wifi: Make WifiMacQueue a subclass of Queue

Being a subclass of Queue, WifiMacQueue gains the ability to trace enqueues, dequeues, and drops. The Cleanup() method has been removed by adopting the following strategy. Keep in mind that packets are timestamped when inserted in the queue, so packets are already sorted by increasing timestamp (from head to tail). When a dequeue or peek operation is requested, packets are processed one-by-one (starting from the head of the queue) until a packet is found that satisfies the requested conditions (e.g., tid and address) and whose time-to-live is not expired yet. Processed packets whose time-to-live is expired are clearly dropped. Given that packets are sorted by increasing timestamp and the time-to-live of the returned packet (if any) is not expired, we manage to remove all the expired packets from the queue. This comes at no additional cost (we necessarily have to walk the list until we find the packet to be returned).

When packets are enqueued, just the oldest item (the one at the head of the tail) is processed. If its timer is expired, it is removed so as to make room for the new packet.

[S. Deronne, 2017-02-02 10:00:38]

Looks fine.
Do we still need a timestamp since you now remove the oldest packet in the
queue?

[Stefano Avallone, 2017-02-02 10:08:38]

On 2017/02/02 10:00:38, S. Deronne wrote:
> Looks fine.

Thanks for your review.

> Do we still need a timestamp since you now remove the oldest packet in the
> queue?

Yep, we need to check how long the packet has been in the queue.

[S. Deronne, 2017-02-02 10:10:05]

On 2017/02/02 10:08:38, Stefano Avallone wrote:
> On 2017/02/02 10:00:38, S. Deronne wrote:
> > Looks fine.
> 
> Thanks for your review.

You're welcome.

> 
> > Do we still need a timestamp since you now remove the oldest packet in the
> > queue?
> 
> Yep, we need to check how long the packet has been in the queue.

I only see a getter, where is this effectively used?

[Stefano Avallone, 2017-02-02 17:11:38]

On 2017/02/02 10:10:05, S. Deronne wrote:
> I only see a getter, where is this effectively used?

The timestamp of a WifiMacQueueItem can only be set at construction time:

WifiMacQueueItem::WifiMacQueueItem (Ptr<const Packet> p, const WifiMacHeader &
header)
  : m_packet (p),
    m_header (header),
    m_tstamp (Simulator::Now ())
{
}

A WifiMacQueueItem is created when a packet has to be enqueued. Clearly, it does
not make sense to allow to modify the timestamp later.

Thanks,
Stefano

[S. Deronne, 2017-02-02 17:54:36]

On 2017/02/02 17:11:38, Stefano Avallone wrote:
> On 2017/02/02 10:10:05, S. Deronne wrote:
> > I only see a getter, where is this effectively used?
> 
> The timestamp of a WifiMacQueueItem can only be set at construction time:
> 
> WifiMacQueueItem::WifiMacQueueItem (Ptr<const Packet> p, const WifiMacHeader &
> header)
>   : m_packet (p),
>     m_header (header),
>     m_tstamp (Simulator::Now ())
> {
> }
> 
> A WifiMacQueueItem is created when a packet has to be enqueued. Clearly, it
does
> not make sense to allow to modify the timestamp later.
> 
> Thanks,
> Stefano

Yes I agree with that, but I expected this to be used in wifi mac queue.
But ok I agree this is better to keep the timestamp.

[Stefano Avallone, 2017-02-02 18:00:31]

On 2017/02/02 17:54:36, S. Deronne wrote:
> Yes I agree with that, but I expected this to be used in wifi mac queue.
> But ok I agree this is better to keep the timestamp.

GetTimeStamp () is called within WifiMacQueue::TtlExceeded (), which is called
by all the enqueue/dequeue/peek methods of WifiMacQueue.

I hope I made myself clear now.

[S. Deronne, 2017-02-02 18:09:55]

On 2017/02/02 18:00:31, Stefano Avallone wrote:
> On 2017/02/02 17:54:36, S. Deronne wrote:
> > Yes I agree with that, but I expected this to be used in wifi mac queue.
> > But ok I agree this is better to keep the timestamp.
> 
> GetTimeStamp () is called within WifiMacQueue::TtlExceeded (), which is called
> by all the enqueue/dequeue/peek methods of WifiMacQueue.
> 
> I hope I made myself clear now.

Ok, now it is fully clear :-)