Bug 2872

Summary:	Packet copy in CsmaChannel breaks netanim tracing
Product:	ns-3	Reporter:	klb <klb>
Component:	csma	Assignee:	ns-bugs <ns-bugs>
Status:	RESOLVED FIXED
Severity:	minor	CC:	tomh
Priority:	P3
Version:	ns-3.27
Hardware:	All
OS:	All
Attachments:	patch to fix

Description klb 2018-02-16 07:48:47 UTC

Since version 3.27 CsmaChannel::TransmitStart() receives a Ptr to a constant Packet. 
As a result Packet must be copied. However, tracing in CsmaNetdevice ( m_phyTxBeginTrace() ) will append its tag to the original packet so that the copy in the channel does not contain the tag. 
Subsequently, the Rx-Trace cannot find a tag on the packet and will not trace the packet.

Comment 1 Tom Henderson 2018-03-05 23:50:24 UTC

If we move the TxBegin trace to before the TransmitStart() call, then it should fix this.  However, the behavior would change slightly; drops would see the TxBegin() trace followed immediately by a TxDrop() trace (rather than just a TxDrop() alone).

      //
      // The channel is free, transmit the packet
      //
      if (m_channel->TransmitStart (m_currentPkt, m_deviceId) == false)
        {
          NS_LOG_WARN ("Channel TransmitStart returns an error");
          m_phyTxDropTrace (m_currentPkt);
          m_currentPkt = 0;
          m_txMachineState = READY;
        }
      else
        {
          //
          // Transmission succeeded, reset the backoff time parameters and
          // schedule a transmit complete event.
          //
          m_backoff.ResetBackoffTime ();
          m_txMachineState = BUSY;
          m_phyTxBeginTrace (m_currentPkt);

          Time tEvent = m_bps.CalculateBytesTxTime (m_currentPkt->GetSize ());
          NS_LOG_LOGIC ("Schedule TransmitCompleteEvent in " << tEvent.GetSeconds () << "sec");
          Simulator::Schedule (tEvent, &CsmaNetDevice::TransmitCompleteEvent, this);
        }

Comment 2 Tom Henderson 2018-03-05 23:52:46 UTC

What I proposed above would make it consistent with how PointToPointNetDevice handles it:

  m_phyTxBeginTrace (m_currentPkt);

  Time txTime = m_bps.CalculateBytesTxTime (p->GetSize ());
  Time txCompleteTime = txTime + m_tInterframeGap;

  NS_LOG_LOGIC ("Schedule TransmitCompleteEvent in " << txCompleteTime.GetSeconds () << "sec");
  Simulator::Schedule (txCompleteTime, &PointToPointNetDevice::TransmitComplete, this);

  bool result = m_channel->TransmitStart (p, this, txTime);
  if (result == false)
    {
      m_phyTxDropTrace (p);
    }

Comment 3 Tom Henderson 2018-03-09 13:27:37 UTC

Created attachment 3086 [details]
patch to fix

Comment 4 Tom Henderson 2018-03-10 12:17:54 UTC

I pushed a change in changeset 13408:07055158a65d; please let me know if it does not resolve the issue for you.  I tested it on NetAnim and it seemed to resolve it.