diff -r 6568bf28a20e src/internet-stack/tcp-socket-impl.cc
--- a/src/internet-stack/tcp-socket-impl.cc	Mon Feb 16 17:16:45 2009 +0000
+++ b/src/internet-stack/tcp-socket-impl.cc	Mon Feb 16 18:24:22 2009 +0000
@@ -665,7 +665,7 @@
 
   header.SetFlags (flags);
   header.SetSequenceNumber (m_nextTxSequence);
-  header.SetAckNumber (m_nextRxSequence);
+  header.SetAckNumber (flags & TcpHeader::ACK ? m_nextRxSequence : SequenceNumber (0));
   header.SetSourcePort (m_endPoint->GetLocalPort ());
   header.SetDestinationPort (m_remotePort);
   header.SetWindowSize (AdvertisedWindowSize());
@@ -1001,7 +1001,7 @@
       TcpHeader header;
       header.SetFlags (flags);
       header.SetSequenceNumber (m_nextTxSequence);
-      header.SetAckNumber (m_nextRxSequence);
+      header.SetAckNumber (withAck ? m_nextRxSequence : SequenceNumber (0));
       header.SetSourcePort (m_endPoint->GetLocalPort());
       header.SetDestinationPort (m_remotePort);
       header.SetWindowSize (AdvertisedWindowSize());