Use the expected sequence number for challenge ACK messages

That avoids avoid endless ACK/RST retransmissions.

FreeRTOS-Plus-TCP issue #1235

Author: mj-servus

source/FreeRTOS_TCP_IP.c

@@ -882,7 +882,7 @@
                                                           pxSocket->u.xTCP.xTCPWindow.xSize.ulRxWindowLength ) != pdFALSE ) )
                             {
                                 /* Send a challenge ACK. */
-                                ( void ) prvTCPSendChallengeAck( pxNetworkBuffer );
+                                ( void ) prvTCPSendChallengeAck( pxNetworkBuffer, pxSocket->u.xTCP.xTCPWindow.rx.ulCurrentSequenceNumber );
                             }
                             else
                             {

source/FreeRTOS_TCP_Transmission.c

@@ -1352,11 +1352,20 @@
  *        unexpected but still within the window.
  *
  * @param[in] pxNetworkBuffer The network buffer descriptor with the packet.
+ * @param[in] ulCurrentSequenceNumber The current sequence number of the connection.
  *
  * @return Returns the value back from #prvTCPSendSpecialPacketHelper.
  */
-    BaseType_t prvTCPSendChallengeAck( NetworkBufferDescriptor_t * pxNetworkBuffer )
+    BaseType_t prvTCPSendChallengeAck( NetworkBufferDescriptor_t * pxNetworkBuffer,
+                                       uint32_t ulCurrentSequenceNumber )
     {
+        ProtocolHeaders_t * pxProtocolHeaders = ( ( ProtocolHeaders_t * )
+                                                  &( pxNetworkBuffer->pucEthernetBuffer[ ipSIZE_OF_ETH_HEADER + uxIPHeaderSizePacket( pxNetworkBuffer ) ] ) );
+
+        /* Correct the sequence number to avoid an endless ACK/RST sequence. When sending the
+         * same number again the next RST will not match as well. */
+        pxProtocolHeaders->xTCPHeader.ulSequenceNumber = FreeRTOS_htonl( ulCurrentSequenceNumber );
+
         return prvTCPSendSpecialPacketHelper( pxNetworkBuffer, tcpTCP_FLAG_ACK );
     }
     /*-----------------------------------------------------------*/

source/include/FreeRTOS_TCP_Transmission.h

@@ -149,7 +149,8 @@ BaseType_t prvSendData( FreeRTOS_Socket_t * pxSocket,
  * case #3. In summary, an RST was received with a sequence number that is
  * unexpected but still within the window.
  */
-BaseType_t prvTCPSendChallengeAck( NetworkBufferDescriptor_t * pxNetworkBuffer );
+BaseType_t prvTCPSendChallengeAck( NetworkBufferDescriptor_t * pxNetworkBuffer,
+                                   uint32_t ulCurrentSequenceNumber );
 
 /*
  * Reply to a peer with the RST flag on, in case a packet can not be handled.

test/unit-test/FreeRTOS_TCP_Transmission/FreeRTOS_TCP_Transmission_utest.c

@@ -2610,7 +2610,7 @@ void test_prvTCPSendChallengeAck( void )
     eARPGetCacheEntry_ExpectAnyArgsAndReturn( eResolutionCacheHit );
     eARPGetCacheEntry_ReturnThruPtr_ppxEndPoint( &pxEndPoint );
 
-    Return = prvTCPSendChallengeAck( pxNetworkBuffer );
+    Return = prvTCPSendChallengeAck( pxNetworkBuffer, 0x3333 );
     TEST_ASSERT_EQUAL( pdFALSE, Return );
     TEST_ASSERT_EQUAL( 1, NetworkInterfaceOutputFunction_Stub_Called );
     TEST_ASSERT_EQUAL( tcpTCP_FLAG_ACK, pxTCPPacket->xTCPHeader.ucTCPFlags );