final int mAssociationId;
+
+        private final InputStream mRemoteIn;
+        private final OutputStream mRemoteOut;
+
+        private final AtomicInteger mNextSequence = new AtomicInteger();
+
+        @GuardedBy("mPendingRequests")
+        private final SparseArray<CompletableFuture<byte[]>> mPendingRequests = new SparseArray<>();
+
+        private volatile boolean mStopped;
+
+        public Transport(int associationId, ParcelFileDescriptor fd) {
+            mAssociationId = associationId;
+            mRemoteIn = new ParcelFileDescriptor.AutoCloseInputStream(fd);
+            mRemoteOut = new ParcelFileDescriptor.AutoCloseOutputStream(fd);
+        }
+
+        public void start() {
+            new Thread(() -> {
+                try {
+                    while (!mStopped) {
+                        receiveMessage();
+                    }
+                } catch (IOException e) {
+                    if (!mStopped) {
+                        Slog.w(TAG, "Trouble during transport", e);
+                        stop();
+                    }
+                }
+            }).start();
+        }
+
+        public void stop() {
+            mStopped = true;
+
+            IoUtils.closeQuietly(mRemoteIn);
+            IoUtils.closeQuietly(mRemoteOut);
+        }
+
+        public Future<byte[]> requestForResponse(int message, byte[] data) {
+            final int sequence = mNextSequence.incrementAndGet();
+            final CompletableFuture<byte[]> pending = new CompletableFuture<>();
+            synchronized (mPendingRequests) {
+                mPendingRequests.put(sequence, pending);
+            }
+            try {
+                sendMessage(message, sequence, data);
+            } catch (IOException e) {
+                pending.completeExceptionally(e);
+            }
+            return pending;
+        }
+
+        private void sendMessage(int message, int sequence, @NonNull byte[] data)
+                throws IOException {
+            if (DEBUG) {
+                Slog.d(TAG, "Sending message 0x" + Integer.toHexString(message)
+                        + " sequence " + sequence + " length " + data.length
+                        + " to association " + mAssociationId);
+            }
+
+            synchronized (mRemoteOut) {
+                final ByteBuffer header = ByteBuffer.allocate(HEADER_LENGTH)
+                        .putInt(message)
+                        .putInt(sequence)
+                        .putInt(data.length);
+                mRemoteOut.write(header.array());
+                mRemoteOut.write(data);
+                mRemoteOut.flush();
+            }
+        }
+
+        private void receiveMessage() throws IOException {
+            if (DEBUG) {
+                Slog.d(TAG, "Waiting for next message...");
+            }
+
+            final byte[] headerBytes = new byte[HEADER_LENGTH];
+            Streams.readFully(mRemoteIn, headerBytes);
+            final ByteBuffer header = ByteBuffer.wrap(headerBytes);
+            final int message = header.getInt();
+            final int sequence = header.getInt();
+            final int length = header.getInt();
+
+            if (DEBUG) {
+                Slog.d(TAG, "Received message 0x" + Integer.toHexString(message)
+                        + " sequence " + sequence + " length " + length
+                        + " from association " + mAssociationId);
+            }
+            if (length > MAX_PAYLOAD_LENGTH) {
+                Slog.w(TAG, "Ignoring message 0x" + Integer.toHexString(message)
+                        + " sequence " + sequence + " length " + length
+                        + " from association " + mAssociationId + " beyond maximum length");
+                Streams.skipByReading(mRemoteIn, length);
+                return;
+            }
+
+            final byte[] data = new byte[length];
+            Streams.readFully(mRemoteIn, data);
+
+            if (isRequest(message)) {
+                processRequest(message, sequence, data);
+            } else if (isResponse(message)) {
+                processResponse(message, sequence, data);
+            } else {
+                Slog.w(TAG, "Unknown message " + Integer.toHexString(message));
+            }
+        }
+
+        private void processRequest(int message, int sequence, byte[] data)
+                throws IOException {
+            switch (message) {
+                case MESSAGE_REQUEST_PING: {
+                    sendMessage(MESSAGE_RESPONSE_SUCCESS, sequence, data);
+                    break;
+                }
+                case MESSAGE_REQUEST_PERMISSION_RESTORE: {
+                    if (!mContext.getPackageManager().hasSystemFeature(PackageManager.FEATURE_WATCH)
+                            && !Build.isDebuggable()) {
+                        Slog.w(TAG, "Restoring permissions only supported on watches");
+                        sendMessage(MESSAGE_RESPONSE_FAILURE, sequence, EmptyArray.BYTE);
+                        break;
+                    }
+                    try {
+                        mListener.onRequestPermissionRestore(data);
+                        sendMessage(MESSAGE_RESPONSE_SUCCESS, sequence, EmptyArray.BYTE);
+                    } catch (Exception e) {
+                        Slog.w(TAG, "Failed to restore permissions");
+                        sendMessage(MESSAGE_RESPONSE_FAILURE, sequence, EmptyArray.BYTE);
+                    }
+                    break;
+                }
+                default: {
+                    Slog.w(TAG, "Unknown request " + Integer.toHexString(message));
+                    sendMessage(MESSAGE_RESPONSE_FAILURE, sequence, EmptyArray.BYTE);
+                    break;
+                }
+            }
+        }
+
+        private void processResponse(int message, int sequence, byte[] data) {
+            final CompletableFuture<byte[]> future;
+            synchronized (mPendingRequests) {
+                future = mPendingRequests.removeReturnOld(sequence);
+            }
+            if (future == null) {
+                Slog.w(TAG, "Ignoring unknown sequence " + sequence);
+                return;
+            }
+
+            switch (message) {
+                case MESSAGE_RESPONSE_SUCCESS: {
+                    future.complete(data);
+                }
+                case MESSAGE_RESPONSE_FAILURE: {
+                    future.completeExceptionally(new RuntimeException("Remote failure"));
+                }
+                default: {
+                    Slog.w(TAG, "Ignoring unknown response " + Integer.toHexString(message));
+                }
+            }
+        }
+    }
+}