diff --git a/xwords4/android/app/src/main/java/org/eehouse/android/xw4/MQTTUtils.java b/xwords4/android/app/src/main/java/org/eehouse/android/xw4/MQTTUtils.java
index 183555366..9227c660b 100644
--- a/xwords4/android/app/src/main/java/org/eehouse/android/xw4/MQTTUtils.java
+++ b/xwords4/android/app/src/main/java/org/eehouse/android/xw4/MQTTUtils.java
@@ -483,6 +483,27 @@ public class MQTTUtils extends Thread implements IMqttActionListener, MqttCallba
         addToSendQueue( context, topic[0], packet );
     }
 
+    public static void ackMessage( Context context, int gameID,
+                                   String senderDevID, byte[] payload )
+    {
+        String sum = Utils.getMD5SumFor( payload );
+        JSONObject params = new JSONObject();
+        try {
+            params.put( "sum", sum );
+            params.put( "gid", gameID );
+            // params.put( "from", senderDevID );
+            // params.put( "to", XwJNI.dvc_getMQTTDevID( null ) );
+
+            HttpsURLConnection conn
+                = NetUtils.makeHttpsMQTTConn( context, "ack" );
+            String resStr = NetUtils.runConn( conn, params, true );
+            Log.d( TAG, "runConn(ack) => %s", resStr );
+        } catch ( JSONException je ) {
+            Log.e( TAG, "ackMessage() ex: %s", je );
+        }
+    }
+
+    // MqttCallbackExtended
     @Override
     public void connectComplete(boolean reconnect, String serverURI)
     {
@@ -498,7 +519,7 @@ public class MQTTUtils extends Thread implements IMqttActionListener, MqttCallba
     }
 
     @Override
-    public void messageArrived( String topic, MqttMessage message) throws Exception
+    public void messageArrived( String topic, MqttMessage message ) throws Exception
     {
         Log.d( TAG, "%H.messageArrived(topic=%s)", this, topic );
         Assert.assertTrueNR( topic.equals(mTopic) );
@@ -533,6 +554,7 @@ public class MQTTUtils extends Thread implements IMqttActionListener, MqttCallba
         }
     }
 
+    // IMqttActionListener
     @Override
     public void onSuccess( IMqttToken asyncActionToken )
     {
diff --git a/xwords4/android/app/src/main/java/org/eehouse/android/xw4/jni/DUtilCtxt.java b/xwords4/android/app/src/main/java/org/eehouse/android/xw4/jni/DUtilCtxt.java
index 1989dd449..e05292e8d 100644
--- a/xwords4/android/app/src/main/java/org/eehouse/android/xw4/jni/DUtilCtxt.java
+++ b/xwords4/android/app/src/main/java/org/eehouse/android/xw4/jni/DUtilCtxt.java
@@ -372,4 +372,9 @@ public class DUtilCtxt {
         Assert.assertTrueNR( from.contains( CommsAddrRec.CommsConnType.COMMS_CONN_MQTT ) );
         MQTTUtils.handleGameGone( m_context, from, gameID );
     }
+
+    public void ackMQTTMsg( int gameID, String senderID, byte[] msg )
+    {
+        MQTTUtils.ackMessage( m_context, gameID, senderID, msg );
+    }
 }
diff --git a/xwords4/android/jni/utilwrapper.c b/xwords4/android/jni/utilwrapper.c
index 13851506c..11fd07e21 100644
--- a/xwords4/android/jni/utilwrapper.c
+++ b/xwords4/android/jni/utilwrapper.c
@@ -29,6 +29,7 @@
 #include "LocalizedStrIncludes.h"
 #include "dbgutil.h"
 #include "nli.h"
+#include "strutils.h"
 
 #define MAX_QUANTITY_STRS 4
 
@@ -919,6 +920,23 @@ and_dutil_onGameGoneReceived( XW_DUtilCtxt* duc, XWEnv xwe, XP_U32 gameID,
     DUTIL_CBK_TAIL();
 }
 
+static void
+and_dutil_ackMQTTMsg( XW_DUtilCtxt* duc, XWEnv xwe, XP_U32 gameID,
+                      const MQTTDevID* senderID, const XP_U8* msg, XP_U16 len )
+{
+    DUTIL_CBK_HEADER( "ackMQTTMsg", "(ILjava/lang/String;[B)V" );
+
+    XP_UCHAR tmp[32];
+    formatMQTTDevID( senderID, tmp, VSIZE(tmp) );
+    jstring jdevid = (*env)->NewStringUTF( env, tmp );
+    jbyteArray jmsg = makeByteArray( env, len, (const jbyte*)msg );
+
+    (*env)->CallVoidMethod( env, dutil->jdutil, mid, gameID, jdevid, jmsg );
+
+    deleteLocalRefs( env, jdevid, jmsg, DELETE_NO_REF );
+    DUTIL_CBK_TAIL();
+}
+
 XW_UtilCtxt*
 makeUtil( MPFORMAL JNIEnv* env,
 #ifdef MAP_THREAD_TO_ENV
@@ -1073,6 +1091,7 @@ makeDUtil( MPFORMAL JNIEnv* env,
     SET_DPROC(onInviteReceived);
     SET_DPROC(onMessageReceived);
     SET_DPROC(onGameGoneReceived);
+    SET_DPROC(ackMQTTMsg);
 
