traffic-control: Align the implementation of Peek methods to Linux

src/traffic-control/model/codel-queue-disc.cc

@@ -474,18 +474,18 @@ CoDelQueueDisc::GetDropNext (void)
 }
 
 Ptr<const QueueDiscItem>
-CoDelQueueDisc::DoPeek (void) const
+CoDelQueueDisc::DoPeek (void)
 {
   NS_LOG_FUNCTION (this);
 
-  if (GetInternalQueue (0)->IsEmpty ())
+  Ptr<const QueueDiscItem> item = PeekDequeued ();
+
+  if (!item)
     {
       NS_LOG_LOGIC ("Queue empty");
       return 0;
     }
 
-  Ptr<const QueueDiscItem> item = GetInternalQueue (0)->Peek ();
-
   NS_LOG_LOGIC ("Number packets " << GetInternalQueue (0)->GetNPackets ());
   NS_LOG_LOGIC ("Number bytes " << GetInternalQueue (0)->GetNBytes ());
 

src/traffic-control/model/codel-queue-disc.h

@@ -192,7 +192,7 @@ private:
    */
   virtual Ptr<QueueDiscItem> DoDequeue (void);
 
-  virtual Ptr<const QueueDiscItem> DoPeek (void) const;
+  virtual Ptr<const QueueDiscItem> DoPeek (void);
   virtual bool CheckConfig (void);
 
   /**

src/traffic-control/model/fq-codel-queue-disc.cc

@@ -295,29 +295,11 @@ FqCoDelQueueDisc::DoDequeue (void)
 }
 
 Ptr<const QueueDiscItem>
-FqCoDelQueueDisc::DoPeek (void) const
+FqCoDelQueueDisc::DoPeek (void)
 {
   NS_LOG_FUNCTION (this);
 
-  Ptr<FqCoDelFlow> flow;
-
-  if (!m_newFlows.empty ())
-    {
-      flow = m_newFlows.front ();
-    }
-  else
-    {
-      if (!m_oldFlows.empty ())
-        {
-          flow = m_oldFlows.front ();
-        }
-      else
-        {
-          return 0;
-        }
-    }
-
-  return flow->GetQueueDisc ()->Peek ();
+  return PeekDequeued ();
 }
 
 void

src/traffic-control/model/fq-codel-queue-disc.h

@@ -151,7 +151,7 @@ private:
 
   virtual bool DoEnqueue (Ptr<QueueDiscItem> item);
   virtual Ptr<QueueDiscItem> DoDequeue (void);
-  virtual Ptr<const QueueDiscItem> DoPeek (void) const;
+  virtual Ptr<const QueueDiscItem> DoPeek (void);
   virtual bool CheckConfig (void);
   virtual void InitializeParams (void);
 

src/traffic-control/model/mq-queue-disc.cc

@@ -68,7 +68,7 @@ MqQueueDisc::DoDequeue (void)
 }
 
 Ptr<const QueueDiscItem>
-MqQueueDisc::DoPeek (void) const
+MqQueueDisc::DoPeek (void)
 {
   NS_FATAL_ERROR ("MqQueueDisc: DoPeek should never be called");
 }

src/traffic-control/model/mq-queue-disc.h

@@ -56,7 +56,7 @@ public:
 private:
   virtual bool DoEnqueue (Ptr<QueueDiscItem> item);
   virtual Ptr<QueueDiscItem> DoDequeue (void);
-  virtual Ptr<const QueueDiscItem> DoPeek (void) const;
+  virtual Ptr<const QueueDiscItem> DoPeek (void);
   virtual bool CheckConfig (void);
   virtual void InitializeParams (void);
 };

src/traffic-control/model/pfifo-fast-queue-disc.cc

@@ -128,7 +128,7 @@ PfifoFastQueueDisc::DoDequeue (void)
 }
 
 Ptr<const QueueDiscItem>
-PfifoFastQueueDisc::DoPeek (void) const
+PfifoFastQueueDisc::DoPeek (void)
 {
   NS_LOG_FUNCTION (this);
 

src/traffic-control/model/pfifo-fast-queue-disc.h

@@ -92,7 +92,7 @@ private:
 
   virtual bool DoEnqueue (Ptr<QueueDiscItem> item);
   virtual Ptr<QueueDiscItem> DoDequeue (void);
-  virtual Ptr<const QueueDiscItem> DoPeek (void) const;
+  virtual Ptr<const QueueDiscItem> DoPeek (void);
   virtual bool CheckConfig (void);
   virtual void InitializeParams (void);
 };

src/traffic-control/model/pie-queue-disc.cc

@@ -474,17 +474,18 @@ PieQueueDisc::DoDequeue ()
 }
 
 Ptr<const QueueDiscItem>
-PieQueueDisc::DoPeek () const
+PieQueueDisc::DoPeek ()
 {
   NS_LOG_FUNCTION (this);
-  if (GetInternalQueue (0)->IsEmpty ())
+
+  Ptr<const QueueDiscItem> item = PeekDequeued ();
+
+  if (!item)
     {
       NS_LOG_LOGIC ("Queue empty");
       return 0;
     }
 
