Main/TCP4 : ACK number in TCP RESET reply to SYN packet (#724)

* Main/TCP4 : ACK number in TCP RESET reply to SYN packet * Typo fix * Add unit-test for coverage; Fix ntohl to htonl * Fix unit-test --------- Co-authored-by: Nikhil Kamath <[email protected]> Co-authored-by: Aniruddha Kanhere <[email protected]>
FreeRTOS · Feb 19, 2023 · 14cf915 · 14cf915
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diff --git a/source/FreeRTOS_TCP_Transmission.c b/source/FreeRTOS_TCP_Transmission.c
@@ -1463,10 +1463,20 @@
                 TCPPacket_t * pxTCPPacket = ( ( TCPPacket_t * ) pxNetworkBuffer->pucEthernetBuffer );
                 const uint32_t ulSendLength =
                     ( ipSIZE_OF_IPv4_HEADER + ipSIZE_OF_TCP_HEADER ); /* Plus 0 options. */
-
+                uint8_t ucFlagsReceived = pxTCPPacket->xTCPHeader.ucTCPFlags;
                 pxTCPPacket->xTCPHeader.ucTCPFlags = ucTCPFlags;
                 pxTCPPacket->xTCPHeader.ucTCPOffset = ( ipSIZE_OF_TCP_HEADER ) << 2;
 
+                if( ( ucFlagsReceived & tcpTCP_FLAG_SYN ) != 0U )
+                {
+                    /* A synchronize packet is received. It counts as 1 pseudo byte of data,
+                     * so increase the variable with 1. Before sending a reply, the values of
+                     * 'ulSequenceNumber' and 'ulAckNr' will be swapped. */
+                    uint32_t ulSequenceNumber = FreeRTOS_ntohl( pxTCPPacket->xTCPHeader.ulSequenceNumber );
+                    ulSequenceNumber++;
+                    pxTCPPacket->xTCPHeader.ulSequenceNumber = FreeRTOS_htonl( ulSequenceNumber );
+                }
+
                 prvTCPReturnPacket( NULL, pxNetworkBuffer, ulSendLength, pdFALSE );
             }
         #endif /* !ipconfigIGNORE_UNKNOWN_PACKETS */

diff --git a/test/unit-test/FreeRTOS_TCP_Transmission/FreeRTOS_TCP_Transmission_utest.c b/test/unit-test/FreeRTOS_TCP_Transmission/FreeRTOS_TCP_Transmission_utest.c
@@ -1867,6 +1867,33 @@ void test_prvTCPSendSpecialPacketHelper( void )
     TEST_ASSERT_EQUAL( 0x50, pxTCPPacket->xTCPHeader.ucTCPOffset );
 }
 
+/* test prvTCPSendSpecialPacketHelper function */
+void test_prvTCPSendSpecialPacketHelper_flagSYN( void )
+{
+    BaseType_t Return = pdTRUE;
+    const uint32_t ulSequenceNumber = 0xABC12300;
+
+    pxSocket = &xSocket;
+    pxNetworkBuffer = &xNetworkBuffer;
+    pxNetworkBuffer->pucEthernetBuffer = ucEthernetBuffer;
+    TCPPacket_t * pxTCPPacket = ( ( const TCPPacket_t * ) pxNetworkBuffer->pucEthernetBuffer );
+
+    pxTCPPacket->xTCPHeader.ucTCPFlags = tcpTCP_FLAG_SYN;
+    pxTCPPacket->xTCPHeader.ucTCPOffset = 0;
+    pxTCPPacket->xTCPHeader.ulSequenceNumber = FreeRTOS_htonl( ulSequenceNumber );
+
+    usGenerateChecksum_ExpectAnyArgsAndReturn( 0x1111 );
+    usGenerateProtocolChecksum_ExpectAnyArgsAndReturn( 0x2222 );
+    eARPGetCacheEntry_ExpectAnyArgsAndReturn( eARPCacheHit );
+    xNetworkInterfaceOutput_ExpectAnyArgsAndReturn( pdTRUE );
+
+    Return = prvTCPSendSpecialPacketHelper( pxNetworkBuffer, tcpTCP_FLAG_ACK );
+    TEST_ASSERT_EQUAL( pdFALSE, Return );
+    TEST_ASSERT_EQUAL( tcpTCP_FLAG_ACK, pxTCPPacket->xTCPHeader.ucTCPFlags );
+    TEST_ASSERT_EQUAL( 0x50, pxTCPPacket->xTCPHeader.ucTCPOffset );
+    TEST_ASSERT_EQUAL( ulSequenceNumber + 1, FreeRTOS_ntohl( pxTCPPacket->xTCPHeader.ulAckNr ) );
+}
+
 /* test prvTCPSendChallengeAck function */
 void test_prvTCPSendChallengeAck( void )
 {