PointToPointNetDevice: Add support for flow control

The (unique) transmission queue is stopped when enqueuing a packet fails, so
that upper layers do not send other packets down. When a packet transmission
is completed, if the queue is empty then wake the upper layers. If the queue
was stopped, there is now room for another packet and hence wake the upper
layers as well.

src/point-to-point/model/point-to-point-net-device.cc

@@ -280,18 +280,26 @@ PointToPointNetDevice::TransmitComplete (void)
   m_phyTxEndTrace (m_currentPkt);
   m_currentPkt = 0;
 
+  Ptr<NetDeviceQueue> txq = GetTxQueue (0);
+
   Ptr<Packet> p = m_queue->Dequeue ();
   if (p == 0)
     {
-      //
-      // No packet was on the queue, so we just exit.
-      //
+      NS_LOG_LOGIC ("No pending packets in device queue after tx complete");
+      txq->Wake ();
       return;
     }
 
   //
-  // Got another packet off of the queue, so start the transmit process agin.
+  // Got another packet off of the queue, so start the transmit process again.
+  // If the queue was stopped, start it again. Note that we cannot wake the upper
+  // layers because otherwise a packet is sent to the device while the machine
+  // state is busy, thus causing the assert in TransmitStart to fail.
   //
+  if (txq->IsStopped ())
+    {
+      txq->Start ();
+    }
   m_snifferTrace (p);
   m_promiscSnifferTrace (p);
   TransmitStart (p);
@@ -510,6 +518,12 @@ PointToPointNetDevice::Send (
   const Address &dest, 
   uint16_t protocolNumber)
 {
+  Ptr<NetDeviceQueue> txq = GetTxQueue (0);
+  if (txq->IsStopped ())
+    {
+      NS_LOG_WARN ("Send should not be called when the device is stopped");
+    }
+
   NS_LOG_FUNCTION (this << packet << dest << protocolNumber);
   NS_LOG_LOGIC ("p=" << packet << ", dest=" << &dest);
   NS_LOG_LOGIC ("UID is " << packet->GetUid ());
@@ -552,6 +566,7 @@ PointToPointNetDevice::Send (
 
   // Enqueue may fail (overflow)
   m_macTxDropTrace (packet);
+  txq->Stop ();
   return false;
 }