diff --git a/src/internet-node/tcp-socket.cc b/src/internet-node/tcp-socket.cc
index 2df2ba7d7..63b33bafb 100644
--- a/src/internet-node/tcp-socket.cc
+++ b/src/internet-node/tcp-socket.cc
@@ -79,7 +79,8 @@ TcpSocket::GetTypeId ()
     m_pendingData (0),
     m_rtt (0),
     m_lastMeasuredRtt (Seconds(0.0)),
-    m_rxAvailable (0)
+    m_rxAvailable (0), //XXX zero?
+    m_maxTxBuffer (65536) //interpret 0 as no limit, //XXX hook into default values
 {
   NS_LOG_FUNCTION (this);
   
@@ -124,7 +125,8 @@ TcpSocket::TcpSocket(const TcpSocket& sock)
     m_lastMeasuredRtt (Seconds(0.0)),
     m_cnTimeout (sock.m_cnTimeout),
     m_cnCount (sock.m_cnCount),
-    m_rxAvailable (0)
+    m_rxAvailable (0),
+    m_maxTxBuffer (0) //interpret 0 as no limit, //XXX hook into default values
 {
   NS_LOG_FUNCTION_NOARGS ();
   NS_LOG_LOGIC("Invoked the copy constructor");
@@ -361,20 +363,19 @@ int TcpSocket::Send (const uint8_t* buf, uint32_t size)
   NS_LOG_FUNCTION (this << buf << size);
   if (m_state == ESTABLISHED || m_state == SYN_SENT || m_state == CLOSE_WAIT)
     { // Ok to buffer some data to send
+      size = std::min(size, GetTxAvailable() ); //only buffer what can fit
       if (!m_pendingData)
-      {
-        m_pendingData = new PendingData ();   // Create if non-existent
-        m_firstPendingSequence = m_nextTxSequence; // Note seq of first
-      }
+        {
+          m_pendingData = new PendingData ();   // Create if non-existent
+          m_firstPendingSequence = m_nextTxSequence; // Note seq of first
+        }
       //PendingData::Add always copies the data buffer, never modifies
       m_pendingData->Add (size,buf);
       NS_LOG_DEBUG("TcpSock::Send, pdsize " << m_pendingData->Size() << 
                    " state " << m_state);
       Actions_t action = ProcessEvent (APP_SEND);
       NS_LOG_DEBUG(" action " << action);
-      // We do not model any limit to the buffer, so report that the
-      // maximum is available
-      NotifySend (std::numeric_limits<uint32_t>::max ());
+      //NotifySend (GetTxAvailable ()); //XXX not here, do this when data acked
       if (!ProcessAction (action)) 
         {
           return -1; // Failed, return zero
@@ -439,8 +440,21 @@ TcpSocket::SendTo (const Address &address, Ptr<Packet> p)
 uint32_t
 TcpSocket::GetTxAvailable (void) const
 {
-  // No finite send buffer is modelled
-  return 0xffffffff;
+  if (m_maxTxBuffer == 0) //interpret this as infinite buffer
+  {
+    return std::numeric_limits<uint32_t>::max ();
+  }
+  if (m_pendingData != 0)
+  {
+    uint32_t unAckedDataSize = 
+        m_pendingData->SizeFromSeq (m_firstPendingSequence, m_highestRxAck);
+    NS_ASSERT (m_maxTxBuffer >= unAckedDataSize); //else a logical error
+    return m_maxTxBuffer-unAckedDataSize;
+  }
+  else
+  {
+    return m_maxTxBuffer;
+  }
 }
 
 int
@@ -757,9 +771,7 @@ bool TcpSocket::ProcessPacketAction (Actions_t a, Ptr<Packet> p,
       if (tcpHeader.GetAckNumber () > m_highestRxAck)
       {
         m_highestRxAck = tcpHeader.GetAckNumber ();
-        // We do not model any limit to the buffer, so report that the
-        // maximum is available
-        NotifySend (std::numeric_limits<uint32_t>::max ());
+        //NotifySend (GetTxAvailable() );  //XXX do when data gets acked
       }
       SendPendingData ();
       break;
@@ -1158,9 +1170,9 @@ void TcpSocket::CommonNewAck (SequenceNumber ack, bool skipTimer)
   NS_LOG_LOGIC ("TCP " << this << " NewAck " << ack 
            << " numberAck " << (ack - m_highestRxAck)); // Number bytes ack'ed
   m_highestRxAck = ack;         // Note the highest recieved Ack
-  // We do not model any limit to the buffer, so report that the
-  // maximum is available
-  NotifySend (std::numeric_limits<uint32_t>::max ());
+  //m_highestRxAck advancing means some data was acked, and the size of free
+  //space in the buffer has increased
+  NotifySend (GetTxAvailable ());
   if (ack > m_nextTxSequence) 
     {
       m_nextTxSequence = ack; // If advanced
diff --git a/src/internet-node/tcp-socket.h b/src/internet-node/tcp-socket.h
index 2cf21c547..f68e19884 100644
--- a/src/internet-node/tcp-socket.h
+++ b/src/internet-node/tcp-socket.h
@@ -161,7 +161,9 @@ private:
   SequenceNumber m_nextRxSequence;
 
   //history data
+  //this is the incoming data buffer which sorts out of sequence data
   UnAckData_t m_bufferedData;
+  //this is kind of the tx buffer
   PendingData* m_pendingData;
   SequenceNumber m_firstPendingSequence;
 
@@ -183,9 +185,12 @@ private:
   
   // Temporary queue for delivering data to application
   std::queue<Ptr<Packet> > m_deliveryQueue;
-  uint32_t m_rxAvailable; 
+  uint32_t m_rxAvailable;
+  
+  // buffer limit for the outgoing queue
+  uint32_t m_maxTxBuffer;
 };
 
 }//namespace ns3
 
-#endif /* UDP_SOCKET_H */
+#endif /* TCP_SOCKET_H */