traffic-control: (fixes #2485) Add ECN support to RED

RELEASE_NOTES

@@ -29,6 +29,7 @@ Bugs fixed
 - Bug 2450 - LogDistancePropagationLossModel is not continuous
 - Bug 2463 - create trace source to trace the TXOP time that is actually used
 - Bug 2477 - DCF manager assert
+- Bug 2485 - Check for queue full should happen before checking RED thresholds
 - Bug 2492 - uan: Make use of RxGain attribute in UanPhyGen class
 - Bug 2511 - HT Greenfield is not working
 - Bug 2513 - ParetoRandomVariable needs a "scale", not a "mean" attribute.

src/traffic-control/doc/red.rst

@@ -34,7 +34,25 @@ weight, MinTh and MaxTh and (ii) adapts maximum drop probability. The model
 in ns-3 contains implementation of both the features, and is a port of Sally
 Floyd's ns-2 ARED model. Note that the user is allowed to choose and explicitly
 configure the simulation by selecting feature (i) or feature (ii), or both.
- 
+
+Explicit Congestion Notification (ECN)
+======================================
+This RED model supports an ECN mode of operation to notify endpoints of
+congestion that may be developing in a bottleneck queue, without resorting
+to packet drops. Such a mode is enabled by setting the UseEcn attribute to
+true (it is false by default) and only affects incoming packets with the
+ECT bit set in their header. When the average queue length is between the
+minimum and maximum thresholds, an incoming packet is marked instead of being
+dropped. When the average queue length is above the maximum threshold, an
+incoming packet is marked (instead of being dropped) only if the UseHardDrop
+attribute is set to false (it is true by default).
+
+The implementation of support for ECN marking is done in such a way as
+to not impose an internet module dependency on the traffic control module.
+The RED model does not directly set ECN bits on the header, but delegates
+that job to the QueueDiscItem class.  As a result, it is possible to
+use RED queues for other non-IP QueueDiscItems that may or may not support
+the ``Mark ()`` method.
 
 References
 ==========
@@ -45,6 +63,9 @@ S.Floyd, K.Fall http://icir.org/floyd/papers/redsims.ps
 ARED queue implementation is based on the algorithm provided in:
 S. Floyd et al, http://www.icir.org/floyd/papers/adaptiveRed.pdf
 
+The addition of explicit congestion notification (ECN) to IP:
+K. K. Ramakrishnan et al, https://tools.ietf.org/html/rfc3168
+
 Attributes
 ==========
 
@@ -62,6 +83,8 @@ policies:
 * LInterm
 * LinkBandwidth
 * LinkDelay
+* UseEcn
+* UseHardDrop
 
 In addition to RED attributes, ARED queue requires following attributes:
 

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

@@ -193,6 +193,16 @@ TypeId RedQueueDisc::GetTypeId (void)
                    TimeValue (MilliSeconds (20)),
                    MakeTimeAccessor (&RedQueueDisc::m_linkDelay),
                    MakeTimeChecker ())
+    .AddAttribute ("UseEcn",
+                   "True to use ECN (packets are marked instead of being dropped)",
+                   BooleanValue (false),
+                   MakeBooleanAccessor (&RedQueueDisc::m_useEcn),
+                   MakeBooleanChecker ())
+    .AddAttribute ("UseHardDrop",
+                   "True to always drop packets above max threshold",
+                   BooleanValue (true),
+                   MakeBooleanAccessor (&RedQueueDisc::m_useHardDrop),
+                   MakeBooleanChecker ())
   ;
 
   return tid;
@@ -382,36 +392,43 @@ RedQueueDisc::DoEnqueue (Ptr<QueueDiscItem> item)
       m_old = 0;
     }
 
-  if ((GetMode () == Queue::QUEUE_MODE_PACKETS && nQueued >= m_queueLimit) ||
-      (GetMode () == Queue::QUEUE_MODE_BYTES && nQueued + item->GetPacketSize() > m_queueLimit))
-    {
-      NS_LOG_DEBUG ("\t Dropping due to Queue Full " << nQueued);
-      dropType = DTYPE_FORCED;
-      m_stats.qLimDrop++;
-    }
-
   if (dropType == DTYPE_UNFORCED)
     {
-      NS_LOG_DEBUG ("\t Dropping due to Prob Mark " << m_qAvg);
-      m_stats.unforcedDrop++;
-      Drop (item);
-      return false;
+      if (!m_useEcn || !item->Mark ())
+        {
+          NS_LOG_DEBUG ("\t Dropping due to Prob Mark " << m_qAvg);
+          m_stats.unforcedDrop++;
+          Drop (item);
+          return false;
+        }
+      NS_LOG_DEBUG ("\t Marking due to Prob Mark " << m_qAvg);
+      m_stats.unforcedMark++;
     }
   else if (dropType == DTYPE_FORCED)
     {
-      NS_LOG_DEBUG ("\t Dropping due to Hard Mark " << m_qAvg);
-      m_stats.forcedDrop++;
-      Drop (item);
-      if (m_isNs1Compat)
+      if (m_useHardDrop || !m_useEcn || !item->Mark ())
         {
-          m_count = 0;
-          m_countBytes = 0;
+          NS_LOG_DEBUG ("\t Dropping due to Hard Mark " << m_qAvg);
+          m_stats.forcedDrop++;
+          Drop (item);
+          if (m_isNs1Compat)
+            {
+              m_count = 0;
+              m_countBytes = 0;
+            }
+          return false;
         }
-      return false;
+      NS_LOG_DEBUG ("\t Marking due to Hard Mark " << m_qAvg);
+      m_stats.forcedMark++;
     }
 
   bool retval = GetInternalQueue (0)->Enqueue (item);
 
+  if (!retval)
+    {
+      m_stats.qLimDrop++;
+    }
+
   // If Queue::Enqueue fails, QueueDisc::Drop is called by the internal queue
   // because QueueDisc::AddInternalQueue sets the drop callback
 
@@ -471,6 +488,8 @@ RedQueueDisc::InitializeParams (void)
   m_stats.forcedDrop = 0;
   m_stats.unforcedDrop = 0;
   m_stats.qLimDrop = 0;
+  m_stats.forcedMark = 0;
+  m_stats.unforcedMark = 0;
 
   m_qAvg = 0.0;
   m_count = 0;

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

@@ -108,6 +108,8 @@ public:
     uint32_t unforcedDrop;  //!< Early probability drops
     uint32_t forcedDrop;    //!< Forced drops, qavg > max threshold
     uint32_t qLimDrop;      //!< Drops due to queue limits
+    uint32_t unforcedMark;  //!< Early probability marks
+    uint32_t forcedMark;    //!< Forced marks, qavg > max threshold
   } Stats;
 
   /** 
@@ -298,6 +300,8 @@ private:
   bool m_isNs1Compat;       //!< Ns-1 compatibility
   DataRate m_linkBandwidth; //!< Link bandwidth
   Time m_linkDelay;         //!< Link delay
+  bool m_useEcn;            //!< True if ECN is used (packets are marked instead of being dropped)
+  bool m_useHardDrop;       //!< True if packets are always dropped above max threshold
 
   // ** Variables maintained by RED
   double m_vProb1;          //!< Prob. of packet drop before "count"