 #undef SET_DPROC
 
diff --git a/xwords4/common/device.c b/xwords4/common/device.c
index 59718d085..effc3c033 100644
--- a/xwords4/common/device.c
+++ b/xwords4/common/device.c
@@ -223,9 +223,14 @@ dvc_parseMQTTPacket( XW_DUtilCtxt* dutil, XWEnv xwe, const XP_U8* buf, XP_U16 le
         XP_LOGFF( "read proto %d, expected %d; dropping packet",
                   proto, PROTO_1 );
     } else {
-        MQTTDevID myID;
-        stream_getBytes( stream, &myID, sizeof(myID) );
-        myID = be64toh( myID );
+        MQTTDevID senderID;
+        stream_getBytes( stream, &senderID, sizeof(senderID) );
+        senderID = be64toh( senderID );
+#ifdef DEBUG
+        XP_UCHAR tmp[32];
+        formatMQTTDevID( &senderID, tmp, VSIZE(tmp) );
+        XP_LOGFF( "senderID: %s", tmp );
+#endif
 
         MQTTCmd cmd;
         XP_U32 gameID = 0;
@@ -233,6 +238,9 @@ dvc_parseMQTTPacket( XW_DUtilCtxt* dutil, XWEnv xwe, const XP_U8* buf, XP_U16 le
         gameID = stream_getU32( stream );
         cmd = stream_getU8( stream );
 
+        /* Need to ack even if discarded/malformed */
+        dutil_ackMQTTMsg( dutil, xwe, gameID, &senderID, buf, len );
+
         switch ( cmd ) {
         case CMD_INVITE: {
             NetLaunchInfo nli = {0};
@@ -245,7 +253,7 @@ dvc_parseMQTTPacket( XW_DUtilCtxt* dutil, XWEnv xwe, const XP_U8* buf, XP_U16 le
         case CMD_MSG: {
             CommsAddrRec from = {0};
             addr_addType( &from, COMMS_CONN_MQTT );
-            from.u.mqtt.devID = myID;
+            from.u.mqtt.devID = senderID;
             if ( CMD_MSG == cmd ) {
                 dutil_onMessageReceived( dutil, xwe, gameID, &from, stream );
             } else if ( CMD_DEVGONE == cmd ) {
diff --git a/xwords4/common/dutil.h b/xwords4/common/dutil.h
index ebb9e8606..4ec1d59a7 100644
--- a/xwords4/common/dutil.h
+++ b/xwords4/common/dutil.h
@@ -91,6 +91,9 @@ typedef struct _DUtilVtable {
                                        const CommsAddrRec* from, XWStreamCtxt* stream );
     void (*m_dutil_onGameGoneReceived)( XW_DUtilCtxt* duc, XWEnv xwe, XP_U32 gameID,
                                        const CommsAddrRec* from );
+
+    void (*m_dutil_ackMQTTMsg)( XW_DUtilCtxt* duc, XWEnv xwe, XP_U32 gameID,
+                                const MQTTDevID* senderID, const XP_U8* msg, XP_U16 len );
 } DUtilVtable;
 
 struct XW_DUtilCtxt {
@@ -160,4 +163,7 @@ void dutil_super_init( MPFORMAL XW_DUtilCtxt* dutil );
 #define dutil_onGameGoneReceived(duc, xwe, gameID, from)         \
     (duc)->vtable.m_dutil_onGameGoneReceived((duc),(xwe),(gameID),(from))
 
+#define dutil_ackMQTTMsg( duc, xwe, gameID, senderID, msg, len )    \
+    (duc)->vtable.m_dutil_ackMQTTMsg((duc), (xwe), (gameID), (senderID), (msg), (len) )
+
 #endif
diff --git a/xwords4/linux/lindutil.c b/xwords4/linux/lindutil.c
index e0f913197..98556caea 100644
--- a/xwords4/linux/lindutil.c
+++ b/xwords4/linux/lindutil.c
@@ -130,6 +130,19 @@ linux_dutil_onGameGoneReceived( XW_DUtilCtxt* duc, XWEnv XP_UNUSED(xwe),
     }
 }
 
+static void
+linux_dutil_ackMQTTMsg( XW_DUtilCtxt* duc, XWEnv xwe, XP_U32 gameID,
+                        const MQTTDevID* senderID, const XP_U8* msg, XP_U16 len )
+{
+    XP_USE(duc);
+    XP_USE(xwe);
+    XP_USE(gameID);
+    XP_USE(senderID);
+    XP_USE(msg);
+    XP_USE(len);
+    XP_LOGFF( "doing nothing" );
+}
+
 XW_DUtilCtxt*
 linux_dutils_init( MPFORMAL VTableMgr* vtMgr, void* closure )
 {
@@ -168,6 +181,7 @@ linux_dutils_init( MPFORMAL VTableMgr* vtMgr, void* closure )
     SET_PROC(onInviteReceived);
     SET_PROC(onMessageReceived);
     SET_PROC(onGameGoneReceived);
+    SET_PROC(ackMQTTMsg);
 
 # undef SET_PROC