diff -r 6ed1556809ea -r 5991df10eb5e src/internet/model/tcp-socket-base.cc
--- a/src/internet/model/tcp-socket-base.cc	Fri Nov 02 15:39:41 2012 -0700
+++ b/src/internet/model/tcp-socket-base.cc	Mon Nov 05 14:07:02 2012 -0500
@@ -1982,7 +1982,16 @@
   // Use m_rtt for the estimation. Note, RTT of duplicated acknowledgement
   // (which should be ignored) is handled by m_rtt. Once timestamp option
   // is implemented, this function would be more elaborated.
-  m_lastRtt = m_rtt->AckSeq (tcpHeader.GetAckNumber () );
+  Time nextRtt =  m_rtt->AckSeq (tcpHeader.GetAckNumber () );
+
+  //nextRtt will be zero for dup acks.  Don't want to update lastRtt in that case
+  //but still needed to do list clearing that is done in AckSeq. 
+  if(nextRtt != 0)
+  {
+    m_lastRtt = nextRtt;
+    NS_LOG_FUNCTION(this << m_lastRtt);
+  }
+  
 }
 
 // Called by the ReceivedAck() when new ACK received and by ProcessSynRcvd()