From a37e7fb06e61a3dd260fb32126d66e34029f5194 Mon Sep 17 00:00:00 2001
From: Tom Henderson <tomh@tomh.org>
Date: Wed, 25 Nov 2020 14:20:21 -0800
Subject: [PATCH] tcp: Update DCTCP to avoid use of ReduceCwnd

---
 src/internet/model/tcp-dctcp.cc | 13 ++++++++++---
 src/internet/model/tcp-dctcp.h  |  2 ++
 2 files changed, 12 insertions(+), 3 deletions(-)

diff --git a/src/internet/model/tcp-dctcp.cc b/src/internet/model/tcp-dctcp.cc
index 7ba1f5a1d..7c450785a 100644
--- a/src/internet/model/tcp-dctcp.cc
+++ b/src/internet/model/tcp-dctcp.cc
@@ -112,12 +112,19 @@ TcpDctcp::Init (Ptr<TcpSocketState> tcb)
   tcb->m_ectCodePoint = m_useEct0 ? TcpSocketState::Ect0 : TcpSocketState::Ect1;
 }
 
+// Step 9, Section 3.3 of RFC 8257.  GetSsThresh() is called upon
+// entering the CWR state, and then later, when CWR is exited,
+// cwnd is set to ssthresh (this value).  bytesInFlight is ignored.
+uint32_t
+TcpDctcp::GetSsThresh (Ptr<const TcpSocketState> tcb, uint32_t bytesInFlight)
+{
+  NS_LOG_FUNCTION (this << tcb << bytesInFlight);
+  return static_cast<uint32_t> ((1 - m_alpha / 2.0) * tcb->m_cWnd);
+}
+
 void
 TcpDctcp::ReduceCwnd (Ptr<TcpSocketState> tcb)
 {
-  NS_LOG_FUNCTION (this << tcb);
-  uint32_t val = static_cast<uint32_t> ((1 - m_alpha / 2.0) * tcb->m_cWnd);
-  tcb->m_cWnd = std::max (val, 2 * tcb->m_segmentSize);
 }
 
 void
diff --git a/src/internet/model/tcp-dctcp.h b/src/internet/model/tcp-dctcp.h
index e06d21bb2..2fcb04ad3 100644
--- a/src/internet/model/tcp-dctcp.h
+++ b/src/internet/model/tcp-dctcp.h
@@ -72,6 +72,8 @@ public:
   virtual void Init (Ptr<TcpSocketState> tcb);
 
   // Documented in base class
+  virtual uint32_t GetSsThresh (Ptr<const TcpSocketState> tcb,
+                                uint32_t bytesInFlight);
   virtual Ptr<TcpCongestionOps> Fork ();
   virtual void ReduceCwnd (Ptr<TcpSocketState> tcb);
   virtual void PktsAcked (Ptr<TcpSocketState> tcb, uint32_t segmentsAcked,