tcp: Add DCTCP trace source for congestion estimate update

Also ensure that DCTCP alpha cannot be externally set, once initialized

src/internet/model/tcp-dctcp.cc

@@ -21,7 +21,7 @@
 
 #include "tcp-dctcp.h"
 #include "ns3/log.h"
-#include "math.h"
+#include "ns3/abort.h"
 #include "ns3/tcp-socket-state.h"
 
 namespace ns3 {
@@ -44,13 +44,17 @@ TypeId TcpDctcp::GetTypeId (void)
     .AddAttribute ("DctcpAlphaOnInit",
                    "Initial alpha value",
                    DoubleValue (1.0),
-                   MakeDoubleAccessor (&TcpDctcp::SetDctcpAlpha),
+                   MakeDoubleAccessor (&TcpDctcp::InitializeDctcpAlpha),
                    MakeDoubleChecker<double> (0, 1))
     .AddAttribute ("UseEct0",
                    "Use ECT(0) for ECN codepoint, if false use ECT(1)",
                    BooleanValue (true),
                    MakeBooleanAccessor (&TcpDctcp::m_useEct0),
                    MakeBooleanChecker ())
+    .AddTraceSource ("CongestionEstimate",
+                     "Update sender-side congestion estimate state",
+                     MakeTraceSourceAccessor (&TcpDctcp::m_traceCongestionEstimate),
+                     "ns3::TcpDctcp::CongestionEstimateTracedCallback")
   ;
   return tid;
 }
@@ -61,17 +65,18 @@ std::string TcpDctcp::GetName () const
 }
 
 TcpDctcp::TcpDctcp ()
-  : TcpLinuxReno ()
+  : TcpLinuxReno (),
+    m_ackedBytesEcn (0),
+    m_ackedBytesTotal (0),
+    m_priorRcvNxt (SequenceNumber32 (0)),
+    m_priorRcvNxtFlag (false),
+    m_nextSeq (SequenceNumber32 (0)),
+    m_nextSeqFlag (false),
+    m_ceState (false),
+    m_delayedAckReserved (false),
+    m_initialized (false)
 {
   NS_LOG_FUNCTION (this);
-  m_ackedBytesEcn = 0;
-  m_ackedBytesTotal = 0;
-  m_priorRcvNxt = SequenceNumber32 (0);
-  m_priorRcvNxtFlag = false;
-  m_nextSeq = SequenceNumber32 (0);
-  m_nextSeqFlag = false;
-  m_ceState = false;
-  m_delayedAckReserved = false;
 }
 
 TcpDctcp::TcpDctcp (const TcpDctcp& sock)
@@ -86,7 +91,8 @@ TcpDctcp::TcpDctcp (const TcpDctcp& sock)
     m_ceState (sock.m_ceState),
     m_delayedAckReserved (sock.m_delayedAckReserved),
     m_g (sock.m_g),
-    m_useEct0 (sock.m_useEct0)
+    m_useEct0 (sock.m_useEct0),
+    m_initialized (sock.m_initialized)
 {
   NS_LOG_FUNCTION (this);
 }
@@ -110,6 +116,7 @@ TcpDctcp::Init (Ptr<TcpSocketState> tcb)
   tcb->m_useEcn = TcpSocketState::On;
   tcb->m_ecnMode = TcpSocketState::DctcpEcn;
   tcb->m_ectCodePoint = m_useEct0 ? TcpSocketState::Ect0 : TcpSocketState::Ect1;
+  m_initialized = true;
 }
 
 // Step 9, Section 3.3 of RFC 8257.  GetSsThresh() is called upon
@@ -143,21 +150,23 @@ TcpDctcp::PktsAcked (Ptr<TcpSocketState> tcb, uint32_t segmentsAcked, const Time
     }
   if (tcb->m_lastAckedSeq >= m_nextSeq)
     {
-      double bytesEcn = 0.0;
+      double bytesEcn = 0.0; // Corresponds to variable M in RFC 8257
       if (m_ackedBytesTotal >  0)
         {
           bytesEcn = static_cast<double> (m_ackedBytesEcn * 1.0 / m_ackedBytesTotal);
         }
       m_alpha = (1.0 - m_g) * m_alpha + m_g * bytesEcn;
+      m_traceCongestionEstimate (m_ackedBytesEcn, m_ackedBytesTotal, m_alpha);
       NS_LOG_INFO (this << "bytesEcn " << bytesEcn << ", m_alpha " << m_alpha);
       Reset (tcb);
     }
 }
 
 void
-TcpDctcp::SetDctcpAlpha (double alpha)
+TcpDctcp::InitializeDctcpAlpha (double alpha)
 {
   NS_LOG_FUNCTION (this << alpha);
+  NS_ABORT_MSG_IF (m_initialized, "DCTCP has already been initialized");
   m_alpha = alpha;
 }
 

src/internet/model/tcp-dctcp.h

@@ -24,6 +24,7 @@
 
 #include "ns3/tcp-congestion-ops.h"
 #include "ns3/tcp-linux-reno.h"
+#include "ns3/traced-callback.h"
 
 namespace ns3 {
 
@@ -72,6 +73,15 @@ public:
    */
   virtual void Init (Ptr<TcpSocketState> tcb);
 
+  /**
+   * TracedCallback signature for DCTCP update of congestion state
+   *
+   * \param [in] bytesAcked Bytes acked in this observation window
+   * \param [in] bytesMarked Bytes marked in this observation window
+   * \param [in] alpha New alpha (congestion estimate) value
+   */
+  typedef void (* CongestionEstimateTracedCallback)(uint32_t bytesAcked, uint32_t bytesMarked, double alpha);
+
   // Documented in base class
   virtual uint32_t GetSsThresh (Ptr<const TcpSocketState> tcb,
                                 uint32_t bytesInFlight);
@@ -113,11 +123,11 @@ private:
   void Reset (Ptr<TcpSocketState> tcb);
 
   /**
-   * \brief Sets the value of m_alpha
+   * \brief Initialize the value of m_alpha
    *
    * \param alpha DCTCP alpha parameter
    */
-  void SetDctcpAlpha (double alpha);
+  void InitializeDctcpAlpha (double alpha);
 
   uint32_t m_ackedBytesEcn;             //!< Number of acked bytes which are marked
   uint32_t m_ackedBytesTotal;           //!< Total number of acked bytes
@@ -130,6 +140,11 @@ private:
   bool m_delayedAckReserved;            //!< Delayed Ack state
   double m_g;                           //!< Estimation gain
   bool m_useEct0;                       //!< Use ECT(0) for ECN codepoint
+  bool m_initialized;                   //!< Whether DCTCP has been initialized
+  /**
+   * \brief Callback pointer for congestion state update
+   */
+  TracedCallback<uint32_t, uint32_t, double> m_traceCongestionEstimate;
 };
 
 } // namespace ns3