-  Ptr<const QueueDiscItem> item = GetInternalQueue (0)->Peek ();
-
   NS_LOG_LOGIC ("Number packets " << GetInternalQueue (0)->GetNPackets ());
   NS_LOG_LOGIC ("Number bytes " << GetInternalQueue (0)->GetNBytes ());
 

src/traffic-control/model/pie-queue-disc.h

@@ -152,7 +152,7 @@ protected:
 private:
   virtual bool DoEnqueue (Ptr<QueueDiscItem> item);
   virtual Ptr<QueueDiscItem> DoDequeue (void);
-  virtual Ptr<const QueueDiscItem> DoPeek (void) const;
+  virtual Ptr<const QueueDiscItem> DoPeek (void);
   virtual bool CheckConfig (void);
 
   /**

src/traffic-control/model/queue-disc.cc

@@ -870,12 +870,43 @@ QueueDisc::Dequeue (void)
 }
 
 Ptr<const QueueDiscItem>
-QueueDisc::Peek (void) const
+QueueDisc::Peek (void)
 {
   NS_LOG_FUNCTION (this);
   return DoPeek ();
 }
 
+Ptr<const QueueDiscItem>
+QueueDisc::PeekDequeued (void)
+{
+  NS_LOG_FUNCTION (this);
+
+  if (!m_requeued)
+    {
+      m_requeued = Dequeue ();
+    }
+  return m_requeued;
+}
+
+Ptr<QueueDiscItem>
+QueueDisc::DequeuePeeked (void)
+{
+  NS_LOG_FUNCTION (this);
+
+  Ptr<QueueDiscItem> item = m_requeued;
+
+  if (item)
+    {
+      m_requeued = 0;
+    }
+  else
+    {
+      item = Dequeue ();
+    }
+
+  return item;
+}
+
 void
 QueueDisc::Run (void)
 {

src/traffic-control/model/queue-disc.h

@@ -384,7 +384,15 @@ public:
    * DoPeek function, which must be implemented by derived classes.
    * \return 0 if the operation was not successful; the item otherwise.
    */
-  Ptr<const QueueDiscItem> Peek (void) const;
+  Ptr<const QueueDiscItem> Peek (void);
+
+  /**
+   * Extract from the queue disc the packet that has been dequeued by calling
+   * PeekDequeued.
+   *
+   * \return 0 if the operation was not successful; the item otherwise.
+   */
+  Ptr<QueueDiscItem> DequeuePeeked (void);
 
   /**
    * Modelled after the Linux function __qdisc_run (net/sched/sch_generic.c)
@@ -535,6 +543,21 @@ protected:
    */
   bool Mark (Ptr<QueueDiscItem> item, const char* reason);
 
+  /**
+   * Dequeue a packet and retain it in the queue disc as a requeued packet.
+   * The packet is not traced as requeued, nor is the total count of requeued
+   * packets increased. The dequeued packet is not counted in the backlog of
+   * the queue disc and is actually extracted from the queue disc by calling
+   * DequeuePeeked. Queue discs can point their DoPeek method to this one. This
+   * is recommended especially for queue discs for which it is not obvious what
+   * is the next packet that will be dequeued (e.g., queue discs having multiple
+   * internal queues or child queue discs or queue discs that drop packets
+   * after dequeue).
+   *
+   * \return 0 if the operation was not successful; the item otherwise.
+   */
+  Ptr<const QueueDiscItem> PeekDequeued (void);
+
 private:
   /**
    * \brief Copy constructor
@@ -570,7 +593,7 @@ private:
    * This function returns a copy of the next packet the queue disc will extract.
    * \return 0 if the operation was not successful; the packet otherwise.
    */
-  virtual Ptr<const QueueDiscItem> DoPeek (void) const = 0;
+  virtual Ptr<const QueueDiscItem> DoPeek (void) = 0;
 
   /**
    * Check whether the current configuration is correct. Default objects (such

src/traffic-control/model/red-queue-disc.cc

@@ -885,7 +885,7 @@ RedQueueDisc::DoDequeue (void)
 }
 
 Ptr<const QueueDiscItem>
-RedQueueDisc::DoPeek (void) const
+RedQueueDisc::DoPeek (void)
 {
   NS_LOG_FUNCTION (this);
   if (GetInternalQueue (0)->IsEmpty ())

src/traffic-control/model/red-queue-disc.h

@@ -246,7 +246,7 @@ protected:
 private:
   virtual bool DoEnqueue (Ptr<QueueDiscItem> item);
   virtual Ptr<QueueDiscItem> DoDequeue (void);
-  virtual Ptr<const QueueDiscItem> DoPeek (void) const;
+  virtual Ptr<const QueueDiscItem> DoPeek (void);
   virtual bool CheckConfig (void);
 
   /**