tcp: (fixes #2921) Add min_cwnd variable

RELEASE_NOTES

@@ -32,6 +32,7 @@ New user-visible features
 
 Bugs fixed
 ----------
+- Bug 2921 - tcp: Add min_cwnd variable to LEDBAT
 - Bug 2911 - aodv:  Binary exponential backoff can become unlimited
 - Bug 2901 - Add CommandLine::Parse (const std::vector<std::string>> args)
 - Bug 2891 - netanim:  dumbbell-animation breaks when RightCount > LeftCount

src/internet/model/tcp-ledbat.cc

@@ -61,6 +61,11 @@ TcpLedbat::GetTypeId (void)
                    MakeEnumAccessor (&TcpLedbat::SetDoSs),
                    MakeEnumChecker (DO_SLOWSTART, "yes",
                                     DO_NOT_SLOWSTART, "no"))
+    .AddAttribute ("MIN_CWND",
+                   "Minimum cWnd for Ledbat",
+                   UintegerValue (2),
+                   MakeUintegerAccessor (&TcpLedbat::m_minCwnd),
+                   MakeUintegerChecker<uint32_t> ())
   ;
   return tid;
 }
@@ -93,7 +98,8 @@ TcpLedbat::TcpLedbat (void)
   m_lastRollover = 0;
   m_sndCwndCnt = 0;
   m_flag = LEDBAT_CAN_SS;
-}
+  m_minCwnd = 2;
+};
 
 void TcpLedbat::InitCircBuf (struct OwdCircBuf &buffer)
 {
@@ -116,6 +122,7 @@ TcpLedbat::TcpLedbat (const TcpLedbat& sock)
   m_lastRollover = sock.m_lastRollover;
   m_sndCwndCnt = sock.m_sndCwndCnt;
   m_flag = sock.m_flag;
+  m_minCwnd = sock.m_minCwnd;
 }
 
 TcpLedbat::~TcpLedbat (void)
@@ -210,7 +217,7 @@ void TcpLedbat::CongestionAvoidance (Ptr<TcpSocketState> tcb, uint32_t segmentsA
 
   max_cwnd = static_cast<uint32_t>(tcb->m_highTxMark.Get () - tcb->m_lastAckedSeq) + segmentsAcked * tcb->m_segmentSize;
   cwnd = std::min (cwnd, max_cwnd);
-  cwnd = std::max (cwnd, tcb->m_segmentSize);
+  cwnd = std::max (cwnd, m_minCwnd * tcb->m_segmentSize);
   tcb->m_cWnd = cwnd;
 
   if (tcb->m_cWnd <= tcb->m_ssThresh)

src/internet/model/tcp-ledbat.h

@@ -192,6 +192,7 @@ private:
   OwdCircBuf m_baseHistory;   //!< Buffer to store the base delay
   OwdCircBuf m_noiseFilter;   //!< Buffer to store the current delay
   uint32_t m_flag;                   //!< LEDBAT Flag
+  uint32_t m_minCwnd;                //!< Minimum cWnd value mentioned in RFC 6817
 };
 
 } // namespace ns3