Created attachment 930 [details]
Simulation scripts reproducing the bug

WifiMacQueue sender queue drops around 120 secs.
Scenario tested:
wifi adhoc net, 2 nodes, sender: node 1, receiver: node 2, OnOffApplication (always on).

Specifically:
  - after 120 secs, no packet is enqueued.
  - at 121-122 secs, a packet is enqueued and the size of the queue drops. Then, the queue grows again and everything turns normal.
  - the behavior happens cyclically (period ~120 secs).

To record the behavior I have put a couple of NS_LOG instructions in the Enqueue method:
void
WifiMacQueue::Enqueue (Ptr<const Packet> packet, const WifiMacHeader &hdr)
{
  Cleanup ();
  if (m_size == m_maxSize)
    {
      NS_LOG_LOGIC ("qsize " << m_size);
      return;
    }
  Time now = Simulator::Now ();
  m_queue.push_back (Item (packet, hdr, now));
  m_size++;
  NS_LOG_LOGIC ("qsize " << m_size);
}

Example w/ MaxPacketNumber=400,MaxDelay=2:
121.001 400
121.002 400
121.002 400
121.661 1
121.661 2
121.661 3
...

Example w/ MaxPacketNumber=1000,MaxDelay=10:
121.001 1000
121.002 1000
121.002 1000
122.002 392
122.645 2
122.645 3
122.645 4
...

AFAIK, the behavior is:
  - independent from the L4 protocol: tested with both TCP and UDP
  - independent on the MaxDelay attribute: tested with 10 (default), 2, 20
  - dependent on the MaxPacketNumber attribute: tested with 400 (default), 100, 1000). With 1000 the queue first drops by ~600 packets and after another second it drops to zero. With 100 and 400 it drops directly to zero.

Revision used to test this bug: 6e83ec53ba0a.

Attached there's a sample script to reproduce the bug.