bool
PointToPointNetDevice::TransmitStart (Ptr<Packet> p)
{
  NS_LOG_FUNCTION (this << p);
  NS_LOG_LOGIC ("UID is " << p->GetUid () << ")");

  //
  // This function is called to start the process of transmitting a packet.
  // We need to tell the channel that we've started wiggling the wire and
  // schedule an event that will be executed when the transmission is complete.
  //
  NS_ASSERT_MSG(m_txMachineState == READY, "Must be READY to transmit");
  m_txMachineState = BUSY;
  m_phyTxBeginTrace (m_currentPkt);
  m_currentPkt = p;



It seems to me that the order of those last two lines needs to be switched, else the upcall is made with nonsense or with a null packet pointer.  Craig?