--- a/src/internet-stack/tcp-socket-impl.cc	Fri Nov 28 09:17:31 2008 +0100
+++ a/src/internet-stack/tcp-socket-impl.cc	Fri Nov 28 12:31:18 2008 +0100
@@ -656,6 +656,11 @@ 
   NS_LOG_FUNCTION (this << (uint32_t)flags);
   Ptr<Packet> p = Create<Packet> ();
   TcpHeader header;
+
+  if (flags & TcpHeader::FIN)
+    {
+      flags |= TcpHeader::ACK;
+    }
 
   header.SetFlags (flags);
   header.SetSequenceNumber (m_nextTxSequence);