Corrected acknowledge number in challenge ACK packets

Author: mj-servus

source/FreeRTOS_TCP_IP.c

@@ -882,7 +882,8 @@
                                                           pxSocket->u.xTCP.xTCPWindow.xSize.ulRxWindowLength ) != pdFALSE ) )
                             {
                                 /* Send a challenge ACK. */
-                                ( void ) prvTCPSendChallengeAck( pxNetworkBuffer, pxSocket->u.xTCP.xTCPWindow.rx.ulCurrentSequenceNumber );
+                                ( void ) prvTCPSendChallengeAck( pxNetworkBuffer, pxSocket->u.xTCP.xTCPWindow.rx.ulCurrentSequenceNumber,
+                                                                 pxSocket->u.xTCP.xTCPWindow.ulOurSequenceNumber );
                             }
                             else
                             {

source/FreeRTOS_TCP_Transmission.c

@@ -1353,18 +1353,26 @@
  *
  * @param[in] pxNetworkBuffer The network buffer descriptor with the packet.
  * @param[in] ulCurrentSequenceNumber The current sequence number of the connection.
+ * @param[in] ulOurSequenceNumber The SEQ number to send out.
  *
  * @return Returns the value back from #prvTCPSendSpecialPacketHelper.
  */
     BaseType_t prvTCPSendChallengeAck( NetworkBufferDescriptor_t * pxNetworkBuffer,
-                                       uint32_t ulCurrentSequenceNumber )
+                                       uint32_t ulCurrentSequenceNumber,
+                                       uint32_t ulOurSequenceNumber )
     {
         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. */
+        /* In https://tools.ietf.org/html/rfc5961#section-3.2 the field values are defined as follows:
+         *  <SEQ=SND.NXT>
+         *  <ACK=RCV.NXT>
+         *
+         * The prvTCPSendSpecialPacketHelper function uses the sequence number of the packet as the
+         * ACK number and the ACK number as the sequence number, therefore the values are set swapped
+         * here to match the RFC. */
         pxProtocolHeaders->xTCPHeader.ulSequenceNumber = FreeRTOS_htonl( ulCurrentSequenceNumber );
+        pxProtocolHeaders->xTCPHeader.ulAckNr = FreeRTOS_htonl( ulOurSequenceNumber );
 
         return prvTCPSendSpecialPacketHelper( pxNetworkBuffer, tcpTCP_FLAG_ACK );
     }

source/include/FreeRTOS_TCP_Transmission.h

@@ -150,7 +150,8 @@ BaseType_t prvSendData( FreeRTOS_Socket_t * pxSocket,
  * unexpected but still within the window.
  */
 BaseType_t prvTCPSendChallengeAck( NetworkBufferDescriptor_t * pxNetworkBuffer,
-                                   uint32_t ulCurrentSequenceNumber );
+                                   uint32_t ulCurrentSequenceNumber,
+                                   uint32_t ulOurSequenceNumber );
 
 /*
  * 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

@@ -2610,11 +2610,13 @@ void test_prvTCPSendChallengeAck( void )
     eARPGetCacheEntry_ExpectAnyArgsAndReturn( eResolutionCacheHit );
     eARPGetCacheEntry_ReturnThruPtr_ppxEndPoint( &pxEndPoint );
 
-    Return = prvTCPSendChallengeAck( pxNetworkBuffer, 0x3333 );
+    Return = prvTCPSendChallengeAck( pxNetworkBuffer, 0x3333, 0x4444 );
     TEST_ASSERT_EQUAL( pdFALSE, Return );
     TEST_ASSERT_EQUAL( 1, NetworkInterfaceOutputFunction_Stub_Called );
     TEST_ASSERT_EQUAL( tcpTCP_FLAG_ACK, pxTCPPacket->xTCPHeader.ucTCPFlags );
     TEST_ASSERT_EQUAL( 0x50, pxTCPPacket->xTCPHeader.ucTCPOffset );
+    TEST_ASSERT_EQUAL( 0x3333, FreeRTOS_ntohl( pxTCPPacket->xTCPHeader.ulAckNr ) );
+    TEST_ASSERT_EQUAL( 0x4444, FreeRTOS_ntohl( pxTCPPacket->xTCPHeader.ulSequenceNumber ) );
 }
 
 /* test prvTCPSendReset function */