--- i/src/internet/model/tcp-socket-base.cc	
+++ i/src/internet/model/tcp-socket-base.cc	
@@ -1700,7 +1700,7 @@ TcpSocketBase::ReceivedAck (Ptr<Packet> packet, const TcpHeader& tcpHeader)
 
   // RFC 6675 Section 5: 2nd, 3rd paragraph and point (A), (B) implementation
   // are inside the function ProcessAck
-  ProcessAck (ackNumber, scoreboardUpdated, oldHeadSequence);
+  ProcessAck (ackNumber, scoreboardUpdated, oldHeadSequence, packet->GetSize ());
 
   // If there is any data piggybacked, store it into m_rxBuffer
   if (packet->GetSize () > 0)
@@ -1715,7 +1715,7 @@ TcpSocketBase::ReceivedAck (Ptr<Packet> packet, const TcpHeader& tcpHeader)
 
 void
 TcpSocketBase::ProcessAck (const SequenceNumber32 &ackNumber, bool scoreboardUpdated,
-                           const SequenceNumber32 &oldHeadSequence)
+                           const SequenceNumber32 &oldHeadSequence, uint32_t size)
 {
   NS_LOG_FUNCTION (this << ackNumber << scoreboardUpdated);
   // RFC 6675, Section 5, 2nd paragraph:
@@ -1742,10 +1742,16 @@ TcpSocketBase::ProcessAck (const SequenceNumber32 &ackNumber, bool scoreboardUpd
    *     HighData (m_highTxMark)
    */
 
-  bool isDupack = m_sackEnabled ?
-    scoreboardUpdated
-    : ackNumber == oldHeadSequence &&
-    ackNumber < m_tcb->m_highTxMark;
+  bool isDupack = scoreboardUpdated;
+
+  if (!m_sackEnabled)
+    {
+      // SYN and FIN are both OFF thanks to the state processing
+      isDupack = UnAckDataCount () > 0 &&           // condition (a)
+	         size == 0 &&                       // condition (b)
+	         ackNumber == oldHeadSequence &&    // condition (d)
+                 ackNumber < m_tcb->m_highTxMark;  // safety consideration
+    }
 
   NS_LOG_DEBUG ("ACK of " << ackNumber <<
                 " SND.UNA=" << oldHeadSequence <<
--- i/src/internet/model/tcp-socket-base.h	
+++ i/src/internet/model/tcp-socket-base.h	
@@ -962,10 +962,11 @@ protected:
    * \param ackNumber ack number
    * \param scoreboardUpdated if true indicates that the scoreboard has been
    * \param oldHeadSequence value of HeadSequence before ack
+   * \param size Packet size
    * updated with SACK information
    */
   virtual void ProcessAck (const SequenceNumber32 &ackNumber, bool scoreboardUpdated,
-                           const SequenceNumber32 &oldHeadSequence);
+                           const SequenceNumber32 &oldHeadSequence, uint32_t size);
 
   /**
    * \brief Recv of a data, put into buffer, call L7 to get it if necessary